Preheating water feed to gas hot water heater

frank_25

[ Real experience ] The call was not enough hot water for the good ole J. There were two 75 gal. gas fired hot water heaters properly piped for maximum output. I replaced 'em w/ 80 gal stainless Superstore. Now that's what I install for every J. Never a problem. If the boiler can't handle the combined load, just use a priorty relay. I'll bet you'll love that J. ;~0 . bwdik?ijap

moltenmetal

Preheating water feed to gas hot water heater

Hi All:

I'm building a hydronic system for my own house. It will heat an addition via radiant floors, while heating the existing house using the existing radiators. The new system will have a modern modulating boiler, replacing the antiquated coal thermosiphon boiler with NG conversion burner.

The existing house has a relatively new gas hot water heater. We're adding a whirlpool bath which will require some additional hot water capacity. So, rather than going the full distance and installing an indirect hot water system with a separate tank, my thinking was to simply use an exchanger and circulator on the hydronic system to pre-heat the feed water to the existing gas-fired water heater. This should give us the extra capacity we need without adding enormous additional cost or requiring me to retrofit existing hot water plumbing in the house.

Are there any problems with this approach that I'm missing? Any other information you need me to provide to answer this question? Is there a risk of premature failure of the hot water heater?

Thanks in advance for your advice- it's greatly appreciated!

Brad White_9

DHW Preheat

The notion of using a brazed plate exchanger to pre-heat domestic hot water is a good one. It also serves to pre-cool your Mod-Con return water which can increase it's efficiency. What you are thinking of is fine but it is not a substitute for more significant volume. It reduces or rather shifts the cost of preheating.

Your conventional heater has a fixed output and throughput, probably between 1.5 and 2.0 times the volume in gallons for the first hour. Pre-heating that only lessens it's run time to the same setpoint. An indirect would be a nice way to add real generating capacity, with or without pre-heat.

Say you have a 40 gallon indirect with an output of, oh, 240 gallons from 50 degrees to 140 degrees (90 degrees rise). This is a far cry from your 80 gallon potential of your standard DHW Heater.

Your preheat might get the inlet water to say 80 degrees. Now your indirect will take that 80 degrees and boost it to setpoint, another 60 degrees to 140. It works less hard, yes. Your flow rate has not appreciably changed. You could run more flow through it but the end total of the number of BTU's you can deliver to the domestic hot water is roughly the same. You are squeezing the orange a bit tighter and getting more juice perhaps, but it is the same orange.

Now, if you added another tank in parallel and shuttled the water for additional storage, you can buy some time for your larger hot water volumes. You can do this with either your conventional heater or an indirect. The HTP Ultra SuperStor booster is a good product made for economical storage of HW generated elsewhere. There are others.

Marty

why ?

If thats a standard gas water heater why not use it as a tempering tank feeding an indirect? By the time you buy the heat exchanger and controls thats well on your way to getting an indirect.

Larry Weingarten

Although...

...probably not as efficient as an indirect, this could be a good application for a tankless heater just to supply the tub, while keeping your tank type heater for all other uses. Might be interesting to compare costs; both upfront and lifecycle costs to see how this arrangement compares to an indirect

Yours, Larry