Running Pellet and Oil Boilers in Parallel

[velvetfoot]

I am thinking of downrating my pellet boiler, via dip switch, from 15 KW to 10 KW and potentially running the oil boiler at the same time if the pellet boiler can't keep up with the load on a real cold day.

What I was thinking when I set things up was that the oil boiler would start up if the pellet boiler runs out of pellets. If the buffer tank (110 gal) sensor gets a couple degrees cooler than the temp that starts up the pellet boiler, the oil boiler starts up.

Each boiler, after started, runs until the boiler's aquastat is satisfied.

I am thinking that the heat produced by the pellet boiler will be moved via constantly running distribution pumps to the baseboards, and the house will be at a cooler state while the tank still stays warm enough not to turn on the oil boiler.

I am not sure what would actually happen. Maybe the tank temp would eventually go down, I don't know.

I think my thermostats have an auxiliary heat option to it, but I've looked at that in the past and rejected it, for reasons I don't recall now. Something similar could be the ticket, where the fact that the rooms aren't coming up to temp is sensed and the oil heat is started.

Any ideas? Thanks.

 

[NateB]

How do you have you PB and OB plumbed together?

 

[velvetfoot]

How do you have you PB and OB plumbed together?

In parallel. Here is a crude diagram that doesn't have all the isolation valves, etc:

 

[NateB]

That is a tough one since you are always heating your storage. So if the OB is supplying heat it will heat the storage also. Ideally you would want an aquastat to monitor the output of the tank, and when it got below a set point. Turn on the oil boiler. But in your setup your oil boiler will run until your tank is up to temperature.

 

[velvetfoot]

The tank sensor, at 1/3 of way from top, is connected to a controller that turns on the pellet boiler at 135 and turns on the oil boiler at 132. Right now, I have it set up that both boilers are not allowed to run at the same time. If I changed it to allow that, the scenario might be like this on a cold night: pellet boiler runs but can't satisfy zone thermostats, tank temperature drops below 132, oil boiler turns on, both run until their internal aquastats are satisfied (~180 or so) which would probably happen with both boilers running at the same time. Thing is, both of them would then shut off and stay off until the buffer tank (111 gallons or so) is depleted and then the cycle begins again. I guess if the oil boiler's aquastat was turned down somewhat, like below that of the pellet boiler, it would turn off and the pellet boiler would continue to run and then when the tank required another boost at 132 the oil boiler would come on again. Hmmm...

 

[NateB]

Would making your delta t smaller on your pellet boiler help. Give it a chance to stay ahead. Instead of trying to catch up.

 

[velvetfoot]

Setting the tank aquastat higher for the pellet boiler could help I imagine when the temps are real cold, however, in this scenario, I am imagining the derated pellet boiler wouldn't be able to keep up with the heat loss of the house and needs a boost to do so. It is running all the time and would never shut off. One of the goals, generally, in derating it is to have it run longer each cycle, which is one reason why the tank temp could be on the low side when either boiler starts. But, the zones do get first crack at boiler heat when they do start up. Things just might not go as planned when it gets real cold. Of course, if I'm home, I'll be burning the wood insert.

I might try removing the interlock between the two units so that both could run at the same time, maybe turn down the temp where the oil boiler turns on a bit so that it doesn't come on needlessly while the pellet boiler is warming up, and wait til it gets real cold out. I'll read about the staging and aux heat control capabilities of the thermostat, and think about it some more.

 

[Sparky978]

Velvet
I have been thinking of changing my old Taco 007 circa pumps for my heating zones. I see on your drawing you have ecocirc's, what are the advantages of using that type of circ? Are they the auto or the manual speed type?

 

[velvetfoot]

Hi Sparky. I am not an expert, so take what I say knowing that. I also used to have Taco 007 pumps. They are the manual speed type. I figured manual was better since I had zone circs instead of valves. I like that they're infinitely variable, and not three speeds or something like that. ECM motors use less electricity. I'm sure there are other advantages but I can recall them. They look cool.

 

[Sparky978]

I might try removing the interlock between the two units so that both could run at the same time,

Velvet
How do you have the two boilers interlocked?
I want to set up my oil boiler to turn on if the Biowin fails for some reason. I would probably only need to use this scenario if I went away for an extended amount of time. I have my system piped so that I can run either boiler separately or both by turning a few valves. I normally have it valved for PB only. I tried what you said and used my extra Ranco and inserted the sensor in the well of the PB and the temp reading match exactly within 15 seconds of each other. I figured I could run the TT wires that go from my OB to my Taco 6 zone controller end switch through the NO contacts of the Ranco that is sensing the PB temp and if the PB temp goes down below 120*?? the contacts would close allowing the OB to start when any zone or DHW is calling. And for some reason the PB came back on and the temp went up past the differential setting then the contacts would open again and shut of the OB. Am I missing any steps to this interlocking scheme?

Sparky

 

[velvetfoot]

And for some reason the PB came back on

How can you simulate the PB not running if it still can.

I've got this whole other thread where I'm agonizing over how to control aux heat (the oil boiler) with my thermostat, so that could be related.
https://www.hearth.com/talk/threads...cobee-smartsi-thermostat.147270/#post-1982749

In case the Biowin stops running, like if the 1 ton pellet bin I built runs out, that stage 1 Ranco controller tries to turn on the PB when the tank sensor, which is 1/3 of the way down from the top of the tank, gets to 134. If the sensor temp continues to go down to 132, AND the PB's induced fan motor is not running, stage 2 of the Ranco turns on which turns on the OB. I have an RIB relay sensing if there is power going to the induced fan and if there is, it opens. This way, even though the tank temp is running while zone circs are running and the PB is warming up, and its circ pump off, I can get away with that small a temperature fall to know that it's right to turn on the oil boiler.

I have both boilers set up to run until their aquastats are satisfied. They start and don't stop until up to their max temps. I did this to simplify getting a good thermo fill of the buffer tank and to get better run times of the boilers. However, one time when I returned home last year I noticed that both boilers were on. I did not want this because I didn't specify it. I believe there was a momentary power outage, and when power returned the way the controllers are arranged, maybe one normally closed, that the oil boiler kicked on, and as I said, didn't kick off til its aquastat was satisfied. Anyway, I put in another relay that senses the PB induced fan power to cut off the OB. With both of these relays, the boilers can't run at the same time.

HOWEVER, now I am attempting to use the oil boiler to supplement the PB during cold snaps, since I have the PB turned down to 10kW. I have a wood insert, and I don't normally need the higher output until it gets cold and the insert isn't running. But there are issues I'm thinking about, in that thread, like setback recovery. Anyway, right now, I'm thinking the thermostat will get involved in calling for auxiliary heat, so both will run at the same time, bypassing those interlocks above for normal operation

Even though I put together this scheme, and I actually have it documented on a chicken scratch wiring diagram, my eyes still glazed over when I read your post
But seriously, what were you testing? Just seeing if the OB would run and heat the house if the PB failed? I'd turn off the PB and see if it works.
But, I forget if you have a buffer tank and are controlling the PB by a sensor off that. That could be why the PB came on.
Also, if your logic is to turn on the OB if the PB temperature is below 120, realize that the PB temp can get below 120 if it hasn't run for a while (warm outside temps, heating on buffer)

With my setup, the zones pull from the tank until the tank gets depleted and then the PB comes on, with its pump, and water has a tendency to go right from the boiler to the zones, with any excess going to the tank.

In a lame summary, I am using the PB Ranco to control when the PB circ pump starts and stops, and also in some as yet undetermined way to provide boost heat in cold weather. I am using the buffer tank Ranco, along with those interlock relays, to control when the PB and OB come on during normal (not boost) operation.

Maybe you could post a diagram, like my crude one.

 

[Sparky978]

Just seeing if the OB would run and heat the house if the PB failed?

Yes this is my objective.

I forget if you have a buffer tank and are controlling the PB by a sensor off that

I have a 120 gallon buffer tank that kicks on the PB at 160*. I haven't tried this scheme out yet and the 120* to start the OB was just arbitrary.
I just want it low enough to ensure me that the PB is not coming on. But I agree with you that its not foolproof unless after it goes through the PB Ranco I do the induced fan relay like yours. Or just do it like your set up. Here is my crude diagram. Nothing fancy just showing valves I use to change flow and my valve operating chart.

 

[velvetfoot]

Your system is definitely different than mine, so maybe my solution isn't like yours should be. Although, I do seem to remember that at one time, my protection scheme involved the PB temperature.

I'm thinking if the pellet boiler isn't running, the thermobloc also isn't running so there's no circulation through the PB. So, the temperature will go down very slowly. So, even though it took a while for the PB temp to go down, the PB temp should stay down there if the PB isn't running and this will keep the OB running.

I wonder if the PB temp can get below that 120 setpoint on a transient basis (I'm thinking probably not)? That would turn on the OB when you don't want it, ie, when the PB is running, and it would turn off as soon as the PB got over 120.

Thing is, if you're testing how the setup works when the PB fails, why not just turn it off and see how it works?

 

[Sparky978]

I plan on testing it this weekend when I have more time to monitor it. Have you ever tested your scheme that makes your OB come on when you run out of pellets?

 

[velvetfoot]

Yes I have. It tries to blow pellets 10 times or so before it gives up.

 

[velvetfoot]

I plan on testing it this weekend when I have more time to monitor it. Have you ever tested your scheme that makes your OB come on when you run out of pellets?

Success?