Pre-heating (warming) water to water heater

[Billy_Bob]

Being as an electric hot water heater can be 30% of an electric bill and the rates just keep going up through the roof, I'm thinking about doing things which will reduce my electric bill.

One thing is that the water coming into my house from the city is very cold...

And the walls surrounding my wood stove are very hot!

So.... I was thinking about installing an old fashioned steam heating radiator or some sort of coil of pipes going back and forth along the wall next to my wood stove. Then running the cold water from my main through this and THEN out and into the cold water inlet of my electric water heater.

So when burning wood, the heat from the wood stove would "pre-heat" or warm this water before it gets to my electric water heater. So less heating would be required from my water heater and a lower electric bill.

Anyone done anything like this?

(I don't expect this to get the water hot, just to warm it up a bit...)

 

[HUSKYMAN]

Since the water only moves when you open a tap, the coils cant get too hot or it will boil the water. I also think you do not want a bunch of waterlines running to the wood stove and back to the hot water tank.

If you have gas then you will save a lot of money by going to an on demand hot water heater. The system will save you a lot of space, and you wont waste money heating a bunch of water in a tank

 

[Billy_Bob]

Actually my electric water heater is on the other side of the wall from where my wood stove is. So just need to pipe through the wall.

Perhaps I need an "expansion tank" should the water boil?

(There is also a pressure relief valve on the hot water heater.)

 

[Billy_Bob]

The boiling temperature of water at sea level is 212°F...

Under pressure from city water boiling point?

I have a temperature gauge on the wall near and above the wood stove (eye level) and this rarely ever gets up to 115 degrees.

So I would not imagine that the wall around the stove down lower would get much more hot than this...

 

[bassman]

just do it.
I try stuff like this all the time and it works .
I see no reason why you cant get some copper flex line and a small taco pump and heat the water .
My local wood stove shop has a stove with a coil in it for this reason .

 

[badtry]

im thinking about a coil in my wood furnace to heat water in tubing to heat my subfloors in my bathrooms, any body do anything like this???

 

[ray benson]

I considered something similar. A 25' or 30'coil of soft copper tubing behind the stove . But my stove is not close to the hot water heater.

 

[Geez]

im thinking about a coil in my wood furnace to heat water in tubing to heat my subfloors in my bathrooms, any body do anything like this???

It seems to me that for safety sake, you would have to set it up like a regular hot water system, ie; expansion tanks, safety valves, etc. Otherwise how would you control the dangerous pressures involve should the water overheat?
How about if you used a water to water heat exchanger like we use with the OWBs? Then you could treat the heating coil as an open system that could just boil off, and that would heat the closed water system to your water heater, via the heat exchanger?
Flow control valves and thermometers would allow you to control the closed system temperature.

 

[bassman]

this is all leading to an indoor wood boiler that would then be used to heat domestic hot water and then floor heat ect.
pressure and expansionn has to be considered or a hot water boom can and will happen.
then there is the issue of insurance ...

 

[hanko]

Since the water only moves when you open a tap, the coils cant get too hot or it will boil the water. I also think you do not want a bunch of waterlines running to the wood stove and back to the hot water tank.

If you have gas then you will save a lot of money by going to an on demand hot water heater. The system will save you a lot of space, and you wont waste money heating a bunch of water in a tank

.. water will hydro sypeniate as long as intake and discharge heights in relation to the tempering (storage tank) are followed. check charmaster.com, they have a system to follow with their furnace. They advise not to use a pump in case of power failure that would stop the gravity effect or hydro syphoniate (spelling). I used it on my charmaster for years. works great. My gas HW never fired.

 

[hanko]

I would also like to add that witha 50 gal storage tank I never had my pop off valve go off. we allways tried to do a load of laundry in the morning and showers in the afternoon to posh cold water in to the storage tank. This is an approved method by several mfg's

 

[hanko]

I remembered I had a photo

 

[hanko]

****

 

[oneoldbanjo]

The first thing to do is check the temperature in the area where you are thinking about putting the pipes....if they are not above 212 degrees - you will not have a problem with boiling.

I really don't think you will have much success with a series of pipes. The surface area of water pipe compared to the volume of water inside the pipe is not very good - and an air/water heat exchanger will not be very efficient. You will heat the water up that is inside the pipes while the water is not flowing - but as soon as you start flowing water through the pipe the hot water will be gone and the exchanger will not be able to heat water as fast as it is flowing through the pipes. Water can store a lot of heat energy and transfers it quickly while air stores very little. Put a cold bucket of water in the room and see how long it takes to warm up, then put another bucket of cold water in your bathtub with hot water in the tub - and you will get an example of how much better a water/water heat exchanger will work than an air/water exhanger. That is why radiators have so many fins on them - they need lots of surface area to be able to have the air take the heat away. If you still want to use this method - perhaps some form of radiator or heater core would work best. (One reason they don't use this method for heating cold water is the fins will sweat horribly as the warm air passes over the cold fins - and the heat transfer does not work well whent he fins are full of water).

If you find that the temperature does not exceed 212 degrees - I think you will have decent performance by just installing a water storage tank near the woodburner. It will be able to heat up a supply of water while the water is not flowing and when it is flowing it will have a supply of warm water already made. You should have the cold water from the supply line enter the tank at the bottom and the water leave the tank at the top and then head toward your water heater supply side. It is advisable that you install an expansion tank on your existing water heater if you don't already have one - as the water will change volume a little bit as it becomes warm. You could also install a pressure/temperature valve on the tank if overheating or overpressure is a concern. You may experience sweating of the tank when cold water comes into the tank (warm moist air in the room will condense on the side of the tank) - so a drip pan underneath the tank may be necessary. I am sure you will save some money doing this - as the water entering your house in the winter is probably very cold - and at a minimum you will be heating it up to room temperature before it enters the water heater. I am not sure how long it will take you to recoup the cost of the tank and materials needed to create the pre-heater.

 

[johnha]

If you have gas then you will save a lot of money by going to an on demand hot water heater. The system will save you a lot of space, and you wont waste money heating a bunch of water in a tank

I agree with you on the space savings, but can you point me to any data that actually bears out the cost savings? I find lots of data that makes this claim, but it all seems to be supplied by manufacturers of on demand units. Any claims made by others seem to be purely anecdotal.

I don't know why but for some reason I have a problem with those types of sources.

 

[logbutcher]

I agree with you on the space savings, but can you point me to any data that actually bears out the cost savings? I find lots of data that makes this claim, but it all seems to be supplied by manufacturers of on demand units. Any claims made by others seem to be purely anecdotal.
I don't know why but for some reason I have a problem with those types of sources.

DITTO for the "tankless" on-demand sytems. Our Rinnai has been a perfect machine for years now; the on demand heaters have been in use in Europe and Japan for at least a generation. You had to put money into the water heaters in northern Europe-the flame for the heater in your face in the showers!
Cost savings is a wash compared to electric or even gas water heaters: the system is at least 2X to 3X the cost of the better tank heaters, and costs more to install since it needs LP or Natural Gas connections, electric tie-in, plumbing of course, and a vent system.
HOWEVER: you never run out of hot water. Important when Goodbody needs that soothing warm flush to ease aged aches and pains. And, you're not heating an unused storage tank 24/7. We do use less LP than with a the long gone gas water heater, but then use more hot water.
With the Rinnai tankless, we can simultaneously run a shower, the dishwasher, and the washing machine with no decrease in pressure or water temperature. Try that with an 80 gallon gas water heater.
The yearly minor maintenance flush of the Rinnai is simple.

 

[johnha]

Okay logbutcher, thanks for another anecdotal data point, I've seen quite a few of them.

The tankless water heater sellers claim that a tank unit wastes thousands of kWh in standby losses. I have seen government statistics that say the actual number is 300 kWh for an efficient tank. At my provider's electric rates this means I'm 'wasting' $45/year by having a tank system.

They claim that I won't run out of hot water. I have a 50 gallon tank and two teenage daughters. I've never run out of hot water with a tank. Also, I've stayed with cousins in Ireland and England who all seem to have to have several tankless units in their houses to keep up with demand.

I'd love to jump on the tankless bandwagon, but can't see any reason to do so.

 

[woodlumn]

I installed the domestic hot water coil on my Heatmor and it's great. Basically, it's a coil of copper that is inserted into the water jacket. I plumbed it so that, when called for, the water runs from the well, out to the coil in the water jacket, and then back to the house and dumps into the electric water heater. You can always tell when you're starting to use "heatmor" hot water because the shower temp bumps up a few degrees. It's comforting.

The important things here are that a) the well water and furnace water never mix and b) our furnace is set to a high temp of 180, so we're not talking about boiling water.

 

[oneoldbanjo]

Woodlumn: Billy Bob does not have an OWB - he has an indoor stove that heats with air - not water. The coils and system you use or an OWB do not apply to his situation.

I have a Woodmaster and the hot water goes though a plate heater to preheat my supply water for my heater - then goes to a tube in tube system to keep the water in the water heater hot - then the water goes to my furnace to heat the house. The water is so hot in my house that I keep the tube in tube system turned off.

 

[Geez]

I installed the domestic hot water coil on my Heatmor and it's great. Basically, it's a coil of copper that is inserted into the water jacket. I plumbed it so that, when called for, the water runs from the well, out to the coil in the water jacket, and then back to the house and dumps into the electric water heater. You can always tell when you're starting to use "heatmor" hot water because the shower temp bumps up a few degrees. It's comforting.

The important things here are that a) the well water and furnace water never mix and b) our furnace is set to a high temp of 180, so we're not talking about boiling water.

Yes and that's what a lot of us OWB owners have some version of. But in our case the water temp never gets above the controlled temp of our boiler. In the OP's case, he has a regular woodburner inside the house (no water jacket) so the potential is there for the water inside any kind of coil to boil (and blow!). :jawdrop: