Boilers - Steam and Hot Water Systems - Expansion Tank Size

I have a high-efficiency boiler that supports both baseboard and radiant heat. The boiler is 80,000 BTU's and control the boiler temperature through an outdoor reset. The highest temperature is 160 degrees.

What is the best way to size an expansion tank for this type of system?

Take a look at:
Amtrol - Product Sizing System (http://www.amtrol.com/sizing.htm)
and
http://www.amtrol.com/pdf/extrolbrochure.pdf

My old system had an old model 30, and based on this data I replaced with a new model 30

Should I base the size, of the expansion tank, on "Temperature and System Content" or "Boiler Net Output and Radiation Type"?

I'm not an expert on this, but I think both those numbers should match up. I went on Boiler Net Output and Radiation Type as my new setup is pretty straight forward. However, the tank I replaced was the same size, and my old setup was much larger than current ... so maybe I oversized. By BTU output, I needed model 30. Looking at temp, I think at 180-200F range, I am well under the water volume as my boiler is only about 11gal.

Size by the water volume of the system. That includes the boiler, the radiation, and the piping. Be in the ballpark.

If you want a pretty close measure and you have to drain the system anyway, drain it into 5 gallon buckets-full.

xiphias,

Would draining the system, in 5 gallon buckets work? How would all the water in the system be able to get out? My radiant circuits are over 1000 ft combined.


JamesNJ,

The numbers were not teh same to me:
- Temperature and System Content is calls for a 15
- Boiler Net Output and Radiation Type calls for a 30

The 30 size is by far the most used size. I should think that with a high efficiency boiler, the water volume is low. Even with all that radiant tubing, I'd be surprised if you had more than 20 gallons (and that's a stretch) in the whole system. I seriously doubt that you would need bigger than a 30...

There are tables out there that will tell you how much water is held in the piping, and your boiler specs should tell you the amount of water in the boiler... there's no need to drain it and actually count every last gallon... don't even think about it.

When you say your boiler 'supports both baseboard and radiant', does that mean that you actually have both types installed?

1000 feet of radiant tubing, what size tubing?

How many feet of baseboard and associated piping? (and I assume it is 3/4")

What make/model boiler?

I have about 15 gallons of water in my system. The boiler (Peerless Purefire 80 BTU) will fire at 180 degrees on the coldest day (have outdoor reset). My system has both baseborads and radiant circuits in it.

So that's 15 gallons in the entire system, correct?

If you heat that 15 gallons from room temp to 180, you now have approximately 15.6 gallons in the system. Your expansion tank only needs to take .6 gallons. The 30 size is fine.

Thing about using the 30 even though you can get away with the 15 is the fact that if you need to replace it on Sunday, Home Depot, Lowes, etc usually stock that size. So even if you do use the 15, make sure you have room to fit a 30 in the same space.

A larger tank hurts nothing... too small is where the problems start.

OH, by the way, 3/4" copper holds about 2.5 gallons / 100 feet.
1/2" copper about 1.25 gallon / 100 feet.

Pex I believe is slightly smaller inside diameter, so your radiant circuits might hold say 10 gallons or so.

15 gallons was rough calculation. I have:

3/8" Pex - 1,200'
3/4" Pex - 150'
1.25" copper (header) - 8'
1" copper (header) - 5'
3/4" copper - 20 '

You it be better to go with an expanision tank that has a plastic liner, since they are designed for radiant/high-efficiency systems?

And your boiler has less than one gallon in it... I would say your estimate of 15 is pretty close.

You only need the plastic lined expansion tank if your PEX tubing is NOT of the oxygen barrier type. If you need the plastic lined tank, then you also need BRONZE pumps, etc...

So, as long as your tubing is the oxygen barrier type (as it really should be), then you can use a standard expansion tank.

What does it say on your tubing?

My tubing has an oxygen barrier. You are good.