air trapping in 3rd floor hydro-air unit

[Dave4242]

We had our air handler for our attic space hooked up last week. It has 3/4" copper pipes from our basement up to our attic (which is about 35 feet above our basement boiler). A Taco 007 pump pushes the water in this very long run which has also has a long horizontal run of about 40 feet.
We keep having problems with an air lock. After the thermostat in the area reaches temperature in the 3rd floor space, and the air handler shuts off, when we go to turn on the system again, the water won't flow unless we purge the pipe in the basement.
There is an automatic vent on the 3rd floor air handler, but the contractor wants to increase the size of the T&P valve on the boiler from 30 to 40 psi, as he says the boiler needs to be run at a higher pressure.
Does this seem right? Are there any other ways to eliminate this air trapping on this long run?
What about a bigger pump for this zone?

[Ed Imeduc]

Lets go this way. Not sure what the psi should be for you there. But with the pump turned to off and the boiler can be hot. Open that top bleeder valve up on that top floor coil.Now you should get water out of it. If not turn up the Psi on the boiler slow till you get water out of it. that should be the Psi the boiler should work at . You dont need a bigger pump I dont think.
ED ;)

[Dave4242]

Ed,
I'm trying to understand your reply. I should leave the pump for the 3rd floor air handler off when I open the bleed valve?
It is one of those automatic ones (looks like a 3 inch tall cylinder with a schrader (tire) valve on the top.)

[Ed Imeduc]

Hold it down till you get water out of it. The water PSI has to be as high as it takes to make water come out of that valve stem up there. The pump on a boiler dont say pump the water to any psi. All they do is make the water go around and around in the pipes and the boiler.

ED ;)

[KField]

If your circulating pump is on the return side (old time way) of the boiler, you may actually be drawing air into the air vent on the hw coil. I would suggest closing the cap on the air vent after the system is filled with water and the heat is not running. There is no other place for air to enter the system and it is always best to keep that cap closed in an attic anyway. It prevents an indoor rain storm when the vent fails. That should cure the problem. Don't go nuts with pressure and extra expense.

Ken

[arkayassoc]

I agree with Ken, don't go nuts over pressure. 14.7 psi is the same as a column of water 30 feet high, so you only need 14.7 psi to force water to 30 feet elevation. If you attic is only 35 feet above the boiler, you don't need more than 20 psi to get water to it.

[Dave4242]

Currently, my boiler pressure is 20-25 psi. With the 3rd floor unit running, I pushed on the schrader valve on the Amtrol automatic vent, and only water came out, so I screwed down the cap tightly.
What would be the cause of air trapping in this circuit when it shuts off during the day?
It seems like when the air handler is off for long periods of time, air accumulates in the circuit. There are no water leaks anywhere, and the circulating pump for this circuit is on the outflow (not return) portion of the circuit.

[arkayassoc]

Any air in the system will always accumulate at the highest point. Any boiler makeup water introduced to the system will contain entrained air, which will end up at the highest point.

If you recently had new piping added to your system, it will take a while for the air to purge.

[KField]

I'm glad to see your pump is on the supply side. That is helpful. A good air eliminator at the boiler could also prevent air from getting past the boiler too. If all the air is out of the circuit to the 3rd floor, it must be coming back from other zones and going to that zone. Is the pipe for the 3rd floor zone the first tee in the supply off the boiler? Or is it the first fitting that points up off the supply? There is always a reason for one rdiator or one zone to be the problematic one. Once the air is all out the problem will be forgotten, until the next time you need to drain the system. One question I have is, with that air vent, why doesn't it take the air out automatically?

Ken

[Dave4242]

The loop for the 3rd floor is not the first one off the supply circuit. There is an air scoop on the primary boiler circuit.
As far as the automatic vent on the 3rd floor air handler, it is one of those small 3 inch tall cylinders (made by Amtrol). The installer said that these are only supposed to remove a small amount of air and are overwhelmed by laarge amounts.
The piping for the circuit comes out of the boiler, runs horizontally across the basement ceiling for about 25 feet, goes vertically up a wall about 20 feet in height, across our living room ceiling about another 20 feet, up into the attic about 10 feet vertically, and then about another 40 feet horizontally to the air handler. Maybe it's just such a long run?

[KField]

Most air scoops are not installed correctly. The inlet needs to be about 2 feet from an elbow and that usually isn't done. If you can picture air in the pipe, it will ride along the top of a pipe until it rounds a bend. Then it will swirl all around until it comes to another straight pipe. It will ride the top of the pipe until it finds a fitting that will let it go higher. Like a tee pointed up or one pointed sideways that has some flow. As for your air vent in the attic, if the air gravitates up there, it should hang out right at the vent waiting to be left out. It sure won't go back down the pipe. Is the vent at the highest point of the attic piping? And is it installed on the highest pipe in the hot water coil?

Ken

[Dave4242]

The air scoop/vent is on the suction side of the main circulating pump, but is about a foot or less from the elbow that is before the scoop.
On the attic piping, the vent is definitely at the highest point in the piping, and is installed on the upper pipe in the coil.
I wonder if there just was so much air that it took several attempts at bleeding the circuit until there was no more air, as we don't seem to have any more problem in the last couple days.
I thought we were having reaccumulation of air, but that seems to have stopped, so I'm wondering if we just never got all the air out the first time.

[KField]

You may be correct about not getting all the air out the first time. For that height, you really have to run the water fast to carry air back down 25 feet of height. It wants to go up. When we purge, we put the hose in a bucket of water and don't stop purging until there are no more bubbles. Glad you got it fixed and I hope it stays fixed. I don't remember if you sais where you live but if it gets real cold, you should consider putting anti-freeze in the system.

Ken

[Dave4242]

Well, I went upstairs again, and the pipe going into the coil was hot, but the one coming out was lukewarm at best.
With the air handler running and pump operating, I pushed down on the air relief valve on the air handler and nothing came out.
I then thought that maybe the valve wasn't working, so I unscrewed it from its fitting with the air handler running. Nothing came out at all. I would have thought that with the valve removed, the pump in the basement would push the air out through the open pipe. The boiler pressure is set at about 25 psi.
Again, the air handler is about 35 feet vertically above the pump in the baseemnt, but total length of the piping loop is probably about 200 feet.
The pump in the basement is a Taco 007.
Ed Imeduc had said to turn up the boiler pressure with the boiler hot and the pump off upstairs. Would the water be able to go up to the coil upstairs with its circulating pump off?
Also, Ken wrote:
"For that height, you really have to run the water fast to carry air back down 25 feet of height. It wants to go up. When we purge, we put the hose in a bucket of water and don't stop purging until there are no more bubbles."
Where do you hook up the hose in the attic or in the basement? There is a hose bib in the suipply pipe in the attic, and one in the basement on the return. The problem is that everytime I purge with the hose, I must add more water from the autofill which then entraps more air.

[Ed Imeduc]

Can the psi gauge be off there on you?? Have to think like this it dont matter how far over or long the pipe is. Its all in how high up it is . Now how it should work

I pushed down on the air relief valve on the air handler and nothing came out.

With like say a new boiler. We put the water in it till water just comes out the highest bleeder on the top floor no pump on. That is what you have to set the psi at. The pump just makes the water go round and round it dont pump it up there.

ED ;)

ED

[Dave4242]

OK, Ed. I will open the bleeder in the attic and have a helper add water to the boiler until water comes out of the open bleeder, and that will be the working pressure. Is this a good way to bleed the system also (open the feed valve until water comes out)? How do you go about bleeding it? The automatic bleeder doesn't seem to be doing the job it is supposed to do. It is on the highest part of the circuit. I thought its whole job was to release any trapped air automatically.
Also, the boiler PRV is set at 30. What if the pressure is close to that in order to get the air all the way to the atic?

[Dave4242]

I've been running the boiler at 24-25 psi and there seems to be no problem with air anymore. If I were to shut off the automatic bleeder in the attic, and the pressure in the boiler possibly dropped below 20 or so, would air get entrained in the other vents on lower floors or would shutting off the highest vent prevent it?

[KField]

You would not get air in anywhere that the water pressure exceeded the atmospheric pressure. The only reason you may have gotten it in at the aiitc was when the pump started. It may have caused a quick pressure drop until all the water started moving and drawn in a small amount. I would leave all vents closed and only open them when checking for air. Glad you got it fixed.

Ken

[Dave4242]

Thanks, Ken.
What do you think as far as my current scenario....the working pressure is now about 24-25 psi with the water heated. The relief valve on the boiler is set to 30 psi. So far, it has not released any water. Would you just let the system run the way it is or look into possibly increasing the PRV to 40 psi if possible?
Also, I guess I need to increase the autofill pressure to 24 and the expansion tank pressure also to 24 psi empty?
I have an Extrol tank, which I guess I have to remove (take off) completely from the circuit before charging it?

[KField]

If it was mine, I would back the hot pressure down to 20 psi and keep the vent closed in the attic. This would put your pressure where it is better and not admit air anywhere. I would not go to a 40lb. relief.

As for the expansion tank, if you isolate the supply and return lines out of the boiler and drop the boiler pressure to 0 you won't get any air in the system but will get a true reading on the expansion tank. I would set it for 18-20 psi. if you plan on running the system around those pressures.

Ken

[Dave4242]

So I would shut off the supply line and the return line. How would I drop the boiler pressure to 0? Would that involve removing water from the boiler primary loop until the pressure on the gauge is 0? I'd rather not lose any water as it now has glycol in it.
Also, you said to set the expansion tank pressure to 18-20 psi. I'd assume that I would also increase the autofill pressure also to 18-20. Is that right?
Thansk again, Ken.

[KField]

You are correct. You can get the water out of the relief valve on the boiler or the drain on the boiler. You should only get a quart or two. (Just what is in the expansion tank) And you could increase the pressure setting of the auto feed valve but no harm will come if you don't. When I have a system with glycol, I prefer to keep the auto feed closed so that if a leak develops I can see it in the pressure gauge istead of watering down the antifreeze without knowing it.

Ken