Potable expansion tank needed with reverse indirect?

Gordan

I suppose this question would be pertinent with tankless heaters, only perhaps more so. Considering the fairly low water volume of the potable side of a reverse indirect, is there any need to provide a potable expansion tank if a whole house pressure reducing valve is used on the cold water line? I'd much rather avoid having this failure-prone component if it serves no real purpose.

Robert O'Connor_12

Short answer is.....It depends.
If you have a whole house pressure reducing valve. Some are designed like check valves, and if there isn't an integral bypass to bleed high pressure back into the city main created by water expanding then yes, you would need a properly sized domestic potable water expansion tank.
However, if the PRV has internal bypass then no, you wouldn't need it.

Gordan

Robert, thanks for your answer. I'm not sure that it takes into account the low volume of the water in the potable system that will be heated (compared to the overall volume in the system.)

SWEI

There is math for that, of course.

Even with a bleed-back, you still need to accommodate the possibility of pressure up to that of the supply side. Some low-lying areas here have as much as 150-160 PSI.

Jamie Hall

Low volume isn't an issue. Pressure is. Even a very small volume will expand and the pressure can soar, and if there is any kind of check valve on your system on the cold water side of any kind of hot water heater, you MUST have an expansion tank unless you like to live very very dangerously.

Gordan

For the sake of reference, the Turbomax 23 has on the order of 2 gallons of water in the heat exchanger coil (potable side.) The only time water in the distribution piping would heat up is when a faucet is open, which is a pressure drop situation (and after the faucet is shut off, the cooling water in the distribution piping should actually be a pressure reducing factor.)

SWEI

So buy the little one.

Gordan

Jamie, I'm not questioning the need for a TPRV. Consequently, living very very dangerously would in this case mean the potential for a few drops of water on my basement floor.

Likewise, yes, a small volume of water can present a big danger when heated if it is CONFINED. There's no check valve between this relatively small amount of heated water and a considerably larger amount of water that's not heated, elsewhere in my potable water distribution system. Still confined, sure, but now we're talking about raising the average water temperature in the confined portion by a fraction of the temperature rise in the reverse indirect. I'd actually expect that just the temperature rise in cold pipes between inlet temperature and indoor temperature would, given the larger affected volume, have a bigger effect on static water pressure.

Kurt, even little expansion tanks fail. :-) A catastrophic failure flooding my basement is a risk I don't need, unless it outweighs a bigger risk. But yes, there's math, and I'm digging it up.

SWEI

Large amounts of PEX or PE pipe upstream can absorb a certain amount of expansion, but metal pipe will not, and as a result direct almost all of that energy to the weakest link in the system. A conventional metal tank will take a certain amount of it, but will eventually fail. With no place to go, a sweated joint can sometimes take the fall IME.

Gordan

So I did a bit of research and the pressure increase associated with heating a constant volume of water even a few degrees is pretty substantial. Based on that, I'd think that even for cold-water-only distribution systems an expansion tank would be in order. I did a calculation for the heated volume (2 gal) with a 100 F rise and also for unheated volume (estimated at 10 gal) with a 30 F rise, both with a max 10 psi static pressure increase, and got less than .15 gal of total tank volume required in each case, or a total of .3 gal. The smallest tank out there that I can find is 2 gal. Any recommendations for a durable tank in that capacity or smaller? Are the well tanks likely to last longer than the common expansion tanks? Any reason not to use those?