Boiler Control

[Bill Bennion]

I am looking for a simple way to control my oil boiler to keep it from coming on when my PB 105 Boiler is active. I have a simple series set up with 3 circulators one for each zone and a Honeywell triple aqua-stat on the oil boiler. I have disabled DHW coil (not using) and disconnected blue wire in aqua stat to run as cold start. Currently I have temp set very low on aqua stat so that it won't come on until boiler temp drops below 120. PB 105 has 180 for high temp and 150 for low. I would like to raise the temp on aqua-stat on oil boiler so that if pellet boiler doesn't run oil boiler would be more efficient. Looking at possibly running NC relay powered by combustion blower to interrupt power to oil burner?

 

[sgschwend]

Does the oil boiler have an on/off switch? Does it come up properly from lose of power?

If there is an on/off switch then you could add a relay and switch the boiler from the pellet boiler's control, if no switch then use a contactor style that come in enclosures and switch the entire device at the panel/breaker.

Your diagram show the hot water circulates through the two boilers. Is that what you want them to do? If you use them both often I can see how that might be handy, but if your rarely use one then I would think a different design would be better.

 

[Bill Bennion]

A simple series set up seemed to be the least expensive and easiest to do.

 

[Gooserider]

A simple series set up seemed to be the least expensive and easiest to do.

Agreed a simple series can be easy to do, and cheap, but it tends to be somewhat inefficient - as you are sending heat through the unused boiler, you are in effect making it a "radiator". It will tend to lose heat to the outside of the boiler, which might or might not be a problem depending on if it's in an area where you want heat anyways. What can be even more problematic, and a bigger loss however, is that if the boiler is a conventional design with a straight through combustion chamber - chimney connection is that you can get a quite substantial convection air flow through the burner pulling heat out of the firebox and sending it up the chimney. This isn't a problem with a "mod-con" style burner, or one that has dampers on the intake and exhaust to block airflow when not firing, however these boilers tend to have high flow restriction, which increases your pumping energy requirements...

I would think you'd probably be better off with a parallel hookup and a 3-way zone valve at one of the connections so that you are able to bypass the unused boiler...

Gooserider

 

[Rory]

My brain hurts whenever I try to follow the intricacies of other people's designs (strange for a person that has spent decades working with other people's software designs). However, I'll share what I have, which my vendor and plumber seemed to think was the best solution for me. My oil furnace now has a new circuit board that switches a zone valve open, fires the boiler and turns on a circ pump when the temperature in my unpressurized storage tank drops below the "set point". I set the set point with a Johnson controller which reads a probe set in a dry well, placed below the uppermost probe attached to my wood boiler. I also have a manual switch on the oil burner itself so that I can let the valve open and exercise the pump, etc. occasionally without actually burning any oil if I so desire. Lately I've had the setpoint at 135 with the burner on and it fires occasionally but cycles off as soon as the water in the oil furnace jacket reaches 180. It seems to take a little while to transfer the heat into the tank, so I typically have the wood furnace up to temp before any significant amount of oil is consumed.

 

[sbleiweiss]

If the boilers were in parallel, you could use the pellet boiler temperature to decide when to allow the oil boiler to run. I use a Johnson heating control for this, which is just an electronic aquastat. Since they are in series ,the oil boiler will heat the pellet boiler making this control scheme ineffective.

You should wire your oil boiler control so the boiler will only fire with a call for heat. I am assuming you have a zone circulator control relay which has this end switch connection to create this heat demand signal. Take this heat demand signal and run it through a normally closed relay contact controlled by the pellet boiler power. A Taco SR501 relay will work for this, but there are probably simpler relays that can do this also. With this control wiring, the oil boiler will only fire when there is a call for heat and the pellet boiler power is off, so you can set the oil boiler limits to anything that you want. You just need to make sure that the point that you use to power the relay is not powered when the pellet boiler is out of fuel.

I strongly agree with Gooserider that your boilers will be constantly convecting air up the stack pipes losing a substantial amount of BTU's and also dragging excess outside air into your house. Field controls makes an "oil vent damper" to stop this convection.

 

[WoodNotOil]

If the boilers were in parallel, you could use the pellet boiler temperature to decide when to allow the oil boiler to run. I use a Johnson heating control for this, which is just an electronic aquastat. Since they are in series ,the oil boiler will heat the pellet boiler making this control scheme ineffective.

Typically there is a circ on the return line of the pellet boiler in this piping scheme and not one on the return of the oil boiler. Bill - does your have a circ on the pellet return? If there is one on the pellet and not on the oil, then something like the device Scottb suggested might still work.

Also, on a series hookup the return usually comes in on the other line between boilers as that is where they would have been when their was only an oil boiler. The biggest problem with series hookups is the mixing of hot water from the pellet with zone return. In my experience a reverse flow parallel is just as simple and avoids this issue in your situation.

 

[Rory]

I guess my setup is a series, but it's not exactly simple. The original zone distribution valves are just outside my oil furnace. My original circulator pump is now used only for drawing water into the oil furnace when it has a call for heat. A new circulator pump drives the heating zones. I have a Termovar and mixing valve in close proximity to the oil furnace, both of which my plumber thought were unnecessary. They were recommended by Revision Energy, who has around 10 years hooking up wood boilers and storage systems.

 

[RobC]

Rory, Revision Energy's use of the tremovar is good on your oil boiler for charging your tank. It prevents thermal shock just like it does for a wood boiler install. Their designs are well thought out. Heating systems, and, heat storage heating systems are designed and function differently. What you have is a very efficient system. Yes there are some "extra" parts but once understood they have a value for the long haul.

 

[Rory]

Yeah, I guess I missed the fact that he doesn't have storage in his system, making it a much simpler proposition.

 

[Bill Bennion]

If the boilers were in parallel, you could use the pellet boiler temperature to decide when to allow the oil boiler to run. I use a Johnson heating control for this, which is just an electronic aquastat. Since they are in series ,the oil boiler will heat the pellet boiler making this control scheme ineffective.

Typically there is a circ on the return line of the pellet boiler in this piping scheme and not one on the return of the oil boiler. Bill - does your have a circ on the pellet return? If there is one on the pellet and not on the oil, then something like the device Scottb suggested might still work.

Also, on a series hookup the return usually comes in on the other line between boilers as that is where they would have been when their was only an oil boiler. The biggest problem with series hookups is the mixing of hot water from the pellet with zone return. In my experience a reverse flow parallel is just as simple and avoids this issue in your situation.

No circulator on return, If no zone is calling for heat the check valve between supply and return allow for thermoconvection between boilers to keep temp even.

 

[Bill Bennion]

I am also looking at installing Field Controls Oil Vent Damper to save on heat going up oil boiler stack.