Hot water coil on wood stove

commongroundJanuary 27, 2011

I am looking for a good design to plumb a hot water coil installed in a wood burning cookstove.

I am installing a Bakers Choice wood burning cookstove and I am going to plumb a 45 gallons (200 l) hot water tank 7' directly above the stove on the second floor.

I need to know where to locate the pressure relief valve(s) and what kind of valve I need to use for steam.

I also need to know if my system will need an expansion tank. I am drawing water from a shallow dug well with a jet pump.

Thanks

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brickeyee

"I also need to know if my system will need an expansion tank."

If the system is closed (sealed) it will need an expansion tank.

    Bookmark   January 28, 2011 at 9:26AM
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bus_driver

For a wood stove, the coil should be on the top of the stove, outside the fire box. In my area, the acidity of the typical well water will eat up copper pipe used for such coils.

    Bookmark   January 28, 2011 at 6:47PM
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commonground

My current system with an electric hot water tank does not feature an expansion tank.
What would be the difference between an electric and a wood heated system?
Is the pressure relief valve not an opening in the circuit?
Where does the extra volume of the water go when it gets heated?

In that particular stove the coil is located inside the fire box on the top corner, parallel with the logs. It is U shaped and made out of stainless steel. My water might be quite acid, I'll have to check that.

Thank you

    Bookmark   January 28, 2011 at 7:06PM
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brickeyee

"My current system with an electric hot water tank does not feature an expansion tank. "

Are you on a well or municipal system?

Only a sealed system requires an expansion tank.

If you are on a well you bladder tank is probably taking the expansion, or the water just goes back out of the cold water line filling the tank.

Insatlling a pressure rducing valve on a municapal system isolates the house side and an expansion tank is then commonly required.

Some places are demanding check valves to prevent water from flowing back into the municipal system and this also then requires an expansion tank.

    Bookmark   January 29, 2011 at 9:24AM
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alphonse

It sounds like you are constructing a thermosiphon system.
I also assume you are using a bladder tank to control the jet pump cycling.

If my assumptions are incorrect, please disabuse me.

"I need to know where to locate the pressure relief valve(s) and what kind of valve I need to use for steam."

DANGER WILL ROBINSON!!!

Steam??!! You don't want steam! Well, maybe you do, but that can't be addressed without advising any insurance providers, the stove and coil manufacturers and your attorney.

If you are intending hot water, the T&PR valve should be at the hottest point in the system, typically the top of the tank for a thermosiphon.

The bladder tank (or air/water tank) is acting as the expansion tank. Thermal expansion will affect drawdown.

A continuous fired wood stove in this scenario could easily develop water temps over the boiling point in a static (no water use) situation. I strongly advise redundant safeties in your system, devices that do not rely on electrical power. In addition to the T&PR, fusible links and rupture discs come to mind.

    Bookmark   February 1, 2011 at 6:33AM
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commonground

"It sounds like you are constructing a thermosiphon system."

Yes, the tank will be located on the next floor up 7' vertically above the stove (close to the chimney).

"I also assume you are using a bladder tank to control the jet pump cycling."

Yes I have a bladder tank. I am fairly new to wells and did not know that the bladder was acting as a pressure absorber in the system. Thanks for clarifying.

"If my as
sumptions are incorrect, please disabuse me.

"I need to know where to locate the pressure relief valve(s) and what kind of valve I need to use for steam."

DANGER WILL ROBINSON!!!"

I had to look this one up to understand, I did not grow up in North America!

"Steam??!! You don't want steam! Well, maybe you do, but that can't be addressed without advising any insurance providers, the stove and coil manufacturers and your attorney."

I talked to the manufacturer of the stove (Amish people who use the stove daily throughout the year) who said that a 45 gallons tank would have enough buffer to absorb the continuous heating from the coil (U-coil).
I am not intending to produce steam, of course, but what will happen if the water in the tank gets to boiling temperature?

"If you are intending hot water, the T&PR valve should be at the hottest point in the system, typically the top of the tank for a thermosiphon."

The GSW hot water tanks (commonly available in our area) show a TPR valve located not at the top but on the side.
Should I try to get a tank that have a TPR at the very top of the tank?
Is the TPR commonly mounted on electric heated tanks meant to release water, air or steam?

"The bladder tank (or air/water tank) is acting as the expansion tank. Thermal expansion will affect drawdown."

If the bladder is there to absorb the different volumes of water in the system due to the different temperatures, what is the function of a pressure relief valve other than releasing steam after the tank water has reached the boiling temperature?

"A continuous fired wood stove in this scenario could easily develop water temps over the boiling point in a static (no water use) situation. I strongly advise redundant safeties in your system, devices that do not rely on electrical power. In addition to the T&PR, fusible links and rupture discs come to mind."

    Bookmark   February 1, 2011 at 8:22AM
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alphonse

"I talked to the manufacturer of the stove (Amish people who use the stove daily throughout the year) who said that a 45 gallons tank would have enough buffer to absorb the continuous heating from the coil (U-coil)."

Nothing against Amish, but the statement lacks relevance without qualifiers. If there are people constantly using hot water, then likely yes (there is buffer capacity).

But if the main use of the stove is to provide ambient heat with ancillary water use at typical times (morning or evening), then questions arise. You have a closed system, X amount of BTU will get X amount of water to boiling bearing radiation loss etc. in mind

Domestic hot water systems that develop steam will see tremendous pressure increase due to expansion.Lots of force and hazards presented.

"Should I try to get a tank that have a TPR at the very top of the tank?"

Bodies of water are stratified...the warmest being at the top. Side mount T&PR's sense the upper portion. There are a few degrees of safety in the thermal trip part of the valve.

"Is the TPR commonly mounted on electric heated tanks meant to release water, air or steam?"

AFAIK, T&PR valves are the same on electric/gas heaters. Meant to release water ( pressure relief), typically at 150 PSI, and water (thermal relief) at around 210F.

"If the bladder is there to absorb the different volumes of water in the system due to the different temperatures, what is the function of a pressure relief valve other than releasing steam after the tank water has reached the boiling temperature?"

The bladder tank functions to save the life of the pump by enabling captured air to deliver water within a pressure band, decreasing pump on/off cycles. Maybe you mean expansion tank (which often has a bladder).

The T&PR valve attempts to prevent pressure/temp from exceeding set points. In this case, prior to steam development.

    Bookmark   February 1, 2011 at 9:38AM
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commonground

" But if the main use of the stove is to provide ambient heat with ancillary water use at typical times (morning or evening), then questions arise. You have a closed system, X amount of BTU will get X amount of water to boiling bearing radiation loss etc. in mind

Domestic hot water systems that develop steam will see tremendous pressure increase due to expansion.Lots of force and hazards presented.

'If the bladder is there to absorb the different volumes of water in the system due to the different temperatures, what is the function of a pressure relief valve other than releasing steam after the tank water has reached the boiling temperature?'

The bladder tank functions to save the life of the pump by enabling captured air to deliver water within a pressure band, decreasing pump on/off cycles. Maybe you mean expansion tank (which often has a bladder)."

I have a jet pump that starts pumping in the bladder tank at 20 PSI and stops at 40 PSI.

" The T&PR valve attempts to prevent pressure/temp from exceeding set points. In this case, prior to steam development."

I am trying to understand:
if the temperature in the hot water tank almost reach the boiling point it will open,
if the pressure reaches 150 PSI (why this pressure and not another one?) it will open,
so why would it not work with a coil in a wood stove as opposed to an electric coil inside the tank?

The water is supposed to circulate in a loop between the stove and the tank.

Thanks for bearing with me

    Bookmark   February 1, 2011 at 10:26AM
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alphonse

Yes, understanding is important. The system as proposed can be quite hazardous without safeguards in place.

The circulation (movement) in a thermosiphon takes place by heated water having less density than cool.

If you have a closed system, i.e. no open taps (no usage) and the wood stove is firing, the limits to temperature rise are the T&PR valve and heat loss by radiation et al; or putting the fire out.

T&PR valves can fail...either the seats corrode, springs fatigue, minerals in the water affecting operation etc.

Your electric water heater will cut off power over a certain temp (often around 150F), but that isn't a personal safety with wood providing heat. (Something to keep in mind however.)

150PSI is somewhat arbitrarily used by manufacturers as compliance with aspects of ASME ratings for tank fabrication. It provides a fair safety margin for most typical domestic/municipal water pressures.

    Bookmark   February 1, 2011 at 5:22PM
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weedmeister

And expansion tank is in the system in order to absorb the extra pressure created when the hot water heater (gas/electric) is active (flame on). The pressure in the tank (and the rest of the system) can increase from a typical 50-60psi to over 100psi during this time. This continuous cycling of pressure can cause premature failure of the tank and other components in your house.

High pressures can cause other spring-loaded valves in your house (toilets, dishwasher, cloths washer) to pop open or not close completely.

Gas and electric HW heaters have thermostats to turn off the heat. The pressure valve is there in case the thermostat fails. When using a wood stove as the heat source, you have no thermostat. If the WH gets hot enough for the PR valve to open, you will have gallons of hot scalding water under pressure going somewhere.

    Bookmark   February 1, 2011 at 10:17PM
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commonground

Alphonse,

"Yes, understanding is important. The system as proposed can be quite hazardous without safeguards in place.

The circulation (movement) in a thermosiphon takes place by heated water having less density than cool.

If you have a closed system, i.e. no open taps (no usage) and the wood stove is firing, the limits to temperature rise are the T&PR valve and heat loss by radiation et al; or putting the fire out."

I understand that.

"T&PR valves can fail...either the seats corrode, springs fatigue, minerals in the water affecting operation etc."

I will start off with a brand new 45 gallons tank.
Could I install 2 TPRs instead of one to obtain redundancy?
Are there better quality TPRs around?
Is it possible to obtain a TPR with a regulation on the temperature to have it open at a lower temperature?

"Your electric water heater will cut off power over a certain temp (often around 150F), but that isn't a personal safety with wood providing heat. (Something to keep in mind however.)"

I will install a mixing valve to avoid scalding water at the taps.

As I currently visually monitor my wood stove for overheating and chimney fire with a spiral thermometer located on the stove pipe right above the stove when I put a big load of very dry wood, could I possibly also monitor the temperature of the water inside the tank with a probe thermometer and a remote display located next to the stove?
I think I would be satisfied with those extra safeguards.

"150PSI is somewhat arbitrarily used by manufacturers as compliance with aspects of ASME ratings for tank fabrication. It provides a fair safety margin for most typical domestic/municipal water pressures"

Having a potentially higher water temperature than with an electrically heated tank I will also end up with higher pressure in my system. How can I account for this difference in the design of the system? I suppose with an expansion tank.

    Bookmark   February 6, 2011 at 6:56PM
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commonground

Weedmeister,

"And expansion tank is in the system in order to absorb the extra pressure created when the hot water heater (gas/electric) is active (flame on). The pressure in the tank (and the rest of the system) can increase from a typical 50-60psi to over 100psi during this time. This continuous cycling of pressure can cause premature failure of the tank and other components in your house."

There is no expansion tank in my system at the moment. I am on the shallow dug well with a jet pump, a bladder tank and a 104l (27g) electric hot water tank. The system has been installed by a plumber AFAIK before I bought the house.

"High pressures can cause other spring-loaded valves in your house (toilets, dishwasher, cloths washer) to pop open or not close completely."

"Gas and electric HW heaters have thermostats to turn off the heat. The pressure valve is there in case the thermostat fails. When using a wood stove as the heat source, you have no thermostat. If the WH gets hot enough for the PR valve to open, you will have gallons of hot scalding water under pressure going somewhere."

I can pipe the TPR valve straight into my dirt floored basement.

    Bookmark   February 6, 2011 at 7:06PM
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alphonse

"Could I install 2 TPRs instead of one to obtain redundancy?"

You could. One installed near the stove coil, the other on the tank. Note that T&PR valves have a plastic stem for a thermal sensor which should be in the hottest water area. You may need to configure some fittings to attain that in your piping if the stove coil doesn't have a port.

My own setup has a tempering tank prior to the water heater, which has a 75PSI relief. No check valves in the system.

"Are there better quality TPRs around?"

I'm assuming the valve meets governing code req's, and is likely a Watts. They make a reliable valve, but if you have acidic or mineral laden water, best to verify operation periodically or install new.

"Is it possible to obtain a TPR with a regulation on the temperature to have it open at a lower temperature?"

Industrially, yes, but I don't know of an easy homeowner solution. Modifying the valve requires knowledge and a test bench and absolutely waives insurance.

If your system develops sufficient temperature to pop the relief, you can use a circulator pump, thermostatically controlled, to bring the temperature down by mixing water strata in the system or by infusing colder water from a tank or the cold side (supply).

I avoided electrically controlled or operated components as we consistently lose power at the coldest times of year. My design is such that constant firing can't exceed the pop off point (unless there is no water usage, period.-but then there would be no inhabitants and no need of heat).

The mixing valve is a very good idea.

A solution to these safety concerns would be to make the system atmospheric. Re-pressurise at the reservoir outlet.

Sure, you can visually monitor the temps, but then you are tied to the area (which may not be a problem).

"Having a potentially higher water temperature than with an electrically heated tank I will also end up with higher pressure in my system. How can I account for this difference in the design of the system? I suppose with an expansion tank."

Your existing bladder tank will continue to do the same thing as long as there are no check valves between it and your distribution. Although high pressure cycling isn't a desirable thing, safety ratings for components are (generally!) in excess of 100PSI. The danger of water becoming steam is of prime importance.

    Bookmark   February 7, 2011 at 8:46AM
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commonground

" A solution to these safety concerns would be to make the system atmospheric. Re-pressurise at the reservoir outlet."

I really like this idea and it would safer and simpler.
I only have 7' of head between the top of the hot water tank and the shower head.
Do I need to pressurise?
If yes do I need a separate pump and bladder tank again or is it possible to somehow use the existing jet pump and bladder tank used for the cold water?

I suppose I can't have a tempering valve if the cold and the hot are not at the same pressure.

Having an open tank would allow me to add a solar panel later on.

Is an open tank an off-the-shelf item or do I need to build one myself?

    Bookmark   February 10, 2011 at 9:28PM
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