Controls for wood boiler and oil boiler in parallel

[dennisd499]

Apologies if the answer to this inquiry already exists, but I have limited hi-speed Internet access at work, and only lo speed at home. My ability to conduct long Internet searches is severely limited.

I am looking for a layman's (or equivalent) schematic of the piping, devices and controls for a 2 boiler (wood and oil) hot water system connected in parallel. I have this combination but I think I am missing a basic component, or something is not set correctly, or something is not working properly. My oil burner cycles on even when the wood boiler is at temp and the boiler loop circulator is running (supposedly moving water through both boilers). This happens when there is heat demand and when there is no heat demand. There is the standard Honeywell L4148 aquastat on the oil boiler to control hi temp, circulators, and burner firing. There is an overheat aquastat which starts a circulator and opens the relief loop zone valve.

Rather than go into more detail, best for me to answer clarification questions based on a mutual model (schematic).

The goal is to solve the problem of unnecessary and unwanted oil burner operation when wood boiler is at or near operating temperature.

Thanks.

 

[Eric Johnson]

Well Dennis, we've got one-stop shopping here. Welcome to the Boiler Room.

I'm sure your question will be answered, perhaps in more than one way, before sundown.

 

[ISeeDeadBTUs]

Pretty much the problem I had last year. The Viessmann didn't realize the GW was going to send heat, so the oil would fire. Viessmann makes a 'V-Module', but since the Germans won't tell anyone exactly how it works, I haven't gone that route yet.

Unplugging the burner, however, has worked quite well ;-)

 

[sparke]

There are more sophisticated controls others will probably share with you. I will share the way I did it. I put on a strap on aquastat on the supply line from wood boiler to oil boiler. I then separated my oil burner circuit from my circulater circuit. When the supply temp from the wood reaches 160* (set where you want) - the normally closed contact opens. This turns off the oil gun as long as my wood boiler is circulating water.

I have changed that set up with my new boiler but that is another story...

 

[Eric Johnson]

I do what Jimbo does--switch off the gas burner because there's no need for it.

 

[Nofossil]

I have exactly that situation. I've documented my setup on my site - link in my signature below. I also got around to making a cleaner plumbing diagram

 

[sparke]

Keep in mind the original poster may have an older wood boiler that will not provide an overnight burn. He may need the oil to kick in early A.M. ...

Dennis something else that works is to reset your temp setting on the oil burner to 160* or 170* instead of 180* That will help prevent the oil burner fom coming on unless you let the temp drop in the wood boiler...

 

[Nofossil]

From a controls point of view, there are a lot of options. I like th idea of using an aquastat in the wood boiler and a relay. When the wood boiler is hot, it can activate the relay. The relay contacts can disable the oil boiler when the wood is hot, This would provide automatic failover to oil when the wood goes out. Simply wire the 'normally closed contacts in series with the oil burner power, for instance, or in series with the demand signal from the thermostats.

 

[BrownianHeatingTech]

From a controls point of view, there are a lot of options. I like th idea of using an aquastat in the wood boiler and a relay. When the wood boiler is hot, it can activate the relay. The relay contacts can disable the oil boiler when the wood is hot, This would provide automatic failover to oil when the wood goes out. Simply wire the 'normally closed contacts in series with the oil burner power, for instance, or in series with the demand signal from the thermostats.

A simple aquastat (eg, Honeywell L4006A1017) can be used, without a relay. Hook it up to the wood boiler and splice it into the power wire to the oil burner (in this case, since it sounds like the oil boiler circulator still needs to run), and set it for 180 with a 25-degree differential (for example), so that it kicks on when the wood boiler gets up to temp, and allows the oil burner to fire if the wood boiler cools excessively (either due to going out, or not being able to keep up with demand).

Actual setting of the temperature and differential will depend on the heat needs of the house. You need to do (or have done) an accurate heat loss analysis for the house, and then measure the amount of radiation (for example, how many feet of baseboard element). Based on that, you can calculate what temperature water is needed to provide the required heating to the house, using that many feet of baseboard. The cut-in temperature should probably be in the 180-190 range, and you set the differential so that it never drops below the required water temperature without letting the oil boiler fire.

So, if you actually needed 160-degree water, and you chose to set the aquastat for 180, then you would set the differential for 20.

You also probably don't want to let the low end of that get too low (below 160) or else you may not be able to make domestic hot water - and cold showers aren't fun.

Joe

 

[dennisd499]

Thank you all for your responses.

To add a bit more info based on comments seen:

I do have a Honeywell L6006 strap on aquastat. It is on the feed from the wood boiler and is connected to the T terminals of the Honeywell R7184 oil burner control. The L6006 is set at 140. I am not sure the L6006 is wired to the correct place.

I do not want to switch off the wood boiler even thought there is enough wood capacity to go 6 - 10 hours at night.

The documentation for the Honeywell L8148 includes diagrams with an L6006 low limit controller connected to the L8148A. Why will this not work?

When the wood boiler temp is adequate (160 or greater) and there is no call for heat, the circulator comes on and passes water between the two boilers as designed and desired. A call for heat frequently causes the oil burner to ignite even when the wood is at operating temp . This is what I am trying to prevent, while still preserving oil heat when the wood boiler temp falls below a pre-determined setting.

I am repeating part of my initial post with some apology. The responses indicate understanding. I think there is either an obvious missed piece of information or consideration from me, or the system is out of balance. On paper, when the wood boiler is hot enough, the oil burner should not ignite - right?

Thanks for all your kind and prompt responses. Winter is not over, so there is test time to make changes.

Dennis Donahue
Thetford VT

 

[Eric Johnson]

You have a big loss in efficiency in your system because you are heating the oil boiler vessel with wood-heated water and much of that heat is probably going up the stack of the oil burner. That's a big, invisible hole in your system. I suspect that if you disconnected the power to your oil boiler and blocked the chimney, you'd get more mileage out of the wood you burn and perhaps not need the oil boiler at all when the wood side is running.

You can certainly get your controls to do what you want, but you'll always have that heat loss up the oil burner's chimney.

If your boilers are piped in a parallel configuration, I don't understand why you are pumping hot water from the wood side into the oil boiler. Ideally, they would remain separate and the oil burner would only come on when the wood side dropped below a set temp, say 140. To do that, I'd wire the oil burner up to to an aquastat in the wood boiler (in a well, not strapped on if you can avoid it), so that the oil only fires when the wood side goes below 140. And in a truly parallel configuration, there would be no reason to pump hot water into the oil boiler--and in fact, every incentive not to, for the reasons cited above.

 

[BrownianHeatingTech]

Just to be certain... did you remember remove the jumper from the T-T terminals, before connecting the aquastat? (I wouldn't ask if I hadn't seen it done - or not done, actually)

Are you connected to the correct terminals on the 6006? It should be connected for "break on rise" operation.

Joe