Expansion tank requirement and size (indirect hot water)?

Lurkin' Murkin'

You will need a potable hot water expansion tank on the indirect's cold water inlet, in that situation. I don't see a reason for another check valve on that line alone, though. Watts has a model DET-5, that is 1.25 gallons acceptance volume - if that is enough to handle the 40 gallon indirect tank?

Charles_8

How big? Is it even needed?

My tankless coil setup is soon to be an indirect + electric system as in my previous posts.

There is a backflow preventer (check valve) from the city water to the boiler supply. It may be required by local codes, and it seems prudent to eliminate any possibility of that nasty boiler water from getting into my drinking water.

But is there any reason to have a check valve in the cold water line to the indirect setup? (In order for any contamination to occur, not only would the coil have to leak but the boiler pressure must be higher than the incoming water pressure which would be basically impossible).

Anyway if a check valve in that line is required, then I need an expansion tank too, otherwise as the water heats there is nowhere for the pressure to go. The smallest one I can find is 2 gal which seems excessive - 40 gal of water heated from 50 to 140F won't expand nearly that much. Any thoughts?

thanks

Unknown

WIth that critera

assuming 125-150psi min/max pressure your system would require a ST-12 Amtrol tank (PT-12 B&G)

Al Letellier_9

Xtank

Check you local plubing code, but here in Maine, only a vacuum breaker is required on the cold inlet to any hot water storage vessel. The domestic xtank will be necessary as the cold water system is a closed system with the check valve at the service entrance.

Al Letellier_9

xtank

Dave Yates (GrandPAH)_2

the facts

Charles,

You do need the larger 4.5-gallon tank. "99% of the failed water heaters we see are the direct result of contractors having installed the 2-gallon TXT." Quote from York Water Co and our own experience mirrors theirs.

It's Boyle's Law that renders the facts regarding thermal expansion, acceptance volume and the role that plays with stress created via increased pressure.

P1 x V1 must equal P2 x V2 - always.

http://www.contractormag.com/articles/0800/cplumbtemp.asp

Charles_8

Great article...

Thanks for the great article. I assume you wrote it, unless it's by another plumber named Dave Yates

I had assumed 1 gal. or less expansion but I don't know what my street water pressure actually is. Water is also quite cold in winter here in far northern Maine... Anyway I will definitely buy the larger one (although I would NEVER plug a relief valve). There is enough room in the basement for a 4.5 gal tank!

Does it matter if it's mounted high or low? As long as the bladder is intact I would not think it matters.

-Charles

Dave Yates (GrandPAH)_2

Hi or lo?

Not sure what you mean by that. Hi=upside-down? Lo=hanging down?

Anywhere in the cold water line & it doesn't actually need to be close to or next to the water heater. Water is essentially incompressible, so the thermal expansion kick will be 'felt' anywhere within the system. It's best to keep it between the shut-off vlv and the water heater just in case the tank is ever isolated by the vlv.

The overnight pressure spikes get trapped in your home's system due to the dual-check vlv, which compromises the initial acceptance air volume & skews the 'perfect-world' calculations. In addition, all residential water heaters of 75-gallons, or less, are permitted by codes to see elevated water temperatures as high as 190F and be within regs. That too knocks the 50-140 initial calcs for total thermal expansion volume off of its pins.

Manufacturer lit typically focuses on lab-type steady-state conditions that ignore the reality we face in the field. Bench-mark testing. I had the ever lovin poops of being spoon-fed info that didn't seem to square with what I'd long been seeing & that's what let to my destruction of three air compressors while running them to 150-PSI(G)

Charles_8

Tank position

Actually I was thinking above or below the water heater. But as you say it doesn't matter.

However, should the bladder/diaphragm fail, if the tank is mounted so that the air remains trapped in the top, it will still work. If the air escapes into the system during pressure drops, the tank won't cushion pressure rises.

Incidentally, I have been wondering why the T&P valve (on the outlet of the existing tankless coil) leaks at irregular intervals, as much as a quart every few weeks. My lights finally came on during this discussion once I realized there was no expansion tank installed!

thanks

-Charles

Dave Yates (GrandPAH)_1

keeping the bladder wet

I've been told by several manufacturers that hanging the tank is preferred because the bladders last longer if kept wet. The bladders are semi-permiable and will lose air-oressure over time. If inverted, it would make sense that air to air would increase the transfer rate vs air to water. Don't know, but it makes sense to me.

Kal Row

watts size calculators ad products

non potable:
http://www.watts.com/pro/divisions/watersafety_flowcontrol/support/support_ETsizing.asp?catId=64


potable"
http://www.watts.com/pro/divisions/watersafety_flowcontrol/support/support_DETsizing.asp?catId=64