Circulation Pump Operating Cost

Discussion in 'Electrical Forum discussion & Blog' started by molo, Mar 20, 2008.

1. moloMember

Joined:
Sep 23, 2006
Location:
Cold New York
This is a technique to heat water with a wood-stove. I would consult a few professional plumbers prior to installing a system like this.
Question: How do I determine the cost to operate the pump? I'm wondering if it is going to be an unjustified conversion because of pump operating costs.

Thanks for any input

http://www.hilkoil.com/Technical.htm

2. jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
Most circulators are farily small pumps, drawing less than 100W. Taco makes a bunch, and you can view their spec sheets www.tacohvac.com (there might be a hypen between taco and hvac).

Joined:
Oct 20, 2005
Location:
New Hampshire
The pump power is going to be one of the least of your problems.

The problem with putting a coil in a wood or coal stove is that there is no control of the temperature.

Water boils at 212 F, meaning that the steam pressure at 212 F equals atmospheric pressure of 14.7 psi absolute.

At 350 F, which is well within the range of a fire in a wood stove, the gauge pressure is 120 psi. That is much greater than the pressure of your water supply so the steam will force water back into your supply system. In a hot fire of 450 F the pressure will exceed 400 psi and there is considerable danger of bursting copper piping and/or failing solder joints.

The relief valve will prevent overpressure but that will dump a lot of hot water in an uncontrolled way.

The temperature/pressure relief valve in a water heater relies on having the hot water at the temperature-sensing stem, which will not be in the hottest part of the wood stove.

The safety issue is why many residential wood-fired boilers use vented (atmospheric pressure) tanks that can't build up any pressure. You could safely run the water-heater coil through such a tank, but it is not safe to simply put it in the firebox.

A tankless coil in an oil-fired boiler works because the boiler has a temperature control that limits the heating and the tankless coil is immersed in the water of the boiler.

5. jwelectricElectrical Contractor/Instructor

Joined:
Jun 14, 2007
Occupation:
Instructor
Location:
North Carolina
Not to mention the endless hours of cutting wood unless you plan on buying it and if you buy it what have you saved.

6. moloMember

Joined:
Sep 23, 2006
Location:
Cold New York

Hello Bob and thank you for your response regarding safety.

I am aware of the potential disaster and death that an improperly designed and installed wood heated water system can cause. This particular technique (putting a tube in the firebox) does require alot of attention. Would the use of an aquastat work to insure the water circulates and stays cool enough?

If this idea is just not feasibly safe, I would consider an "open" system.
Here is a unit made in vermont that is patent pending and attaches to the outside of the stove, perhaps safer. I would like to know your or anyones opinion on the use of this or other techniques to heat water.

I have an ample supply of firewood here in the Northeast, and don't mind the exercise to cut it.

http://www.wipo.int/pctdb/en/wo.jsp?WO=1984/04576&IA=WO1984/04576&DISPLAY=DESC

http://cgi.****.com/Hot-Water-Tank-...yZ115968QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Thanks,
Molo

Joined:
Oct 20, 2005
Location:
New Hampshire
The Aquastat could control circulation but the problem is that the heat must be dissipated or the whole water heater could exceed the safe temperature.

The Aquastat in a normal boiler controls the temperature of the boiler by controlling the fuel. It turns off all of the fuel when the water in the boiler reaches the controlled temperature.

A wood stove can be shut down by limiting the air but the temperature doesn't drop quickly.

An open boiler is safe because it evaporates the water when the temperature reaches 212 F and dissipates the heat with steam. Each pound of steam lost carries away about 1000 BTUs of heat.

If you put a coil in the stove and connect it to a non-pressurized or low-pressure tank outside the stove, preferably working as a thermosiphon, and then circulate your domestic hot water through a coil in that tank, you could do it safely. The external tank could also supply a circuit for heating the house through low-pressure steam or circulating hot water.

8. moloMember

Joined:
Sep 23, 2006
Location:
Cold New York
Thanks for the response Bob,

The thermosipohon is an attractive option. It would eliminate the need for a pump. Do you have any suggestions for finding engineering info. for thermosiphon designs?

TIA,
Molo

9. BigLouNew Member

Joined:
Nov 7, 2007
Molo,
have you considered adding a large storage tank ? ots a great way to deal with the excess heat from a wood fire.
www.stsscoinc.com

Lou

10. Bill ArdenComputer Programmer

Joined:
Sep 30, 2006
Occupation:
computer programmer
Location:
MN, USA
The key to safety is to make sure that the water loop can dissipate heat faster than the wood stove can heat it. This means that a pump based system is safer since the pump can remove heat faster than a thermosipohon.

A large tank also helps since it averages out the heat serges that the wood stove can create due to fast burning items.

In terms of efficiency you would be better off placing the tubing around the box and around the chimney and keeping the fire box hot.

11. jwelectricElectrical Contractor/Instructor

Joined:
Jun 14, 2007
Occupation:
Instructor
Location:
North Carolina
Would placing the relief valve on the inside of the fire box help to control the heat?

12. moloMember

Joined:
Sep 23, 2006
Location:
Cold New York
Now that is a great idea JW, put the pressure relief valve INSIDE the fire box. Would you warranty that installation please?

Thanks for the good advice Bill Arden, I have seen someone take an Fuel Oil hot water tank that had a 6" flue. He ran his stove pipe chimney through it, and heated the water in the tank. The water got dangerously hot. He had a thermosiphon set up to take the hot water to a tank above this one. It still wasn't a safe design. I won't do anything until I feel I have a safe, working design.

Molo

13. Bill ArdenComputer Programmer

Joined:
Sep 30, 2006
Occupation:
computer programmer
Location:
MN, USA
No, the heat would cause it to fail

I wonder what would happen if you ran a pipe from the relief valve so that it wet down the fire. This could be set up as a sprinkler type system.

Making it so that the fire can't make it too hot is a better idea, but that brings up the question of what happens if the pump fails.

A relief valve should be used as a way to prevent very bad things from happening when something else fails.

FYI: I'm playing with concentrated solar power and the heat would melt my copper pipe if the pump stopped.

Putting a copper pipe in the firebox will work as long as the pump does not fail. If the pump fails you will have water everywhere.

14. cacrosdaleNew Member

Joined:
Oct 31, 2008
I happened to hit this discussion through scanning the web...

I've been looking for an installed system of this type for some time to see how it would be done.

My idea would be for a new-home application where heat would be needed, but not something that may be absolutely crucial... In my case, a smaller cabin in a forested area near the California coast where temps are typically not much lower than the low 40's with no typical freezing, icing, or snow...

The system would have a sealed (outdoor input air only) thermal-fireplace (say, soapstone) with a water-heating coil inside the the firebox. The heated water from the fireplace would recirculate through an insulated hot-water storage tank by way of a pump. Once the fire was out, and the fireplace was loosing its thermal kick, the stored hot water would then be circulated through a radiant floor heating system (pex in slab) to further warm the house.

This could be a closed system with antifreeze, pressure relief valves, expansion tank, etc....

The obvious concerns are pressure and over-heated water--mostly from pumps failing. But some emergency options have been mentioned:

-- Dousing the fire.

-- If the water in the storage tank gets too hot, then it would sound an "alarm" and the radiant-heat floor pump kicks on to use the house slab as a kind of radiator to cool the tank and let you know to ease up on the fire.

-- Additional water could be added to the tank to cool it.

-- The addition of a small solar water heater panel could be added to the roof to preheat the storage tank (if needed), which could also radiate unwanted heat.

Any ideas or criticisms? All input welcomed.