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How to harness waste heat from a gas hot water system? Answered

I have a outdoor gas hot water system in the courtyard of my rental property.
So much waste heat blows out of a vent at the top of it, I want to harness this somehow but there is a sign on it that says specifically not to cover it.
I was thinking about building a box type arrangment on top of it to use for drying clothes. Even beter would be a way to use it to heat the house as it is winter here in melbourne and freezing cold but.

It seems like venting and flow is a bit of a complex subject. Anybody with any expertise here?


(Edit: I have decided not to go ahead with this project as the amount of heat the water heater emits on standby isn't enough to worth bothering with. Thanks to everyone for their replies)



Best Answer 7 years ago

You should be able to make a heat exchanger similar to the one I use for air conditioning.
Something like this would allow for enough air flow that it shouldn't interfere with the proper venting. The copper might deteriorate over time because of the exhaust fumes, that I don't know about.
You would need to add a circulation pump for the water and a series of baseboard units to put the heat into your room. It would also need a thermostat so that it only runs when the exchanger is hot, otherwise it will move the heat from the house to the outside when the gas is off on the water heater.
For me the biggest question would be if its worth it. Will you recover enough heat to justify the cost of the project? I would do a survey of just how often the water heater is running. If its only a few times a day it might not be worth bothering with. On the other hand if its meant to be a prototype then experiment and see what it will do.

Since the water heater is outdoors then CO risk is minimal. Just do not under any circumstances direct the flue gas into the dwelling, only a heated exchanger of some sort. There is a product made to reuse the waste for wood burning flues. It has a series of small tubes sandwiched together and a fan is place behind to blow off the excess heat. By redirecting the heat around it you would get quite a bit. Only problem is that a water heater only runs for short periods.

About the best use I've seen is a heat exchanger that is installed that will capture some of the heat and put it in the house.  Or can be used to preheat the water going into the water heater.

Any use you make of it should not directly contact the combustion products.  They stink, are probably corrosive to some extent, contain moisture and are deadly.

As a DIY version, possibly re-direct the fumes back inside the house, but through a network of thin copper tubes, so that the heat is given up to the room, before the gases are vented back outside again?

My thought would be to make a spiral heat absorption chimney of copper pipe, and run antifreeze through that, then to a radiator in the house -- one final pas for the exhaust gas.

I really like your idea. If I basically build a coil of copper pipe modeled on the existing flue I don't see how it would cause any problems

Only if the cross sectional area of the pipes is the same as the original flue, and then some, because the flow resistance will be disproportionately higher.


My dad knocked a greenhouse-up around our flue once. As long as there's enough ventilation it's safe enough.


When you say rental property, are you renting or collecting rent? There are some good ideas here, but be careful if you are going to redirect the fumes into the house somehow, especially if it isn't your property. If you burn the joint down, or worse, you will be in a heap of trouble. The venting isn't for heat, it's for gasses that displace the oxygen in the air.


7 years ago

I can't contain any serious amount of heat as I dont understand the venting issue... Anyone?

Seems to me that if you're losing that much heat up the stack, the first thing you need to do is get yourself a better water heater.

As soon as mine dies -- which should be Any Time Now since it's at the end of warranty and I've never replaced the sacrificial anode -- I'm replacing it with an indirect-heated superinsulated tank. This uses my boiler as its heat source, and since the boiler is already a highly efficient unit there should be very little heat lost up the stack. The tank insulation means very little heat lost through the tank walls. It'll set me back a chunk of change, but I'm told it pays back moderately quickly.