How to Isolate My Coal Boiler

I have installed a hand fired Harman SF-260 wood/coal boiler. I current have it running in series with my oil boiler, with a zone pump continually running circulating the coal boiler water between the 2 boilers.

I am thinking about using some kind of temp control unit, say another johnson control unit, to control the pump. This is to stop pumping if my coal boiler should start to burn out. My coal boiler has something like 30 gallons of water, where my oil boiler has only about 10.

The idea is to have the controller control the pump and potentially two mechanical valves that would close and isolate the coal boiler in the event that it drops below a preset temp.

Does anyone know of any valves that can be controlled in this way? Or of any controllers designed to do this type of work? I figured that I could use a Johnson Control in cool mode to control the pump and valves. Almost in the same way the unit controls the dump zone pump.

Any help would be great!!

Thank you,
Paul

I would just install a simple aquastat or remote bulb thermostat with well in the coal boiler that started the pump at 150 deg. f. Simple aquastats typically have a built in differential so the stop point would be somewhere below 150.

You would need three way valves to take the coal boiler out of series to isolate it. There are many types of motorized valves on the market and many are cheap but not low cost. As with any moving part, zone valves have failures. You can buy a zone pump for about the same money as a good zone valve.

Adding zone valves is a costly move to save a few gallons a year of fuel oil spent keeping the coal boiler hot. One would have to estimate the amount of time oil is used and the convective losses up the chimney. If adding the controls saved you 1 million btuh per year, this would be less than 10 gallons of fuel oil. If oil hits $3.00 per gallon, your savings would be 30 dollars per year. I bet you cannot add motorized valve(s) in the size needed, controls and piping for less than $400.00. Paybacks over 10 years start to turn me off, although I still consider them.

If you are really bent on isolating the coal boiler, I would repipe the boilers parrallel, install a second pump for the coal boiler (with flow control valves) and use the aquastat with a DPDT rely in which one side shuts down the coal boiler pump and the other side enables the oil boiler. You might also have to install a second low voltage thermostat in the house next to the existing one, such that one is for oil and one is for coal.

There is a large variety of controls on the market to accomplish various sequences, this is just one suggestion.

A primary-secondary piping loop would do what you want. The primary loop isolates the heat input to it (boilers) and the heat output (radiators, indirect hot water heater, etc.). It requires multiple small circulator pumps, but these pumps only run when there is demand. You would have the coal boiler as the primary boiler and the control circuit such that if it fails to produce sufficient heat (i.e. fire goes out or a very cold day) the non-coal boiler comes on. The control circuit can completely remove power from the coal boiler, stopping combustion blowers, coal feed mechanisms, set an alarm, etc. Implementation requires piping with closely spaced tees and circulators with internal flow checks like the Taco 007 IFC. It almost always will require an almost complete re-do of the plumbing near your boiler. It's less expensive than a motorized valve system because it uses lower cost standard residential heating system components.

If your domestic how water is heated by your non-coal boiler with an internal coil it's not the appropriate choice. As steamup says there are many ways to do this.