Help with how to plumb wood heat into this system

[Eureka]

Adding an outdoor coal/wood unit to heat an outbuilding and supplement house through Rehau underground pipe. I could humbly use some help with how to tap into my radiant floor and domestic hot water system via flat plate exchangers. I had originally hoped to simply preheat the return water to the combi unit but Navien told me that won’t work on either the space heating or DHW side of the unit due to flow switches triggering the burners.

How can I make this work so that the outdoor heat source will provide almost all heat but if fuel runs out or on vacation, etc. the LP gas combi unit will fire to heat the floor as well as DHW? Not as simple as I hoped I’m sure.
There is a recirculating loop for DHW connected to an Aquastat on the right.

 

[E Yoder]

I'm not familiar with that particular unit, but is there a way to break the signal to the burner whenever the heat exchangers are heated by the outdoor furnace?

 

[Eureka]

It’s a Navien NCB-240 mod/con combination boiler. It sounds like the burners are triggered on by a flow switch but it didn’t sound like it could be interrupted. Water will only flow through if the internal pump engages and it sounds like if the pump runs the burners fire. I’m going to try to get more help from tech support today but so far all they could tell me is that my plan to heat return water wouldn’t work. He mentioned heating the radiant floor supply through a separate zone and the DHW through the recirculating loop but didn’t elaborate how whatsoever. I’m a cabinetmaker so I’m doing my best to figure this all out.

 

[maple1]

I still haven't picked out your DHW stuff in your pic.

Do those two circulators have check valves?

I'm wondering about a general approach of placing your wood flat plate to the left of your zone piping, and plumbing it into their manifolds on the opposite ends of where they are plumbed now. So the flat plate could flow through the zones without the gas side seeing that flow - if the existing circs are checked. (New flat plate circ(s) likely also should be checked). If the existing circs aren't checked it should be easy (relatively) to cut some check valves in. You would likely also need a mixing valve to make sure the floor piping doesn't get too hot - not seeing one of those in the pic either (yet) so maybe the gas boiler only heats to temp the floors need?

Not sure how you'd control it though. There's a whole lot of controlling going on in your system though that I haven't seen before and not sure the ins & outs of the current setup. A second aquastat or controller that would simply interrupt the existing controls and start the wood side circ if the wood boiler side piping is hot?

EDIT: You maybe wouldn't need to tie into the opposite ends of the manifolds - rather just T in between the existing circs & manifolds where the 90's are? Similar sorta thing on the returns.

 

[Eureka]

Sorry, hopefully you can see the DHW setup better in these pictures.

The cold (well) supply comes in from the right into the boiler. The recirculating pump is teed into the return loop and hooked to an aqua stat that controls the outlet the pump plugs into. I have a timer to make the recirc come on only during useful hours. The hot supply from the boiler runs up at far right. When the recirc comes on (or a faucet is opened) it pulls water through the boiler (sensing flow, it then fires). All connections in/out of the boiler are at the bottom.

So basically, floor heat is all on the left and DHW on the right.

 

[Eureka]

There ARE check valves in the circulators. The boiler has digital control to set floor heat and DHW output temps, so I do need to add mixing valves.

 

[nhtreehouse]

The radiant loops sure look like Primary/Secondary to me. Following the copper to the left, into the branch of a tee, then to two circulators, presumably controlled by the green Taco box. The returns in clear/white PEX come back to the closely spaced tees below the modcon. So from a plumbing point of view, it looks to me like the radiant loop is a secondary, driven by a single primary - the modcon.

So as far as how to get heat from the wood boiler HX to the floor, I would think you could inject into the secondary loop with another set of closely spaced tees.

In theory you could setup the wood as the lead in a permanently staged two boiler system. If the wood side is injecting enough heat into the secondary loop upstream from the modcon, then the modcon would not fire.

Controls would have to be well thought out though. This is a similar setup to a solar with fossil backup which Siggy presented at one point. I haven't looked in a while, but he did a webinar on that and some of that material might have made it to the sticky.

If you were away is there any reason to worry about heating DHW? Any number of members here will give you stories about how much that complicated the system.

 

[Eureka]

No need for DHW if we’re not home, but I want it to be hands off transitioning from wood to mod/con, depending on demand/supply.

 

[maple1]

The radiant loops sure look like Primary/Secondary to me. Following the copper to the left, into the branch of a tee, then to two circulators, presumably controlled by the green Taco box. The returns in clear/white PEX come back to the closely spaced tees below the modcon. So from a plumbing point of view, it looks to me like the radiant loop is a secondary, driven by a single primary - the modcon.

So as far as how to get heat from the wood boiler HX to the floor, I would think you could inject into the secondary loop with another set of closely spaced tees.

In theory you could setup the wood as the lead in a permanently staged two boiler system. If the wood side is injecting enough heat into the secondary loop upstream from the modcon, then the modcon would not fire.

Controls would have to be well thought out though. This is a similar setup to a solar with fossil backup which Siggy presented at one point. I haven't looked in a while, but he did a webinar on that and some of that material might have made it to the sticky.

If you were away is there any reason to worry about heating DHW? Any number of members here will give you stories about how much that complicated the system.

It sounds though like if there is any flow through the existing boiler loop (primary), that will kick the LP boiler on via flow switches. Which would mean that in order for that to work, the switches would need to be where they see secondary flow. So not sure injecting the wood into the secondary could be done without the LP cutting in? Should be able to do something here but would need a clear understanding on exactly how things are controlled now.

 

[NateB]

I think you will need 2 HX, and some controls. Something that may simplify things would be a unpressurized storage in the basement. You could monitor the temperature of the tank, and turn on the tank pump if there is heat. If there is no heat turn on the modcon. Storage would also make your wood consumption a little less, and your boiler cleaner. Does that idea interest you?

 

[Eureka]

The radiant zones are the basement floor (lower manifold) and the attached garage floor (upper manifold). A wall thermostat at each location tells the Taco box to turn on the circulator to push hot water to that zone, with a wire going to the TT terminal on the boiler (presumably turning on the internal pump and burner). I think I’m starting to see the light a little after looking under the hood of the boiler.
Their tech support just confused me because it seems like I can do just as suggested and tap into the secondary piping with a set of closely spaced tees on a new loop through a flat plat exchanger and pump that wouldn’t be connected to the modcon boiler. If the modcon doesn’t get a signal, the internal pump won’t run and the burner won’t fire. When the wood heat exchanger loop is pumping, water will flow right through the secondary loop bypassing the modcon primary loop altogether, through the closely spaced tees. I’m realizing this as I type it so bear with me, but does this sound logical?
I will just need a controller to turn on either the wood secondary loop pump (and the associated zone circulator calling for heat), or to turn on the modcon primary loop, based on the temperature of the outdoor water coming in. Just an aquastat maybe?
I need to draw this up.

Storage is in the future but just not now because winter is on its way and I have plenty of other projects pressing me on time.

 

[maple1]

I would think the flow switches would be the key thing to find out about. Are there really flow switches? Exactly where are they? What part do they play in the controls?

 

[TCaldwell]

You could use a Tekmar 150 or a Johnson A419 to toggle between wood or gas based upon a sensor on the wood tank supply and programmed contactor open/close temp.
The gas wall hung hwh’s that I’ve seen use a flow switch to fire, usually there is a specified gpm needed to activate.

 

[TCaldwell]

By toggling in this case might be if the wood is hot, the power to the gas hwh is removed and power applied to the wood circ. If the wood is cold, power applied to gas hwh and power to the wood circ is removed, this would eliminate the possibility of the gas firing even if there was slight flow through it?

 

[maple1]

Have you determined there are no flow switches at play?

 

[maple1]

I am thinking a simple stat or controller that is also tied to the circs without going thru the part of the zone controller circuit that sends to TT. Sort of the same way two switches are wired to one light. Or - why not a simple stat or controller that just breaks the TT connection when wood exchanger is hot?

 

[nhtreehouse]

All the control suggestions sound like they are right on.

Your modified drawing doesn't use closely spaced tees to inject heat from the wood boiler and 50 plate HX into the secondary loop. I would have thought that you would want to keep the tried and tested primary/secondary plumbing? I checked the biomass sticky on the homepage and almost all the dual boiler schematics use P/S plumbing with closely spaced tees. There are several systems very much like yours depicted in the sticky. And the wood/pellet/biomass boilers are always upstream in the secondary loop, and are effectively a fixed lead boiler in a lead/lag staged boiler system. You could use a delta-T controller to detect if the wood is "hot" or some other control technique. For me, I think through plumbing first and controls second, then iterate to a solution.

 

[maple1]

You will also need a mixing setup on your wood supply to ensure the water is not too hot going to your floors. That is built-in to the gas boiler as it maintains the right temp (I think). Vs your wood exchanger pushing whatever comes out of it.

That could be a mixing valve added to your drawing, or temp controlled variable speed injecting to secondary if you change to primary/secondary injection.

But aside from that, after a night of sleep I still am not seeing why a simple temp controller that would break the TT circuit when wood is hot wouldn't work? Then the existing controlling can just keep on doing what it is doing no matter which side the heat comes from. Unless there is another control in the gas unit that shuts everything down if the gas boiler doesn't fire when it thinks it should?

 

[Eureka]

I realized that I didn't export the correct saved version. Here are the corrected drawings.

 

[TCaldwell]

All above good suggestions, since you have a proposed schematic I’d call Tekmar controlls, the tech support is fantastic. There is likely many ways to accomplish this.

 

[Eureka]

All above good suggestions, since you have a proposed schematic I’d call Tekmar controlls, the tech support is fantastic. There is likely many ways to accomplish this.

One of the reasons I did the schematic was so I could talk it through with tech support from a couple companies much more easily. Tekmar is on my list to reach out to. I value everybody's opinions and suggestions here just as much because I've found that some companies don't know the capabilities of their own products as well as some of the members here.

Do you all think that zone valves will work well for my DHW zoning plan? I have no experience with them. My main question would be does prolonged actuation harm the valve in any way? Only potable zone valves I have seen are from Taco. Now that I think about it, I suppose that I could put a flow switch in to actuate the valves only when a tap is opened.

 

[maple1]

I think I would clean up & simplify the DHW side, something like this:

 

[Eureka]

I have to prevent the mod/con boiler from seeing flow though. Anything over .5 GPM and the integral flow switch lights the burner

 

[Fred61]

I don normally have suggestions on design of these systems because I don't feel I'm qualified but I happened to click on this one and quickly scanned what the OP was trying to do. I also quickly scanned the replies.

My first thought was to use three way zone valves that would divert the flow and therefore take care of the flow switch problem. They could be activated by a control that closes on drop and could actually open the circuit to the modcon.

 

[Dutchie84]

Are you sure that the flow switch is the only thing that controller when the boiler fires? I would think that there must be a thermostat in that circuit. So that even if you have flow over .5gpm and the water is up to temp then the boiler doesn't fire but if the FPHX didn't get it hot enough then it fires to heat it to the set point. If that's the case then I would probably do it like Maples Drawing.