Boiler/Thermal Storage Expansion Tank Sizing

[DSSA]

I've picked up an older Eko 40 gasification wood boiler, an unused, 2024 production date, 1040 gallon, ASME certified (but I need to insulate) thermal reserve tank, an older 41 Gallon BoilerMate Indirect water heater, and am looking into expansion tanks so that I can start laying out my plans for installing this all.

I've been hearing/seeing that expansion tanks should be sized roughly 10% of the system volume, and understand that there are calculators out there for sizing them, but without knowing the delta temps at this point, nor exactly what volume will be with the lines running to the house from the detatched garage it will all be be housed in, I'm just going off of that "rule of thumb", which is telling me that I basically need a 150-200 gallon expansion tank, which is a hard find.

I have have some height in said garage (I believe it's almost 14' tall ceiling, and I do have a crawl space above it, although it's not an easy access), and see that mounting the expansion tank above it will lessen the size needed--is this correct? If so, by how much?

Is it also acceptable to run several smaller tanks in series-parallel?

Any other solutions to "skinning-the-cat" that I'm missing?

My plans are to hopefully batch-burn this boiler as opposed to letting it idle.

Thank you.

 

[hobbyheater]

My storage tank is 1040 imperial gallons and my expansion tank is a 40 gallon hot water tank ,works fine .

 

[DSSA]

My storage tank is 1040 imperial gallons and my expansion tank is a 40 gallon hot water tank ,works fine .

Is it mounted higher than your storage tank?

 

[salecker]

I have 1000 gallons and got the biggest one i could, i had to add the expansion tank that came with the boiler and it took acre of my issues of loosing a bit of water when i reached max temps, and would have to add a couple liters when it was cold.
Now i haven't added for a few years or had and loss of water.

 

[DSSA]

I have 1000 gallons and got the biggest one i could, i had to add the expansion tank that came with the boiler and it took acre of my issues of loosing a bit of water when i reached max temps, and would have to add a couple liters when it was cold.
Now i haven't added for a few years or had and loss of water.

What size is "the biggest one your could"?

 

[brenndatomu]

Our gas fired boiler (hydronic heater) system at work is supposedly 350 gallons, and the expansion tank is maybe 10 gallons...maybe. Been like that since 1969...works fine.

 

[DSSA]

Our gas fired boiler (hydronic heater) system at work is supposedly 350 gallons, and the expansion tank is maybe 10 gallons...maybe. Been like that since 1969...works fine.

Thanks, but that's a completely different beast. The unless the Delta T on that system is 100+ degrees. If I was letting the boiler idle, I'd probably be able to get away with something like 30-40 gallons, depending on what the system is set to kit on at. I'll be batch burning, so in the summer, or if I don't use it for a while and resort temporarily to heat pump/oil, the Delta T could be as high as 145 degrees or more.

If you're running a wood boiler constantly, or running a gas/oil boiler, and the water temp isn't swinging as much, you can get away with smaller expansion tank as the water volume isn't contracting/expanding as much. Still, I'm not seeing how the "10% rule-of-thumb" expansion tank thing is realistic. Maybe in case of an over-run condition and accounting for steam? You'd think that would be taken care of/safeguarded by the relief valve.

For example (assuming 1000 gallons, temps in Fahrenheit, and cubic feet of space):

Temp/Specific Volume
50 degrees/16.024 ft3/lb.
100 degrees/16.130 ft3/lb.
150 degrees/16.342 ft3/lb.
200 degrees/16.633 ft3/lb.

The end calculations (volumetric thermal expansion) come out to 1000 gallons @ 50 degrees ends up being 1017.8333 gallons (U.S. gallon), so an 18 gallon expansion tank should suffice, but even if we double it to be safe, we're looking at 36 gallon expansion tanks. But everything I'm seeing says to size expansion tanks to 10% of volume, which makes me think I'm missing something obvious with my smooth brain.

 

[brenndatomu]

That's true...it kicks the burner on at 160* then off at 180*...delta T is ~30-40*

 

[OffGridICF]

For 825 gallons of total volume (storage + boiler, etc.) I'm using an SHXT90 and SHXT110 in parallel. Parallel tanks are fine. Once I figured out the needed volume I looked at different size combinations to find the lowest price point. Some people advocate tying the Shrader valves on multiple expansion tanks all together to make sure they are at the same pressure but I did not do this.

Check out the Watts expansion tank calculator. I made a plot of acceptance volume vs. pressure using it which you can see in this post:

https://www.hearth.com/talk/threads...ons-of-storage-offgridicf.204701/post-2736783

More info on the expansion tank acceptance volume, etc. are provided in this earlier post in the same thread:

https://www.hearth.com/talk/threads...ons-of-storage-offgridicf.204701/post-2736574

I placed the expansion tanks on the second floor which provided a few PSI of extra margin. Setting the pressure at the boiler to 12psi cold allows the expansion tanks to be set at about 10psi (2.31' per psi for the height difference of about 5-6'). With everything up to about 185F the system pressure goes to about 25-26psi at the boiler if I recall correctly. The 30psi pressure relief valve does not ever open.

 

[DSSA]

For 825 gallons of total volume (storage + boiler, etc.) I'm using an SHXT90 and SHXT110 in parallel. Parallel tanks are fine. Once I figured out the needed volume I looked at different size combinations to find the lowest price point. Some people advocate tying the Shrader valves on multiple expansion tanks all together to make sure they are at the same pressure but I did not do this.

Check out the Watts expansion tank calculator. I made a plot of acceptance volume vs. pressure using it which you can see in this post:

https://www.hearth.com/talk/threads...ons-of-storage-offgridicf.204701/post-2736783

More info on the expansion tank acceptance volume, etc. are provided in this earlier post in the same thread:

https://www.hearth.com/talk/threads...ons-of-storage-offgridicf.204701/post-2736574

I placed the expansion tanks on the second floor which provided a few PSI of extra margin. Setting the pressure at the boiler to 12psi cold allows the expansion tanks to be set at about 10psi (2.31' per psi for the height difference of about 5-6'). With everything up to about 185F the system pressure goes to about 25-26psi at the boiler if I recall correctly. The 30psi pressure relief valve does not ever open.

I used the Watts calculator and put in suspected volume (had to do quick estimate on the piping volume as I haven't started setting it up yet), and with 1040 gallon tank, Eko 40 (25 gallons), and estimating another 185 in the lines to the house and between the boiler and tank(probably too high, but I'd rather over-estimate and have some headroom to compensate for my stupidity), start pressure of 12, end pressure at 25 (relief will actually be 29-30), start temp at 40, end temp 200, it's telling me 217.4 total and 43.9 acceptance on the expansion tank. I'm guessing that both values need to be met/surpassed?

I just picked up an Extrol SX-90V today (new) for $300, which is 34 gallon acceptance, and 44 tank volume. So would a second tank put me there (68 gallon total acceptance), or would I actually need 4 more (to surpass the total volume)?

I didn't think the difference between the two would be so high in the calculator.

 

[OffGridICF]

The acceptance volume depends on the maximum pressure and the total tank volume. The 68 gallon acceptance for 2 of the SX-90 tanks will not happen until the pressure is 51.5psi, far more than the 30psi PRV setting. Three tanks will drop this to 33.7psi. Four tanks will be required to get down to only 27.8psi with 43.9 gallons of acceptance, 176 total gallons.

I'm running 1x SX-90 and 1x SX-110 in parallel for an 825 gallon system and it is just enough. A pair of SX-90s will not be nearly enough for 1250 gallons at 200F. My boiler setpoint is 185F and it goes into idle at 190F so I used 190F as my max temperature. Depending on the total elevation change in your system (1 story vs. 2 story or elevation change from the garage to the house) you might be able to drop the precharge and target minimum cold pressure to 10psi. Or if you can get the expansion tanks up higher you can shave some psi off the expansion tank cold setpoint. You need enough pressure to prevent pump cavitation (minimum pump input head pressure) and you need to make sure the air vent at the highest point never goes to 0psi or less to prevent sucking air into the system. The pump should pump away from the expansion tank (pretty sure you've already heard that ;-).

Using the Watts expansion tank calculator, I created these tables for you. Knowing the total tank volume, for a given system volume I iterate the maximum pressure until the resulting tank volume matches the specified tank volume (132 gal for 3x SX-90, 176 gal for 4x SX-90) and enter the resulting max pressure and acceptance volume into the table. This will allow you to see how different system volumes and pressures drive the required acceptance volume.

If you change your target to 1250 gallons, 190F max, 10psi minimum then 3x SX-90s will result in a max pressure of 27.2psi at an acceptance volume of 39.1 gallons (132gal total volume). This might be a little tight but probably OK - the PRV might open at less than 30psi. You don't want it to drip. You could try 3 of the SX-90 tanks and add another smaller tank later if needed.

 

[hobbyheater]

Is it mounted higher than your storage tank?

Yes its higher open ended , static pressure. Has worked fine for 40 plus years.

 

[hyfire]

If you go 10 psi and dont need the height it will allow for quite a bit more expansion. You will need about 15 psi for pump cavitation prevention at 200F approx , just keep that in mind.

 

[DSSA]

Well, I just picked up an SX-110V on the cheap ($75, used, but everything checks out) to go along with the SX-90V ($300 new).

I'm keeping my eyes open for any more 'deals' I can find.

With the SX-110V being quite a bit larger (physically), I was puzzled to see that Amtrol specs it at 34 gallon acceptance volume, which is the same as the SX-90V (giving me only 68 gallons acceptance thus far). I'm guessing that it will accept more than the SX-90V at pressures higher than 12psi?

Edit: My two remaining brain cells fighting for third place seem to have just tapped each other. Would the difference be that the SX-110V will take the full acceptance volume at a lower pressure due to the larger overall tank volume?

 

[OffGridICF]

Yes, the larger tank will accept the same volume at a lower pressure (or a greater volume at the same pressure).

Watts gives acceptance volume specs at 12 psi for their tanks similar in size to the Amtrol models:
12 psi precharge
ETSX-90:
Tank Volume = 44 gal
Acceptance at 12psi = 17.7 gal

ETSX-110
Tank Volume = 62 gal
Acceptance at 12psi = 24.9 gal

Playing around with the Watts calculator to match the Amtrol specified 34 gal acceptance volume:
12 psi precharge
SX-90V
Tank Volume = 44 gal
34.1 gal acceptance at 138 psi

SX-110V:
Tank volume = 62 gal
34.1 gal acceptance at 60 psi

The SX-90V numbers don't really make sense because the maximum rated pressure for the SX tanks is 100 psi (their well tanks are rated at 150 psi). The Flexcon numbers match the Watts numbers while the Amtrol SX-90V numbers are very different. The pressures from the Watts calculator with my system configuration closely match the measured pressures in the system so it seems accurate.

 

[DSSA]

Thanks! I was rolling it around in my head, and was having issues grasping it until I realized that the bladder holds the water and not the air. Then I could visualize it. I was always under the assumption that the bladder held the charge air, which is of course, incorrect.

Larger area, same charge, same size bladder = more water volume with more space in the tank for the air to be compressed in=lower pressure. Just missing that key factor threw me off.

So, as opposed to just going off of the 7-10% rule, calculating each tank's volume at system pressure will be a better way to calculate what/how many tanks are needed.

Crazy that if you bought these tanks new, you're almost at the cost of a boiler, just for the thermal expansion aspect--and they tend to see end-of-life somewhat quickly in the scheme of things.

I realize that there are cheaper ways to do it, but I'd prefer to keep the system as sealed as possible. With 1250-ish gallons, and having to treat it before and after filling it (we have crappy, acidic, well water with a decent amount of iron in it..), I'd like to keep the system as sealed as possible without having air infiltration/adding more water.

Thanks again!