Expansion tank replacement - 3 questions

garbid

I'm replacing my old boiler expansion tank with an Extrol #30 bladder tank and have the following questions:

1.  Despite the mfgr's instructions, can I install the tank on a dead-end pipe?  Placing it in-line will be very difficult.

2.  Should I increase the pre-charged tank pressure from 12psi to 15psi as some have suggested?

3.  If the tank (installed on a dead-end pipe) can be isolated from the main system by a ball valve, is it necessary to de-pressurize the boiler prior to installation, or can I just close the valve, install the tank, then opne the valve?

Thanks for you help

Mark Eatherton

Responses

1. Yes. Hopefully all pumps will be pumping away from that point.



2. No, fill it to the minimum required pressure to raise the water above the boiler, plus 5 PSI. Typical low rise residential system would be 12 PSI.



3. No, Yes. Also a good idea to put a drain valve at the tanks inlet so you can check and adjust the diaphragm pressure without disconnecting the tank.



You're welcome :-)



ME

garbid

Clarification re placement of drain valve

Mark: 

Thanks for your suggestions.  Do I have the drain valve you mentioned in the right place on the attached diagram?

Also - is the 12 psi enough for a two-story residence?  I believe that is what the baseline level has been with the old steel tank.

Sorry to be a pest!

Mark Eatherton

Egg Zachary...

That way, you can let the water pressure off of the tank to check the diaphragm for proper pressure during future service.



ANd yes, 12 PSI should be adequate for a 2 story house. Math goes as follows;



.434 PSI per foot of SYSTEM elevation above teh boiler pressure gage. So, assuming boiler mounted in the basement, and BBR on the second floor, then total system height is 4 +10 = 14. 14 times .434 = 6.07 PSI. To this add 5 PSI for purging air and keeping the top of the system under positive pressure to avoid possible steam flash, and you have 11 PSI. Most tanks (theoretically) come pre charged at 12 PSI. You should be good to go.



You're not pestering me. We are here to share our wealth of knowledge. You are here to learn.



Pay it forward.



ME

1sttimeposter

14 ft

I don't quite get where we got 10 for the second floor height as BBR would sit no more than 2 ft off the ground..... I am thinking more like (4+2=6) and not (4+10=14).



I am in the midst of figuring out what my requirement is for the in-floor system in our 2-story house with a crawl space where the boiler locates as per rough sketch attached. Does  (4+10)=14 ft sound about right?



wonder what over/under-pressurized the expansion tank would do to the system?



Thanks for the feedback!

Mark Eatherton

The math...

He said in his post that it was a 2 story building. My assumption is that it is baseboard. If it is vertical cast iron radiators, then their height has to be added to the equation.



If the boiler is in the basement, then it is roughly an 8' tall ceiling, so 1/2 of that would be 4'.



The baseboard is located on the floor of the second floor, so there is roughly 10 feet from the bottom of the first floor (basement ceiling) to the top of the second FLOOR.



Remember, it is the highest part of the system above the boilers pressure gage. If it were a radiant ceiling, you'd have to add the additional height.



Over pressuring the expansion tank will cause the operating pressure of the system to be higher. As long as you are 10% away from the relief valves relief setting (typically 30 PSI for residential apps) then you are OK. Under pressure will cause the expansion tank to lose some of its acceptance volume because some of the fill pressure took up that space.



14 foot sounds about right for your application, so 12 PSI pre charge and fill pressures should be fine.



Welcome to the wall first timer :-)



ME

garbid

Confirming the math...

Mark's math is correct for my installation, 1st timer.  The pressure gauge on my basement furnace sits 5' below the ceiling, and it's roughly a 10' run from there to the floor of my second story, or a 15' total rise to the BBRs.  I'm going to pressurize my diaphram tank to 13# which is well below the 30 psi limit on the PRV.

Mark's a patient teacher!

Nick W

Size is important, too.

Remember, if the tank is too small, the pressure may rise high enough to open the safety relief valve even if you have the correct pre-charge pressure in the tank. I'm sure someone here can tell you how to calculate the size you need.

garbid

Expansion tank sizing

I don't know the details of the calculation, but I am told by everyone I ask, including the plumbing supply shop where I purchased the tank, that the Extrol 30# tank is suitable for my 3,000 sq. ft 2-story residence with a mixture of BBRs and convertors.  Is there something I'm missing before I start sweating pipe?

Mark Eatherton

What was there before...

and how did it work?



If the old tank was a number 30, and pressure seemed fairly stable, then a number 30 will work. Same goes for the old ceiling hung tanks. If the tanks volume was 30 gallons, and the pressure was stable, then a # 30 will work.



Other than the loss of dinero, there is no down side to over sizing an expansion tank, within reason. And if it is undersized, your relief valve will let you know...



ME

garbid

More on tank sizing...

The tank being replaced is an old (probably 25+ yrs) horizontally mounted (in the ceiling joists) Thrush steel tank 49" long x 12" od, and although I'm not certain of the actual capacity, 30 gal. seems about right.  I've no problem getting a bigger tank than the #30, but if the #30 will do the job, I already have it and am ready to install. I certainly don't want to risk undersizing.

garbid

Update on old tank size

I just looked at the Thrush catalog and it may be that the old tank is only 24 gallons rather than 30.  It is, in no case, greater than 30 gallons...

Mark Eatherton

You'll be fine...

ME

garbid

What's the purpose of this valve and is it bi-directional?

I encountered this fitting in the line to which I wish to attach the Extrol expansion tank (it's a line that was formerly used to feed a radiant loop but no longer does so) and wondered if it will affect the performance of the Extrol tank which would be installed downline from the fitting (i.e., in the direction of the flow arrow on the back side of the fitting).  It looks like a check valve of some sort, and it won't flow forward unless the little black plastic knob is in the correct position (as shown in the photo).  Does this fitting permit bi-directional flow?  If not, how does that affect the functioning of the expansion tank (i.e., does water which flows into the tank on the heat cycle need to flow in the reverse direction when the system's cooling down)?  I can cut the fitting out and pipe straight through to the tank if necessary but was hoping not to have to.  Hope this is not too confusing!

Mark Eatherton

Check...

I'd get rid of it if it isn't providing any useful function.



It would cause confusion to some future owner/service agent.



ME

1sttimeposter

ME is great!

Indeed, he is. That's why ME is our leader!



I mistakenly presumed your basement is your first floor.... got it now!

Mark Eatherton

Aw Jeez..

Yer makin' me blush. I'm just here doing the best I can with the given talents I've got. Paying it back and forward as it were...



Thanks for the kudos. There are many more deserving people on this site.



ME