I am installing an outdoor wood boiler to assist my heat pump system. Currently I have a Rheem heat pump with electric strip back up heat. The issue at hand is to prioritize heat from the boiler before kicking in the heat pump. As I see it I come up with a few options:

1 Do nothing and simply use the wood boiler as 'more' heat when the heat pump kicks on. Not accomplishing what I want to happen...
2 There are diagrams around the internet to use 2 thermostats inside, slaving the heat pump to the wood boiler. Not ideal as I don't want the 2 thermostats on the wall.
Now my swags...
3 Install a 3 stage thermostat, I assume I can get full speed blower operation with the first stage simply kicking on the blower (Y1 on stat going nowhere, RG fan and RB which is always on in heat mode), 2nd stage calling for the heat pump compressor (Y2 on stat going to Y on air handler) and 3rd stage calling for the heat strips to kick in (add in RW).
4 Install a thermal switch on the boiler line that will break the yellow compressor wire to the heat pump until the boiler lines fall below a set temp (ie I have not done my job and kept the stove full of wood).

Any help would be appreciated.

 

Forgot to mention, the wood boiler requires no input from the thermostat, the pump runs all the time and has an aquastat mounted on the unit to control the damper.

Currently have a Honeywell TH4210D Thermostat installed.

 

I would get my control scheme working before I installed the wood boiler. You are looking at a boiler explosion if the boiler liquid turns to steam. What happens if the wood boiler circ pump fails?

 

It is an open boiler, it does not create steam or pressure merely circulates hot water and the system is vented to the air at the unit. Kind of a misnomer to a "boiler" I know.

This is similar to my thoughts on the thermal switch in the Y wire to the compressor:
https://www.grainger.com/product/GRA...et-Limit-6UDZ5
open at 160 close at 120, the 'boiler' system is supposed to run at 180

 

Welcome to the forums.

You'll need to install a water to air heat transfer coil inside the air handler too....... correct ?
Are you planning on using the boiler full time as primary heat or on a whim as backup ?

If you have a lot of wood to burn..... the boiler should be primary heat.

 

Yes, wood for primary heat. Unfortunately we have been hit hard by the emerald ash borer, bad for the trees but good for the firewood pile.

Yes, I have an air to water exchanger inside the duct work above the airhandler.

I have figured out my 3 above won't work exactly as written. The airhandler needs Y power to kick the fan up to full speed. I could possible isolate the Y on airhandler from the Y on the condenser with a relay??? Or Y1 to air handler, Y2 to condensor assuming stage 2 powers Y1 and Y2???

 

I would use a 3 stage heating thermostat to control the system. You could use an adjustable time delay relay to energize the 2nd stage (heat pump) in case the boiler is slow to respond to temperature changes, which it may be since you are burning a solid fuel. Operation of the furnace fan and the control of the speeds would be dependent on the wiring of the existing heat pump control system. (The guys on this site can help) I am very old school and not well versed on the newer control systems. You mentioned that the boiler is an open boiler system. I am curious as to the design, type and model. Please post some more information of what you have.

 

www.ezboilers.com
I have their larger Pro model. It circulates 200 and change gallons of water.

The wood boiler operates autonomously from the thermostat, the pump runs continuously and there is an Aquastat controlling the damper on the stove.

 

Finally fired the boiler up yesterday. Wow this thing is awesome. After the boiler heated the water to running temperature you could feel the heat from the duct work even when the blower wasn't running. I slipped the fan on at the thermostat (real low speed) and this warmed the house up 10 degrees over night. Just running as additional heat with the heat pump for now. I think I am going to install a thermal switch in my yellow compressor wire unless someone says I am going to break something by doing it. I picked one of these up: https://www.supplyhouse.com/White-Ro...F-Open-on-Rise Can this be as simple as a $6 part and a few minutes of time?

 

That coil needs a bypass in it so you aren't heating the house when its not calling.

 

I just insalled the switch above to break the yellow wire to the outside unit on my heat pump. It took some experimenting while placing the switch to get enough surface bearing to allow the switch to open , or maybe I just wasn't patient enough, but I ended up placing it on a 'T' in the hot water system closest to the heat exchanger. Works fine to break the connection to the outside, hopefully I don't have to worry about completing the circuit until spring and I am shutting the boiler down.