preheating water to a gas water heater
Hi All:
I’m installing my new Munchkin modulating condensing boiler and the rest of my hydronic heating system at the moment- just in time for winter. I breathed a heavy sigh of relief once I had that old coal-fired, friable asbestos-insulated thermosiphon boiler bagged up and abandoned in place! Maybe I’ll have it removed once the kids are in university, but for now it can stay there!
I’m thinking of setting up a separate circulator and heat exchanger to pre-heat the feed water to my existing natural gas water heater. We’re installing a whirlpool tub which will challenge the capacity of the existing heater a bit, and I thought this would be a cheap and easy way to get some extra capacity. The existing hot water heater is gas fired, relatively new and in good shape. The exchanger is available to me for scrap value (a “scratch and dent” copper-brazed stainless plate type exchanger of adequate size, not new enough looking to put on a project for one of our customers) and the circulator is $100- the rest of the plumbing is peanuts.
We don’t need instant hot water, nor are we interested in ripping out or retrofitting the existing hot water plumbing in the house. I’ll plumb the new tub with a recirc line on the hot water just in case we get fussy in our old age!
The hydronic system consists of a main “hot” circulator circuit serving the existing rads directly from the boiler. Secondary circuits with their own circulators and t/stats supply PEX radiant floor manifolds in the addition. The boiler is condensing type, so there’s no worries about low return water temperature resulting when somebody opens the big faucet to fill the tub.
Any problems with this approach that I’m missing? Am I likely to corrode out my hot water heater prematurely?
I’ve posted over at The Wall too, but know there are smart folks here who don’t post over there. Your help is, as always, greatly appreciated!
Replies
The only thing you didn't describe is what if any means of controls are you going to have. You mentioned a dedicated circ.
Here's an idea. Just run the hot water return from your high temp loads (rads, 180 in 160 out?) through the heat exchanger before hitting the boiler, I think is a good idea. The more you drop that HWR temp, the more efficient that mod/con will be. Let it "run wild" all the time, keep it simple. May need to provide some expansion for that trapped cold water volume to heat up and expand a little.
I can't think of any downside here, unless the dp through the HX is great enough to be noticed. Not going to harm the tank water heater.
My original plan was to have a demand thermostat that turned on the circulator for the exchanger only when hot water was flowing.
So- what you're saying is let it rip and save the $100 on the circulator and the extra $$ for the thermostat. Preheat the HW feed water full time- but only when we're heating the house. Good idea! Now why didn't I think of that?! Thanks for the suggestion!
I can afford the pressure drop on the main loop- the pressure drop through the rads, the huge old supply/return piping to/from them, and the Munchkin itself, are all relatively small, and those little Grundfos three-speed wet rotor circulators offer lots of flexibility- need more head, just run the pump on speed 2 rather than speed 1! The only downside I can see is that with this method I wouldn't be able to use the Munchkin to heat the hot water without also firing the rads- not something I'd want to do in summer. It also might be that the flow I want to run the rads at for noise reasons is lower than I might want to run through the exchanger, but that remains to be seen.
If I go this route, I'll leave closely spaced tees with shutoff valves on the main loop in case I want to revert to the separate circulator later.
The simplicity comes at the cost of demand control, as you noted. Your use and/or need will determine if demand control is necessary. I suppose the first time you need to fill that jetted tub when not heating the house will tell. The Munchkin has DHW terminals on the control panel, if I'm not mistaken. Could treat the HX like an indirect with an aquastat or a flow switch.