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Re: Heat Loss and indirect water heater

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Posted by dana on August 11, 2009 at 16:41:53:

In Reply to: Heat Loss and indirect water heater posted by kenneth DeConti on July 30, 2009 at 09:59:47:

: I had a heat loss done on my home and it came out to 80,000 Btus. If I add a 40 gallon indirect water heater to a gas fired water boiler (80-82% AFUE) do I have to add any Btus to the heat loss for the indirect which will have its own zone? I have received varied opinions on this.

: Thanks Ken

If you're close to the limit, rather than oversize the boiler and suffer lower heating efficiency you can often get around having to upsize it if you have room to install a drainwater heat recovery heat exchanger on the main shower(which is usually the largest draw anyway, unless you're filling a humongous tub, in which case a heat recovery system buys you nothing.) The impact on hot water drawn during showers is huge- it preheats the incoming cold stream (including the cold water feed into the indirect), recovering 50% or more of the heat.

I does require at least 36" of vertical drainpipe downstream of the shower, and it helps if it's located near the indirect. (Fatter and taller is always better on this, since heat exchange efficiency is all about maximizing the surface area.) Major manufacturer/model names are GFX, PowerPipe, and Retherm.

For effectiveness & test info, see:


Short installation instructions from one vendor:


If you get 48"x4" or a 60"x3" it's like adding 25-30k of burner into the indirect during showers (but not tub fills, since the drain flow has to be simultaneous with the potable water flow to get the heat exchange.) Best part about it, that 25-30K "burner" requires no maintenance and uses no fuel. It also turns even the smallest boiler into an endless-shower tankless.

If you have the space, it's probably worth it even though they're not particularly cheap (Figure on spending $500-800 for a decent sized one.)

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