Combi Boiler Backup to Wood Boiler

Hoopman5000

Hi everyone, I recently completed the install of a combi boiler (Weil Mclain Aquabalance AB-120c) that is a backup to a wood boiler.

I've attached my schematic.

The circulator for the primary loop is on all the time (as long as the water temp is sufficient). My question is this: when there Is a call for heat, the combi boiler circulator starts and no matter what the water temp, the combi boiler burner fires for a few minutes. Normally the combi circulator is off unless there is a call for heat.

This is the same issue when I use the domestic hot water, the burner fires for a few minutes and then modulates down, even though the primary loop water temp is already hot.

Is there a way to eliminate that burner coming on in the beginning when the circulator comes on? In my eyes, if the combi circulator runs continuously, then the burner shouldn’t have to fire since the water temp should be at setpoint. Then, I'm a bit out of my element as to how to control the combi circulator when the wood boiler goes out.

I thought about moving the combi boiler temp sensor to the primary piping, any thoughts with that?

thanks!

hot_rod

More about the control logic? When does the combi call for heat? How does it know the wood boiler is off?
If the  burner fires the boiler is not up to temperature, piped as as secondary there is no way for the wood boiler to heat the combi and vice versa, and you would not want the combi heating the wood boiler

One way is with a differential control that fires the combi only when the wood is out

In the back of Idronics 10 are some control and wiring options 

Hoopman5000

I will take a look at Idronics 10 - right now the controls aspect is where I need help.

Right now, whenever a thermostat calls for heat, it activates that zone circulator and it activates the combi circulator. When the combi circulator comes on, the combi burner starts and then after a few minutes, the burner turns off since it is drawing hot water from the primary loop heated by the wood boiler.

This is a new system and I'm still working on the controls. Right now, if the wood boiler isn't up to temp, I'm just unplugging the primary loop circulator. Again - not idea as this would be nice if it were automatic.

hot_rod

Yep, the controls of those systems is always the gotcha. If your only DHW source is them combi, it gets complicated.
Most often the fossil fueled boiler is locked out whenever the wood fired WSB has adequate temperature.
If you want DHW generated by the wood fired boiler, you will need to run thru the combi somehow.
Your drawing doesn't show the DHW portion.

If you are okay firing the combi for dhw, yes it will short cycle every dhw draw.

As for heat, how low can the wood boiler run and still provide adequate heat? At that temperature you switch to the combi and turn off the wood boiler pump. You don't want to run the wood boiler pump when the combi is running or you will sent combi BTUs to the wood boiler outside.

Often wood boilers have storage tank as shown here. But, if not sensor S1 goes on the top of the WB, S2 goes on combi boiler hot side pipe, or return if you run ODR. This is with a solar ∆T control.

If the WB is 150° or hotter the TT on combi via R-2 is open, R 2 is closed.
If the WB falls below 150 then WB pump is turned off, relay 1, R-1 opens, R-2 closes to fire the combi via TT
When the fire comes back at the WB, it raises say 5° to 155, then the contact to the TT (R-2) opens, R-1 closes and sends power to the WB pump.

So basically you need a control the watches temperature in two places, both boilers. It prioritizes the WB, and calls on the combi and drops WB when the WB is too low of a temperature to add heat.

On a call for DHW the combi fires via it's flow switch. Depending on the boiler brand, it should lock out the pump for heat or divert a 3 way valve to only the DHW heat exchanger. So on DHW call you will not, or should not add heat to WB.

One issue is if WB is off for a period of below freezing temperatures you need to protect the piping and boiler from freezing. It's pretty important to keep the WB fired if it is not glycoled.

Hoopman5000

Sounds good, thanks Hot Rod, its a great help! Ill start researching controllers. Any recommendations on something that would fit this bill? Perhaps this?

https://www.supplyhouse.com/Honeywell-T775B2016-Electronic-Temp-Controller-w-2-Temp-Inputs-2-SPDT-Relays-NEMA-4X-Enclosure

Thanks again!

hot_rod

I like Resol controls. Mostly that is what I trained on and are familiar with. Alpha Thermal Systems in MA was the US rep, check with them.
Victor was the Canadian importer and knows those controls very well

https://www.solarnetix.com/index.php/products/solar-thermal-controllers

Steca is another brand. I think Sun Earth Solar may be the dealer. Or google search solar differential controls.

Very little customization needed if you can find a multi relay solar controller.