Sizing new diaphram tank for old system w steel expansion tank

Al Roethlisberger

Thanks, if true, that would indeed make the "conversion" easy, and would seem to make sense.

So, you are saying a #30 diaphragm tank is the correct equivalent replacement for a 30 gallon traditional steel expansion tank?

Thanks!
Al

Al Roethlisberger

Sizing new diaphram tank on old system with steel expansion tank


I have an old, about 80 years, overhead gravity hot water system that was converted to a sealed circulated system back in 1991.

Along with a variety of other updates and fixes I am in the process of working on, I would like to change out the rusty steel expansion tank the previous owner installed.

From the looks of the tank, I believe it was old salvage in the first place and is getting pretty rough looking. So while we are redoing some of the other plumbing, I thought it might be a good time to change out the tank too.

When I have been researching the sizing for a tank, one of the key questions seems to be the overall capacity(gallons) for the hydronic system. This of course makes sense, but given the age of my system, I honestly have no idea. However I do have the capacity for the original open air expansion tank in the attic, and the "new" sealed unit the owner installed in the basement about 15 years ago.

That capacity is about 30 gallons.

So my question is this: What size diaphram expansion tank is equivalent to the old steel "air cushion" expansion tank of 30 gallon capacity?

It could be the same, but before I purchase, I'd like to make sure that one doesn't need a larger, or even smaller, tank when moving to the new diaphram units.

Thanks
Al

al_roethlisberger@yahoo.com

Al Roethlisberger

I thought I had seen some sort of conversion


I thought I had seen some sort of conversion formula for going from an old steel expansion tank to a new diaphram unit. But I'm not certain.

Does anyone have any guidance on this?

Thanks!
Al

Brad White

Al, Try This

Jim_65

Expansion Tank

Al:

If at all possible can you calculate the entire distribution system lengths, pipe size, ie. total system volume? This would be the best way to size the tank.

If not I was taught that a 30 gallon steel compression tank would be replaced with a #30 bladder type. Not quite sure if this is printed somewhere but the info came from a seasoned professional.

Take care.

Unknown

Does this

Help?

Dave

Al Roethlisberger

Calculating volume is possible, but would be very difficult


Yes, it would be ideal if I knew the system capacity, but unfortunately being already installed for 80 years, it is hard to do so. As you can imagine, most of the piping is in the walls, between floors, and other mostly inaccessible areas. So I don't think I could really get a good estimate.

That's why I was hoping for some conversion formula for going from the existing 30 gallon expansion tank to using a new diaphragm based model.

Thanks!
Al

Brad White

Aw, c'mon... give it a try!

Expansion tanks are cheap, relatively speaking. Buy multiple small ones (#30's) and pipe them in battery for redundancy. So you buy two maybe three? Two cost less than a single #60.

The original sizing criteria was based on the old steel tanks but it also had a basis in small-tube copper pipe sizes, I was told. It does not matter really. Throw that out the window and size by your best estimate of system volume.

Seeking to size the new tank by something as arbitrary as the existing tank size is like driving looking in the rear view mirror. People will think you are talented and clever until you crash.

The following data (hope it posts correctly) has the volume of pipe in gallons, per linear foot of pipe.

If you take your largest pipe in the basement as 1/3 the system volume as a guess, that would not be a bad start. If you are wrong, add another tank. No big deal.

Pipe Size Gallons
IPS per Foot


1" 0.04494

1 1/4" 0.077896

1 1/2" 0.105609

2" 0.174517

2 1/2" 0.249417

3" 0.384237

3 1/2" 0.514563

4" 0.662116

5" 1.040361

6" 1.502494

8" 2.660448

Unknown

According to Dan Holohan,

and I quote: Take the size of the steel, air cushion tank and multiply by .55 if the building is two-stories tall, or .44 if the building is three stories tall.
The answer will give you the volume of the diaphragm tank.
(Diaphragm tank manufacturers list the volume of their tanks in their literature.) End of quote.
Edit-So, based on what you have 30 gal(assuming that`s OK), you will likely require an SX 90V freestanding Amtrol tank.
As Mike says, a #30 is way too small!
Does this help??

Dave

Mike T., Swampeast MO

A older rule of thumb is that there were 6 gallons of expansion tank per 100 gallons of system content. In theory only 4 gallons were needed for water expanding between 40F and 200F, but the dead men were rather generous as they wanted to avoid water squirting out the roof (through the overflow) if the system was slightly overfilled.

30 / 6 = 5

5 * 100 = 500 gallons assumed in the system. Sounded huge to me until I re-read your post and saw "overhead gravity system". House is likely large and compared to a standard gravity system the piping holds significantly more water due to the huge "express" risers to the attic.

Using Amtrol's simple sizing charts based on maximum water temperature and volume:

For 180F water temp in the system, you would need three model 90 compression tanks. (This good for 546 gallons at 180F).

If you want to stay with the 200F system temp generally assumed by the dead men, you would need three model 90s and one model 60. Unless you (and others) sleep with windows open in the dead of winter or there have been no thermal improvements (insulation/weatherization), or you have an utterly enormous boiler and use significant daily setback, I'd say the chances of your system even nearing 200F are VERY low and that 180F would be quite generous.

I agree with Brad however. Why not estimate the actual volume? You can probably get a very good idea of the size/length/offsets of "hidden" piping by merely looking at the feeds in the attic.

Here are the volume estimates for various types of standing iron radiators. (Flue radiators look similar to column radiators but each section is in a single column instead of multiples--you'll probably see a bunch of projections in interior sections.)

IN PINTS! AND PER SQUARE FOOT OF EDR!

Column rad: 1.5 pints/sqft EDR

Flue rad: 1.75 pints/sqft EDR

Thin-fin tube: 0.807 pints/sqft EDR

Mike T., Swampeast MO

NO! An Amtrol Model 30 is FAR, FAR, FAR from equivalent to the amount of expansion assumed with a 30-gallon gravity expansion tank!!!!

Mike T., Swampeast MO

Must Ask

Why are you replacing the plain compression tank in the basement? (I assume the old expansion tank in the attic is disconnected.)

Unless the plain compression tank is leaking, a properly installed B&G AirTrol fitting will stop any air problem at the source by preventing gravity flow into and out of the compression tank. Those old heavy, welded tanks are nearly eternal unless you've had a runaway boiler with a defective T&P relief.

IMPORTANT: When the old open expansion tank is removed, the piping leading to it really should be removed back to the point of connection to a pipe that will always be filled with water. If impossible, I'd suggest simple rad bleeder(s) at the high point and possibly even "auto bleeders" if the piping is large and long.

Capped off risers have any annoying habit of trapping air that will migrate through the system. Here today, there tomorrow...

Al Roethlisberger

THANKS EVERYONE!


As usual, the feedback here is always very valuable and much appreciated.

I will take a close look at the blueprints to see if total volume is stated, or if I can easily estimate system capacity.

If possible, it will certainly be interesting to see if my current tank was sized appropriately and will help with the right choice of a new tank.

Regarding the question of "why" to replace the old tank, I simply was concerned about the provenance of the old tank(obviously used) and its condition. I don't want to throw money away, but thought it might be nice to just put in a new tank while doing some other work.

Cheers, and thanks again.
Al