Indoor wood boiler piped with propane boiler

jasonmaher · November 13

hi everyone, I built my house a couple years and and installed a full infloor heat system with an indoor wood boiler and a propane boiler piped in series (it’s a closed loop system, not an outdoor wood boiler). It’s been running okay for the past couple winters but I’m thinking now I need to make a few changes. I have the dump zone for my wood boiler piped out into my front patio outside which works good (I have glycol in the system). I don’t have a mixing valve on my upstairs infloor which is just stapled up to the bottom of the plywood subfloor and I have vinyl plank flooring upstairs. I got the aquastat on the wood boiler set on 150 cut in and 170 cut out which works fairly well but I’d like to lower it some more to get a constant circulation and more even consistent warm floors rather than it not call for heat for while and the floors cool down. I also have an insulpex pipe made by rehau ran out to my detached garage. Just built the garage so I’ve never ran that yet. So I’m thinking if I put upstairs in a mixing valve and the basement manifold is already on a mixing valve, then take a tee off and run it full temp out the the garage and put a mixing valve out there. I’m thinking I may put a unit heater out there as well so there’s constant flow going to the garage and returning.

With all the mixing valves on the system I’m thinking the return water will be fairly cool to the wood boiler so I’m thinking along with all that I may put a tee into the supply and return and a bypass with a pump on it so the supply will always put a litttle hot water into the return to keep it safe from thermal shock. I would also like to put an aquastat In the line somewhere that will cause the propane boiler to cut I. If the temp drops to a certain point. That’s the way I intended it but I wasn’t really sure how to accomplish it. Does this sound good to you guys?

and lastly, would it be a benefit to use a 30 gallon indirect hot water heater as a buffer tank? Run the supply and return of the wood boiler through the buffer tank so it’s its own loop and have constant circulation all the time. And then have the hot pipe from the indirect to go to the return of the propane boiler and then go through it and the system and the return come back to the indirect. Would I still get the heat transfer from the interest to get 180 or so water going through the system to then bump down with the mixing valves?

EdTheHeaterMan · November 13

There are lots of ways to accomplish what you want. you need to define what you want first then the control and piping can be made to accomplish that goal. @hot_rod s the man with all those design answers. You just need to tell us how you want the wood stove and LP boiler to operate. For example:

Primary heat source for space heat, DHW and Garage will always be the wood stove. If the woodstove does not provide enough heat as a result of some vacation time away from the home, the LP will automatically kick in after the wood stove fire is no linger enough to heat the home.

Water is the best heat transfer medium so the dump zone being the outdoor patio may be best operated via a heat exchanger so you do not need to fill the entire system with glycol. I’m not sure how much that will save you over the life of the system but I do know the Glycol is a PIA to work with.

hot_rod · November 13

what type of indoor boiler? Is it a gasification style? How much water does it hold? The newer air tight indoor boilers can ramp down quickly, so a heat dump may not be on the top of the list.

With low temperature radiant, boiler return may be an issue, but easily solved with a 3 way valve.

A common control calls on the backup when the wood drops to a low temperature. A differential control offers more benefit.

parallel piping, not series you never want the back up boiler heating the wood boiler.

Flat plate heat exchangers are a good way to isolate the garage, and may end up cheaper than Glycol depending on capacity.

It would be helpful to know what temperature water the upstairs and basement radiant needs, probably not 180, so several mixers as you mentioned

How do you generate DHW? An indirect tank?

GroundUp · November 13

Pipe the wood boiler with a "boiler loop" and its own circulator that runs 24/7 when the wood boiler is going, then hydraulically separate the loads from that loop using closely spaced tees or a hydraulic separator. This will maintain a safe return temperature to the wood boiler regardless of what is coming back from the radiant. Put an aquastat on the supply side of that loop and interrupt the TT circuit to the gas boiler with it, so that the gas will kick on when that wood boiler loop drops below setpoint. This is a very common system and I have dozens of them out there operating like this

hot_rod · November 13

Here is a piping option using a hydraulic sep to connect all the boilers and loads.

The sep 4 gives you air, dirt, mag and hydraulic separation for both boilers, the entire system really.

Showing 3 temperature.

Highest to unit heater in garage

Medium to staple up

Low to in slab.

A simple 3 way boiler protection valve at the wood boiler.

Control could be a simple as one or the other, you manually switch back and forth or a temperature controller to make the switch.

Also info on how a boiler protection 3 way valve works. It's basically a fixed temperature thermostatic 3 way valve, high flow rate.

\You could maybe use the indirect as your dump. I'd look at how large the firebox of the boiler is, how much energy you would need to dump in a run away condition. If it is a small indoor boiler you may not need to dump much?

jasonmaher · November 14

https://forum.heatinghelp.com/discussion/comment/1872126#Comment_1872126

it’s a Benjamin heating products DO110 model. It’s not a gasification boiler. It holds 31 US gallons. I already have the house filled with glycol / water mix as my dump is in the patio outside and I have enough glycol to add to loop of garage anyways. Is there anyway you could sketch up a schematic as to the way you would pipe it in parralell? Here’s some pics of how it’s piped now.

hot_rod · November 14

How often do you go to dump? If you get the fire management dial in it should be rarely.

If it does need to dump often consider dumping into an indirect or a zone in the home or garage where the heat is more useable? You paid for that energy, may as well use it.

A row of fin tube can often be adequate as a dump. And there are options for a dump in the event of a power outage and a pumps can run. Either with a swing check or normal open zone valve.

Next question is how many temperature zones do you want. You probably send a high temperature to the shop for a unit heater, 140F or above. Maybe 110 to the radiant.

6-12 shows a buffer tank, but that could be a hydraulic sep instead, same 4 ports, much smaller.

Then pick off the various loads on the right side of the sep.

so more similar to fig 6-14 but with the NTI which doesn't need a protection valve. In 6-14 a swing check allows thermosiphon to dump to the sep, then pick the load you want to pump that dump excess to.

Then the sky is the limit for what you can do off the other side of the Sep. Pumps, zone valves, multi mix temperatures.

This shows all the zone possibilities off the sep. Maybe a high temperature zone, an indirect zone, a garage zone, then the mix valve for the radiant.

You have a lot of the components to be used, but it will be a major repipe.

A patient person could disassembe, clean and reuse a lot of the copper fittings. I made some wood jigs to hold the pipe or fitting without squeezing it out of round, so fittings come off easily. Copper tube and fittings are crazy expensive now.

You really do not want the two boilers in series as the NTI (auxiliary) is just sending heat up the flue of the wood boiler, keeping it warm, plus jacket loss to the room.

In your drawing the returns all go through the wood boiler to get back to the NTI.

More ideas and explanations here.

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_10_NA_Hydronics%20for%20wood-fired%20heat%20sources.pdf

Kaos · November 15

There are two main issues with your setup.

The modcon is getting hot water from the wood boiler which means when the modcon fires when the boiler is still hot, you are giving up efficiency.

When the modcon is running you are circulating hot water through the wood boiler so you have additional flue losses.

Since your backup fuel is expensive, you want to be careful with how this is all set up.

Lot of the answer depends on if you are DIYing the fix. I think this is the best setup especially if you have room for a decent buffer tank. You might want to use an indirect as a buffer to reduce the amount of glycol.

Also does your modcon have a built in pump? I

hot_rod · November 15

If you did want to keep the glycol out of the wood boiler (30 gallons) and associated piping, a reverse indirect is an option that adds more buffer capacity, but more glycol. A $$ tank however.

Everything pipes off the B side as show above.

Or a plate heat exchanger which would be higher efficiency and not much additional glycol.

Both sides of heat exchangers need air and expansion components, maybe a fill tank also to maintain pressure on the glycol side.

Indoor wood boiler piped with propane boiler

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