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Incorporating off-peak with outdoor wood boiler

GroundUpGroundUp Posts: 673Member
Happy Saturday all, I had a customer call yesterday and ask about adding an off-peak electric boiler to his existing hydronic system which is currently heated with an outdoor wood boiler. It seems to be piped properly, with a 100 plate HX and single circ feeding 3 zones of radiant slab. 2 zones in the house at 4 loops each, total 2000 sq ft as well as a 1500 sq ft garage on the 3rd zone. While I would typically install the electric boiler in series with the plate as a backup, he wants to retain the wood boiler for the garage while using the off-peak for the house but still keep the wood source for on-peak hours with automatic switchover. I've been pondering it for a day and have drawn it out several times, but short of basically building a second system in parallel with the existing and cutting in tees to feed the zones with added zone valves I am not having much luck with a seamless piping diagram. I feel like there is a better way, possibly implementing a 3 way to bypass the plate HX but if the garage and house happen to call at the same time that will not work with what I have envisioned. Anyone have any great ideas?

Before you ask, I have no idea why he wants to keep the garage on the OWB but not the house. "Because".


  • hot_rodhot_rod Posts: 12,042Member
    Is the off-peak a determined time everyday? Or when the power plant has excessive capacity, and a varying schedule?

    I visited homes in Montreal that had gas and electric boilers with automatic switch-over that worked with the power plant scheduling somehow. I'll bet Henry would know.

    You can accomplish most anything with a 3 way zone valve or two :) Would need a piping drawing to see how it all is working now to be clear.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GroundUpGroundUp Posts: 673Member
    Off-peak has no specific schedule, from what I understand it is quite random and will be no more than 12 hours at a time. He did mention that for 2018, they only had 65 hours of on-peak all year. I have piped systems with both electric and gas in off-peak scenarios with simple injection loops but those are one or the other. With retaining the OWB for the garage at all times, I just can't wrap my head around it! I'll try to figure out how to post pics of what he's got, he sent me some yesterday but I will be visiting the site tomorrow morning and hopefully gather a better picture of what's going on. Thanks!
  • GroundUpGroundUp Posts: 673Member
    Okay, I made a site visit yesterday and looked it over. I put some rough labels on the piping to show flow and drew in the circ which is out of frame. The top zone valve is for the garage and the lower two are for the house. The way I see it, I should be able to place a 3 way in both the red and blue "T" intersections I drew in and run the "C" ports of the ZVs to the electric boiler. Then the garage would continue to function off the existing plate HX (lower left) and use the existing system at all times. The electric would then require another circ, expansion tank, etc to serve the house zones as two separate systems running in tandem unless during peak hours when the 3 ways would open A-B and the system would function as it does now. Right?
  • hot_rodhot_rod Posts: 12,042Member
    All this for 65 hours a year that are on-peak prices?

    Or is the goal to have the shop always and only on wood fired?

    I don't see any mixing device, does the radiant run at wood boiler temperatures?

    What control will automate all this? some isolation relays so all thermostats can make multiple circulator choices.

    How about these drawings, two zone valves (green) close off when the electric boiler is energized. Shop can only get heat from wood HX.

    Wood heats all 3 zones whenever the electric is off. This assumes the wood will always be fired and ready.

    Control wise you never want the wood boiler heated by the electric boiler so the 2 additional ZV isolate the electric to only the house zones.

    Two expansion tanks will be weird, essentially two PONPC when the electric boiler is off (bottom pic). Or zone valve off the electric boiler expansion tank when that boiler is not fired?
    Check required in electric boiler circ.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GroundUpGroundUp Posts: 673Member
    Goal is to have the garage wood fired only, ever. For some reason. I tried talking him out of this but it's what he wants. There is a 3 way mixing valve up high to the left of the air eliminator, it's piped correctly as is. I don't do electrical so the controls are not up to me, but I would imagine some sort of delay relay through an aquastat, if there is no heat from the electric unit in a given amount of time it would switch the 3 ways?

    Your drawing is effectively the same thing I have drawn in, although mine is very rough. The OWB will always be fired and ready as it heats 2 other outbuildings as well as this attached garage in question. Looks like we're on the same page, but I don't see why there would be a need for a check. ZV will only be open to the other system if all 3 are calling simultaneously, at which point both circs would be running anyway right? I would leave the IFC in the circ anyway as it won't hurt, but I don't think it would need to be there. As for the second exp tank, yes it would be weird but I see no other way to regulate pressure. If only the garage is calling and the electric is off, the ZV will be closed and the electric side would be isolated from the other exp tank so any remaining heat in the electric side would (I think) play games with the pressure in that side. Maybe not?
  • hot_rodhot_rod Posts: 12,042Member
    The check, or IFC in the electric boiler circulator is to prevent the wood boiler from flowing backwards thru the electric boiler when it is off line. Being a low pressure drop vessel, most of the flow may take that route and not provide adequate flow to the two zones. I suppose you could calculate that, but a check eliminates the concern.
    Checks on or in circulators are always a good protection against ghost or reverse flow.

    If the system were pressure gauged up, the multiple expansion tank connections would show some unusual dynamic pressure conditions. With low or medium head circs, low ∆P conditions, it probably will not be an issue.

    When is the electric boiler enabled? Only when the wood boiler is off or cannot keep up? AND only when on off peak rates? If wood boiler is off line and electric rates are high, no heat for the house? That control logic and wiring will be complicated, or require manual intervention.

    I like a basic delta T control to make the decision when the electric kicks in, but involving an off-peak/ on-peak logic is a head scratcher.

    When I added solar PV to my shop my power provider replaced my meter with an electronic meter. I suspect that meter, which can communicate to the power provider, could somehow know when the utility is on peak rates and trigger a relay or signal, that is the piece you are missing and need to clear up.
    But for only 65 hours of peak time rates per year, is it really worth it? How many $$ are we talking.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GroundUpGroundUp Posts: 673Member
    The wood boiler is outside, and atmospheric- the plate HX keeps it separate from the load. With both NC 3 way ZV powered open to the plate side, the electric boiler would be isolated from the rest of the system by the valves so there would be no way for ghost flow to happen. Same theory applies to the second exp tank, without it there would be no place for expansion on the house zones unless the off-peak meter was shut down and the whole system was running off the OWB.

    The OWB will still run 24/7 as before, because it will still be heating another house and detached garage on the property as well as the attached garage on this house. He wants to cut down on wood usage, so he wants to utilize the off-peak when possible to save on wood. The off-peak meter was shut down completely for 65 hours last year, during which period we would need a backup heat source. Seeing as the plate HX will already be hot from the constant circ from the OWB to heat the garage, it would be as simple as powering open the NC 3 ways when the off-peak meter is shut down to resume heating all 3 zones with the OWB. The only controls would be to power the 3 ways when the off-peak meter is dead, I believe.

    To clarify, there are 2 separate meters. One runs everything as normal, and the off-peak meter is for heating only. That way they can interrupt the power whenever they need to and only interrupt the electric heat. Most folks around here will either have a dual fuel setup with a forced air gas furnace, or the power will automatically switch and come from the main panel during on-peak hours. This setup is a new one for me, but your drawings have confirmed what I was already thinking, we've essentially drawn out the same thing except I used 3 ways which alleviates the ghost flow and expansion issues from earlier.
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