Boilers - Home Heating Steam and Hot Water Systems - Pump too small --Water starting to boil?

Hello…

I’ve installed a wood fired water heating system for the house. I’m using a Grundfos 15-18 SF that was lying around here. A couple of problems, or at least what appear to be problems have emerged:

- I can hear the boiling inside the stoves water jackets during a pre-heat cycle
- Inbound pressure (inside the house) is 7 to 9 PSI
- Distance between the house and wood fired heater is 200 feet

Other details:

- Temperature at the inbound coupling (inside house) matches the stoves (155 to 160 F)
- Water is fed into a heat exchanger located in my forced air furnace

- Stove sleeps when internal water temp reaches 160 F.

I’m not an expert, but I don’t think you should hear boiling inside the stove at any time. Almost sounds like the water is sitting inside the stove too long, thus nearing or exceeding 212 F.

I heard that minimum pressure for most hot water based systems is 12 PSI. I'm getting decent heat, but pushing water through a total of 400-feet of line could be resulting in a below normal flow rate. I dunno... Something here does not seem right.

For anyone interested, here's the specs on the pump currently installed:

Grundfos UP15-18SU 1/25 HP Recirculator Pump:
• Flow range: 0 - 12 U.S. GPM
• Head range: 0 - 7.5 FEET
• Motors: 2 Pole, Single Phase, 115 Volt
• Maximum fluid temperature: 230°F (110°C)
• Minimum fluid temperature: 36°F (2°C)
• Maximum working pressure: 145 PSI


Should I be using a larger pump, and if so, what should the ‘Head FT’ be? I have a feeling I'd get even more heat if the flow was increased. What do you people think?

Your comments or insight would be appreciated!

Dave

That circ sounds way too small... is your piping large enough?

What piping size is that 200' out and 200' back circuit?

How many BTUs is the wood stove?

----- ----- ----- ----- -----

Typically, you look at the the number of BTUs that you need to transfer, that determines roughly what piping diameter you'll need. You need to flow a certain number of GPMs to move the BTUs and you don't want to move them faster than 4 feet per second or you'll have piping flow noises and premature where inside that loop. So that'll mean that you do X or Y diameter. X might be cheaper to pipe, but then when you calculate the head, you may find you need a monster pump to have sufficient GPM through such a tight diameter pump.

Anyway, your pipe is what it is, but we don't know what it is, so maybe you can give us the above requested info and also point out what fittings are part of that big loop. Any fitting that is constrictive adds to the head since the water needs to speed up through that section to maintain the overall flow.

Sorry I can't just come up with a pump recommendation...

My reply may be a little verbose, but I don’t know of a short way to provide an accurate picture of this situation.

That circ sounds way too small... is your piping large enough?

What piping size is that 200' out and 200' back circuit?

I should clarify. This is a homemade stove. I did not build, or install it, but did lend a casual hand to the development of it over the past year. The builder (the guy I lease this place from) is out of town for 2-weeks. I have a choice… Keep running it like this until he gets back, or see if I can correct the problem based on a combo of observation, elementary knowledge, and hopefully some help from here. I’m concerned about internal warping of the water jackets /pipes, or other potential damage from (what I think) may be a lack of flow. I could be entirely wrong though.

How many BTUs is the wood stove?

That’d be hard to guess, but if I had to, I’d say between 150,000 and 200,000 BTU. It’s a completely new high efficiency design we’re trying out, and man… Does this thing thunder when you run it at maximum air intake. You can feel the subsonic resonance through the very concrete it sits on! :)

Typically, you look at the the number of BTUs that you need to transfer, that determines roughly what piping diameter you'll need. You need to flow a certain number of GPMs to move the BTUs and you don't want to move them faster than 4 feet per second or you'll have piping flow noises and premature where inside that loop. So that'll mean that you do X or Y diameter. X might be cheaper to pipe, but then when you calculate the head, you may find you need a monster pump to have sufficient GPM through such a tight diameter pump.

Anyway, your pipe is what it is, but we don't know what it is, so maybe you can give us the above requested info and also point out what fittings are part of that big loop. Any fitting that is constrictive adds to the head since the water needs to speed up through that section to maintain the overall flow.

It’s 1-inch pipe all the way. Expensive stuff.. $15 per foot. It’s dual line encased in a 6-inch tube, which is encased in a combination of densely packed (what looks like) a urethane, or Styrofoam type of insulation. The whole 200 feet of line is buried 6-feet into the ground. Very nice stuff… 200-feet from the stove and only a 1-degree temperature loss :thumbup:

As for the type, I’m a total novice to this so I need to explain what I see. It’s plastic. You can see through it. It’s speced at a maximum temp of 180 F, and a maximum pressure of 80 PSI. To attach it, you expand the end of the hose with a special tool. Once expanded to desired size, you then quickly place the hose on the male fitting and that’s it! As it shrinks back to size, it pretty much couples itself to the copper pipes –the ones that receive the hot water from the stove, or better termed a 3-way distribution unit that routes the water to the heat exchangers and hot water tank.

As an uneducated guess, I'd say the flow is to slow. The pump is rated at Head Range of 7-feet. What exacty does this mean? Something tells me it's not enough for over 400-feet of line.


Sorry I can't just come up with a pump recommendation...

Heh.. And I thought server-side programming was difficult.

Again.. Sorry for the long replay.

Dave

Dave, that pipe is called PEX (cross linked polyethylene). I'm not certain about this, but aren't there also supposed to be 'crimp rings' on those fittings ? All the store bought outdoor wood boilers I've looked at are not pressurized, they run at atmospheric pressure, open systems, requiring a bronze or stainless steel circulator pump. I think the S suffix on your pump designates stainless ? Is your design different in that it's pressurized ? You using a 'gasifier' design with compressed air injected into the fire, and an 'afterburner' ?

Since you are using a water to air HX coil in a forced air furnace, you don't need to worry AS MUCH about keeping the flow in the piping low enough to prevent the typical whistly, swishing noise that we try to prevent with hot water heating systems. But there is a practical limit to the amount of water that you can pump through 1" pipe.

If this were a closed loop hydronic heating system, 1" pipe is good for about 80,000 BTU ( 8 GPM x 10,000BTU per GPM). Depending on the design of the HX, you may be able to push 12 GPM ... or 120,000 BTU . Is the HX constructed of 1" pipe also ? If not, follow manufacturers recommendation on THAT, because that's your limiting factor.

In other words, no matter what the stove is capable of producing, if you have a 'bottleneck' in the system, that's your limit to the amount of water you can pump.

I'll see if I can find the chart for 'friction losses' (head) on plastic pipe.

Here it is:

Plastic pipe friction chart (http://www.plumbingsupply.com/flowchart.html)

About the 'boiling' noise. Since this is a homebuilt thingy, it's possible that no matter how much water you put through there, you could still have 'hot spots' in the water jacket that no amount of circulation will cure. Could possibly be 'eddy currents' in the internal circulation that only a thorough engineering evaluation and possibly a re-design might cure.

I'm not sure this is even the correct forum... isn't there a 'Wood Heating' forum ? That's where you really need to be. Those guys might have better answers for you.

Dave, take a look at the 'pump curve' for your pump to better understand the relationship between HEAD and FLOW. You will see that you can only pump the max flow at the minimum head.

Your piping has HEAD (from the chart), and so does the HX you are using. They are additive. There is a point on the pump curve that will become your 'operating point', and it's where the flow and head intersect.

The easiest way to MEASURE the flow in your system would be to install a pressure gauge at the suction (you may need a vacuum gauge if the system IS atmospheric) and one at the discharge and calculate the flow from the algebraic difference between the two... or spend a ton of $$$ on an accurate flow meter.

I don't think that pump is up to the task.

Head is a measurement of drag in the water pumps that the pump has to overcome at different water speeds / rates of flow / GPM. Up to 4 feet per second is fine. At that limit 1" is good for 80,000 BTUs. An 1½" is good for 220,000 BTUs and that's what I'd be using or maybe twin the one inch to at least double the capacity. You could do that and fire the second circulator as needed on a setpoint contraller... like Johnson A419. Maybe that and Grundfos 15-58s on high speed.

You really need 1½ hopefullly threaded piping to handle the expansion cycles.

You might want to price Taco 011s, Grundfos 26-99s. Not cheap but unless you get some fat pipe down, your gonna need an expensive pump using 2 amps instead of 0.7 amps.

Who and NJ Trooper... An extended thanks for all info /suggestions you've provided.

I’m limited to making any major modifications to the unit –at least until the builder returns, however a minor increase in flow would be easy enough to accomplish by swapping out the 15-18 with a UP 26-99. Somehow, I think this is all that will be required to solve the original problem of water boiling in the stove –just need a tad more flow.

If however, it’s a design issue, which very well could be the case, then we’ll need to live with that, but I think we can all agree that a pump with a Head range of 0 - 7.5 FEET is inadequate for circulating through 400 feet of line. So at very least, a stronger pump will be required.

I should mention once again that the system is putting out some pretty steamy heat and lot’s of hot water too, so all is well as far as that goes. We’re also heating a 3,000 SF 2-level barn. The barn in just 70-feet from the stove, so the 15-18 may be better suited to this task, while the UP 26-99 can manage the 200-foot run to the log cabin. And yes… Those pumps are not cheap!

Much thanks again!

Dave

..., but I think we can all agree that a pump with a Head range of 0 - 7.5 FEET is inadequate for circulating through 400 feet of line. So at very least, a stronger pump will be required.

Dave, if you had some huge diameter pipes, you could be under 7½' of headloss... but you have the 1" and a lot of heat to move.

In order to move 160,000 BTUs through 1" PEX, you'd have to flow the water at 8 FPS for 16 GPM. The resulting headloss just for the PEX would be 0.148' of head loss per lineal foot or 59.2' plus fittings making it even higher.

http://www.grundfos.com/web/homeus.nsf/GrafikOpslag/superbrute/$File/scoreboard.jpg

As you can see, even a 26-99 won't be able to move that kind of heat unless you put 3 in series.

What's the heatloss on the structure that the boiler is serving? That's all the heat you should need to move.

Unfortunately, I don’t have those specifications. Out here in rural land, it seems that most implementation and rollout is done though a combination of past experience, trial, and error. I think I mentioned before that the water temp at the stove is 160 F.

Despite the 200-foot distance, the gage at the heat exchangers is also showing 159 to 160 F, which is a marginal differential. Even with the forced air running, the gage at the exchangers does not drop –well maybe it might after 20-minutes, or until the next burn cycle, but you get what I mean. It pretty much stays matched with the stove.

So without fully understanding the official formulas here, I must ask this question: If the water temperature at the stove matches the temperature at the exchangers, what discernable advantage would I gain by increasing flow to 16 GPM, which in my guesstimate, would be an increase of up to 11 times what it is at present?

On a side note, I did a temperature test on the forced air system yesterday, (placed thermometer on air register). I fired up the high efficiency propane, which resulted in a maximum air temperature of 117 F. I then cranked up the stove to 160 and measured air temperature again. I measured 117 to 119 F. Is this significant at all?

Btw.. The cabin is small. Only 1,300 SF on 2-levels and very well insulated.

Thanks again Who,

Dave

I mentioned the air to water HX being your limiting factor several times. You will get just so much heat out of it before you reach the point of diminishing returns. You need to look at the manufacturers spec on THAT, before you start trying to pumps tons of flow through it. You can try and pump all the heat you want INTO that thing, increase the flow to the moon. You will only get just so much heat OUT of it. The more water you flow, the hotter it will come back to the boiler.

The HX is your bottleneck.

The HX is your bottleneck.

The HX is your bottleneck.

there, I've said it several more times. :wall:

OK, you've got a 1300 sf cabin, superinsulated. Let's guesstimate and say that the heat loss is what, maybe 50,000 BTU MAX. And now you want to pump HOW MUCH? heat into that little cabin ? Y'all will be running around naked in the dead of winter with the windows open. But, you will never do it with only that one HX . It's got a BTU rating, and that's it ... X sq in" of surface area, Y cubic feet of air per minute.

You've got a boiler that's pumping what, 150,000 BTU into that water ? If this were an oil fired or gas fire conventional boiler, can you say SHORT CYCLING ? on/off............./on/offffffffffffffffffffffff

See what I'm trying to say here ? You only need enough flow through that HX to meet the manf specs. More will do very little for you. If you don't believe this, look at the specs for fin tube baseboard. There are usually two lines on the chart. One is for 1 GPM, the other is for 4 GPM ... how much difference is there between the two ? Not a whole heckuva a lot...

You've gotta find someplace other than that little cabin to dump those extra BTU's. You said there's a barn ? Why not focus on dumping the extra heat in there ?

...but I think we can all agree that a pump with a Head range of 0 - 7.5 FEET is inadequate for circulating through 400 feet of line...

Not necessarily.

You only need to pump enough water through that loop to meet the design spec of the EMITTER. Any more is wasting your money on the bigger pump. So what do you need for that thing ? Maybe MAX of 4-5 GPM ? The pump you have may be able to do that.

When I said in my earlier post that the pump may not be up to the task, what I meant was that it might not be up to the task of pumping 8 GPM, and certainly not 12 GPM, but since I realized that you don't have a place for that heat to go, that point is moot.

Look at the spec for the HX, and MEASURE the flow in the loop. You might be surprised...

Sorry for my ranting, just trying to have you understand my point, so I'm gonna re-state it another way.

If you pump more water through that loop, you need to add HEAT EMITTERS that can deal with that extra water, and extract the extra heat from it. If you don't do this, all you will succeed in doing is wasting money. It won't solve a thing, because the bottom line is that the water will only return to the boiler hotter than it already is returning.

It's physics, not server-side programming ! :D

Sorry… I should have addressed your question earler in the discussion.

The HC, or acronym for heat exchanger (I think) does not have those specs on it. Why? Because the two exchangers came out of a very old school bus. They had to be cleaned up and now work awesome, but I could find no specs on them.

Yes, if we’re not heating the barn, we have a lot of energy to spare, however the stove often goes 4-hours without a cycle. The ‘original concern’ was pushing up the water flow slightly to prevent water from boiling inside the stove, and partially because the burn cycles are so ferocious.

The builder and owner of this properly knew the stove was oversized, but wanted to make sure it would heat the barn when necessary. In any event, maybe there are few options here and boiling water may be something that’s normal on this stove, but still… I can’t see how allowing the water inside the stove to get 'boiling hot' can be a good thing.

Dave

Sorry for my ranting, just trying to have you understand my point, so I'm gonna re-state it another way.

If you pump more water through that loop, you need to add HEAT EMITTERS that can deal with that extra water, and extract the extra heat from it. If you don't do this, all you will succeed in doing is wasting money. It won't solve a thing, because the bottom line is that the water will only return to the boiler hotter than it already is returning.

It's physics, not server-side programming ! :D

Gotcha :)

Grin... Not server-side programming. That's funny :D

Seriously man, I do appreciate the perspective you’re providing here. The above post was actually the most helpful and sums it up quite eloquently.

D.

Glad I could make ya laff ! In person, that's what I usually seem to do best. First when they see me, next when I open my big mouth ! :D

Man, that setup sounds like it's straight outta "Mother Earth News" ... I love it... school bus radiators... kewl...

OK, we all understand now... great.

Lemmee just throw an idea out here. THERMAL MASS . Since you might typically only be extracting say 50 mBH out of the boiler, and it's capable of producing say 150-200 mBH, to keep the boiler from boiling ... (wait, did I just say that? , why DO they call them boilers anyway ?) ... you need to draw more heat out of the water. If you designed and built a large super-insulated 'buffer tank', you could set up another heating loop to also heat that water. Is there a swimming pool nearby ? There could be a temp control on that buffer tank that would keep the boiler from waking up on a call for heat when the tank was hot, and extracting the heat from the stored water, until such time as the water cools below a useable heat source temp. You could leave the piping to the cabin alone, and turn the circs on both loops on, drawing heat from the buffer tank into the boiler, hence into the cabin. If the storage tank were buried and insulated, it would lose little heat even in middle of winter... just some thoughts to chaw on.

You didn't answer the question about whether or not the boiler was pressurized or atmospheric ?

I have one more question I meant to ask...

Supply temp you said is 160*F, do you have a temp gauge on the RETURN from the cabin ? Curious as to what that might be...

Glad I could make ya laff ! In person, that's what I usually seem to do best. First when they see me, next when I open my big mouth ! :D

Sense of humor is always a good thing. As much as I’m enjoying the experience out here in rural land, the humor is, well… I don’t want to say lacking, but you don’t get the good sass you do in the city:)


Man, that setup sounds like it's straight outta "Mother Earth News" ... I love it... school bus radiators... kewl...

Yeah, I’d even go as far as saying that Matt’s Mr. Father earth himself. What he's done with this 100-acre farm is beyond words, including an 1860's log cabin, which he completely dismantled and then rebuilt .This guy can build anything, and despite a few of his unorthodox approaches, his resulting creations never cease to amaze most.

OK, we all understand now... great.

Lemmee just throw an idea out here. THERMAL MASS . Since you might typically only be extracting say 50 mBH out of the boiler, and it's capable of producing say 150-200 mBH, to keep the boiler from boiling ... (wait, did I just say that? , why DO they call them boilers anyway ?) ... you need to draw more heat out of the water. If you designed and built a large super-insulated 'buffer tank', you could set up another heating loop to also heat that water. Is there a swimming pool nearby ? There could be a temp control on that buffer tank that would keep the boiler from waking up on a call for heat when the tank was hot, and extracting the heat from the stored water, until such time as the water cools below a useable heat source temp. You could leave the piping to the cabin alone, and turn the circs on both loops on, drawing heat from the buffer tank into the boiler, hence into the cabin. If the storage tank were buried and insulated, it would lose little heat even in middle of winter... just some thoughts to chaw on.

Some interesting ideas, which have certainly dawned a different perspective on this issue. There are also some microprocessor based solutions I’ve been pondering, however anything binary based is not well received in this camp.

You didn't answer the question about whether or not the boiler was pressurized or atmospheric ?
I would have to guess atmospheric, since the function of this stove is to ‘heat’ the water, not to boil it. Nothing is pressurized on it. There are several air bleed valves on it, but only for the purpose of dealing with associated air lock issues found in most types of circulatory systems.

Some thoughts

In any event, this is yet another way of looking at the equation –maybe there’s so much energy being generated that we’re simply not getting rid of it fast enough. I’m still trying to get an idea of what a reasonable flow rate should be. I find it hard to imagine that a pump with Head range of 0 - 7.5 FEET could be adequate under any circumstances for a loop that’s 400-feet in total.

I mean… That’d be what? About 1 GPM, probably less? That’s a wild guess, but I’ll bet it’s not too far off. Let me ask this question:

For the sake of boiling, let’s say we increased the GPM by a modest factor of 1.5. Let’s also assume this is just enough to move that water through the stove fast enough so it does not boil. What potential downside could there be here? Theoretically I suppose, it could return the water back to the stove too quickly, thus resulting in the same boiling issue as now.

Thinking aloud about something else I tried here…

If I fire up that stove (with the pump off), you can hear the water starting to boil when it’s approaching 147 F. Ok well… It will boil even when the pump is actually running.

But in this case, the circuit was full of cold water. When I activate the pump, the sudden inflow of cool water into the stove should immediately stop the boiling process correct? Well it does not… This may suggest that although cold water is being pumped in, its rate of input is so slow, it can’t replace the boiling water with cool water fast enough to halt the superheating process. Wouldn’t this suggest an ‘abnormally’ slow flow rate?

Yeah I know… That reasoning doesn’t sound terribly scientific –it sounds speculative or deductive, but is there any merit to it at all?

Sorry for the long ramble..

D.

I have one more question I meant to ask...

Supply temp you said is 160*F, do you have a temp gauge on the RETURN from the cabin ? Curious as to what that might be...

There’s no gage on the return line, but I’ll bet the difference is within 2 to 3-degrees. I want to get one of those laser based heat scanners, as there’d be more than enough uses for it with this application.

D.

IR guns are great unless you're shooting copper tubes... :wall:

Anyway, if the water going back is nearly the same temp, then your little heat transportation route is sending the trucks back nearly full. I would think that with a 200' run that you'd have a difference in temperature of at least 20° and that's before insulation losses.

Any considerations of setting up a warehouse (buffer tank) to store the BTUs? You could get better flow through the boiler to help extract all the BTUs and keep the hot spots from boiling and then supply the house from the BTU storage tank. Sizing the tank right could allow perhaps a burn of X hours to supply the BTUs for a day or 2 at low outdoor temps...

IR guns are great unless you're shooting copper tubes... :wall:

Anyway, if the water going back is nearly the same temp, then your little heat transportation route is sending the trucks back nearly full. I would think that with a 200' run that you'd have a difference in temperature of at least 20° and that's before insulation losses.

Any considerations of setting up a warehouse (buffer tank) to store the BTUs? You could get better flow through the boiler to help extract all the BTUs and keep the hot spots from boiling and then supply the house from the BTU storage tank. Sizing the tank right could allow perhaps a burn of X hours to supply the BTUs for a day or 2 at low outdoor temps...

Indeed, I’ve been thinking about that. It would also make sense (at least in outside temperatures above 33 F) to shutdown the pump, as calls for heat could be as much as 4-hours a part. Otherwise, the stove continues to cycle for the sake of keeping the 400-feet of line heated, albeit it very well insulated. Nothing's being lost in the outside run --only at the exposed plumbing inside.

Now... On the downside, it may result in the cabin cooling down faster. The reason why the forced air blower is rarely activated is because the heating lines, hot water side arm, and the plumbing around the heat exchangers omit a tremendous amount of heat, which rises to the main floor. I mean the basement maintains 78-degress at all times. We can maintain main floor temperature of 69 F with an outside temperature of 31 –no blower. Or use the blower every few hours and you can maintain 71.3 F.

Interesting question… Would it take more energy to allow those lines to cool down, and then heat them back up, or leave the pump running, which maintains the heat in the loop? If the pump was off, the 75-gallons of water in the stove would not likely drop below 150 F. So when the pump fired back up, the system would have an 80-gallon head start.

I dunno… It’d probably balance out to the same amount of fuel being used and this method would likely be limited to outside temperatures ‘above’ 33 F. Just rambling aloud…

Another unrelated problem is that wonke thermostat being used. It’s an old stat out of a hot water tank. It opens and closes the air-intake on the combustion chamber to maintain a water temp of 160 F. The problem is the low threshold is 30, to 33 degrees! So the stove shuts off at 160 F, and then cools down to 127 to 130 F before firing again. I’m not an expert, but that’s a sizable amount of variance. I would have thought a low threshold of 143 F would be more energy efficient, but what do I know…

D.

Dave, I'm gonna try to explain why I don't think the return water is coming back as hot as you think it is. But first...

Who, those lines they've got buried 6' in the ground aren't going to lose ANY BTU's to speak of. Google up "outdoor wood boilers" and find the company that sells that duplex super insulated underground piping. It's amazing stuff...

The Heat Train

Think of the water as a train carrying heat. You load the train at the boiler, and unload the train at the fan coil in the furnace. The rules of thumb are - for every GPM of flow, if you remove 10000 BTU, you will drop the temp of the water 20*F.

So, let's say that your fan coils in the furnace are good for maybe 40000 BTU. This means that you would want to pump 4 GPM through those coils, and the return water would be 20*F cooler in the return.

Pumps / Pump Curves / Operating Point

Your guesstimate of 1 GPM is probably way low.

Look at the chart link that I posted, and the curves that Who posted.

You see in the chart that the HEAD is directly proportional to the FLOW. Even in 400 feet of pipe, if you only flow 1 GPM, you will see that the HEAD is almost non-existent.

The pump curves are inversely proportional, the more HEAD, the less FLOW.

When that pump is operating, it will 'settle' on an OPERATING POINT where it's ability to pump water will match the head that it is pumping against. At some point, the pump curve, and the SYSTEM CURVE (which is an unknown, because you don't have data on the school bus radiators) will intersect. That's your operating point.

Look at the pump curves. For this example, imagine that you have ONLY the pipe loop, out and back 400 feet. Take the numbers for 1" pipe from the chart, and plot them on the curve. You should have a line that starts low on the left, and goes up toward the right. This would be your SYSTEM CURVE.

At some point, those two curves/lines intersect. That is your operating point.

Now, unless you have a way to UNLOAD THE HEAT from the train, it doesn't matter a dang bit how fast that train goes. In fact, the faster it goes, the harder it is to remove the heat! Imagine trying to unload a train going 80 MPH ... it ain't easy, is it ?

BUT, it sounds to me as though your problem could be two-fold. Boiler design, AND system design. You've got surfaces on that heat exchanger that are extremely hot. It appears that the design of the boiler and system is not moving COOLER water past those hot spots to remove the heat from them. Circulating MORE HOT WATER isn't really gonna do the trick, THE TRAIN IS FULL ! NO ROOM FOR MORE ! You need to empty the train (cool the water) before it comes back to the heat warehouse.

Does this make any sense ? or have I had one or three or five too many Hefeweizen's tonight ?

The Heat Train

Think of the water as a train carrying heat. You load the train at the boiler, and unload the train at the fan coil in the furnace. The rules of thumb are - for every GPM of flow, if you remove 10000 BTU, you will drop the temp of the water 20*F.

So, let's say that your fan coils in the furnace are good for maybe 40000 BTU. This means that you would want to pump 4 GPM through those coils, and the return water would be 20*F cooler in the return.

Ok, but assuming /let’s just say the heat exchangers are 40,000 BTU. The above calculations would also be based on 3” piping, correct? Or put another way… You’d need 3” piping to achieve this objective with any degree of success?

Wow… 20*F cooler. I can hold both the inbound and outbound lines and while I cannot guesstimate the ‘actual’ temp differential, I’d say they feel almost the same (within a few degrees). Yep... Water may be returning too hot. An awful lot of guessing here, but suffice to say, we have a tremendous amount of heat to manage /allocate.


Pumps / Pump Curves / Operating Point

Your guesstimate of 1 GPM is probably way low.

Look at the chart link that I posted, and the curves that Who posted.

You see in the chart that the HEAD is directly proportional to the FLOW. Even in 400 feet of pipe, if you only flow 1 GPM, you will see that the HEAD is almost non-existent.

The pump curves are inversely proportional, the more HEAD, the less FLOW.

When that pump is operating, it will 'settle' on an OPERATING POINT where it's ability to pump water will match the head that it is pumping against. At some point, the pump curve, and the SYSTEM CURVE (which is an unknown, because you don't have data on the school bus radiators) will intersect. That's your operating point.

Look at the pump curves. For this example, imagine that you have ONLY the pipe loop, out and back 400 feet. Take the numbers for 1" pipe from the chart, and plot them on the curve. You should have a line that starts low on the left, and goes up toward the right. This would be your SYSTEM CURVE.

At some point, those two curves/lines intersect. That is your operating point.

Now, unless you have a way to UNLOAD THE HEAT from the train, it doesn't matter a dang bit how fast that train goes. In fact, the faster it goes, the harder it is to remove the heat! Imagine trying to unload a train going 80 MPH ... it ain't easy, is it ?

BUT, it sounds to me as though your problem could be two-fold. Boiler design, AND system design. You've got surfaces on that heat exchanger that are extremely hot. It appears that the design of the boiler and system is not moving COOLER water past those hot spots to remove the heat from them. Circulating MORE HOT WATER isn't really gonna do the trick, THE TRAIN IS FULL ! NO ROOM FOR MORE ! You need to empty the train (cool the water) before it comes back to the heat warehouse.

Does this make any sense ? or have I had one or three or five too many Hefeweizen's tonight ?

Makes absolutely perfect sense. It’s beginning to appear as if the main problem may lie in the inability to remove a sufficient amount of heat from the inbound water flow. Unless this can somehow be corrected (and it won’t be easy), the water is being returned to the stove just a few degrees less than when it left. Basically reiterating your train analogy...

Hmm… I’m going to grab a basic digital thermometer with a probe so we can see the temperature of the outbound line –both at the wall, and at the outbound pipe on the heat exchanger. And also at the stove. I highly doubt we’re going to see a 20 F difference, but it will be interesting nevertheless.

What do you think the return temperature differential should be at very least? Maybe 14 degrees?

And no, the Hefeweizen's are further enhancing your ability to conduct this class of Thermodynamics 101:p

Dave

... The above calculations would also be based on 3” piping, correct? Or put another way… You’d need 3” piping to achieve this objective with any degree of success?

I'm not sure where the 3" pipe came from ? No, you can pump 4 GPM through 3/4" pipe. In fact given a big enough pump, you can pump 50 GPM through 3/4" pipe, but I'm not sure the pipe or the pump would survive more than a few minutes !

Wow… 20*F cooler. I can hold both the inbound and outbound lines and while I cannot guesstimate the ‘actual’ temp differential, I’d say they feel almost the same (within a few degrees)...

I'm not sure you could discern the difference between 160* and 140* with your hand. Anything above about 120-130 and your brain is gonna say " HOT! LET GO ! " It goes binary at that point, it either is, or it isn't. Below 120 or so, you have some amount of analog capability.

Hmm… I’m going to grab a basic digital thermometer with a probe so we can see the temperature of the outbound line –both at the wall, and at the outbound pipe on the heat exchanger. And also at the stove. I highly doubt we’re going to see a 20 F difference, but it will be interesting nevertheless.

That will be good data, but keep in mind that while you may be able to somewhat accurately measure the DIFFERENCE, the absolute temps you measure may not be all that accurate. The SURFACE of a pipe more often than not is significantly cooler than the contents.

What do you think the return temperature differential should be at very least? Maybe 14 degrees?

The differential isn't fixed in stone. The main reason that 20*F is a target figure is because it makes the math very easy to do in your head, as long as you remember the rules of thumb. "For every GPM of flow, if you extract 10000 BTU, the water will be 20* cooler returning".

When you design a series run of fin-tube baseboard as an example, it [delta T] becomes very important, because as the water runs through that loop, it unloads heat the entire distance. If you don't have enough flow, or you have too much b/b on the loop, the b/b at the end of the loop won't be able to emit as many BTU (because the water will be cooler) and the system will be 'out of balance'. Rooms at the end of the run may be too cool for comfort.

In your case, with a single set of coils in the plenum of your scorched air furnace, the delta T won't be as important, as long as you have enough flow to provide the needed heat. Even if your return water was 1*F cooler, and you had enough heat output from your emitters, you would be OK, but it would also mean that you could flow a LOT less water (and you would probably want to, since you would save electricity pumping less water, with less wear and tear on the system) and STILL be OK. Keep in mind that the SLOWER the FLOW, the MORE BTUs will be extracted from that water, and the COOLER it would return.

Your case isn't really about more flow, it's about extracting more heat from the boiler. The flow is only tangentially related. (wow! big word! must be the caffeine! :coffee: )

And no, the Hefeweizen's are further enhancing your ability to conduct ...

That's good! Tonight, it's gonna be Irish Red ... :thumbup:

Dave, has anyone ever measured the STACK TEMPERATURE on that boiler ? If the exiting flue gases are above say 350-400, maybe even as high as 450-500, your efficiency is gonna go WAY DOWN on the system.

Taking more heat out of the water will lower the stack temps, and raise the system efficiency (unless you 'dump' the extracted heat to atmosphere, then yer back on the same train).

... I can hold both the inbound and outbound lines ...

You can ? For how long ? 160*F you should just barely be able to touch for less than a second !

I thought you were a server-side programmer... or maybe you are a lumberjack with 1" thick callouses on your hands ?