Integrating dampers on parallel furnace setup.

[andym]

I'm currently enjoying my Drolet Heatmax2 furnace. I've got it installed in parallel with my LP furnace sharing supply and return ducting. I have a backdraft damper installed immediately after the wood furnace. It is normally open/power close and works great. I have a gravity damper coming that will be installed above the LP furnace. Returns are shared with no obstructions.
Here's the deal: the recent cold snap revealed that this furnace will not be sufficient on its own. (Airsealing, insulating, etc is on the list.) What is the best way to make the 2 furnaces work together seamlessly? The 2 things I want to avoid are 1: closing the damper while wood furnace blower is running. 2: having the damper open when the LP runs (short cycling the gas furnace.)

Can I wire the motorized damper to only close if there is a call for heat and the wood furnace blower is off? One problem here is that the wood furnace sends power through 4 different wires depending on plenum temp.

My current idea is to let the LP furnace always close the damper when there is a call for heat. I would then add a barometric bypass damper (like 10" round or something) to the HMX2 plenum that would allow the blower to 'dump' the heat.

Any thoughts? Installing separate ductwork is not an option. I love the setup I have, just need to address this one issue.

 

[andym]

One thing I should add: currently I have a motorized damper above the LP furnace. It is normally closed/power open. That was a mistake. It opens in time, but closes before the blower stops. That means the blower is trying to push air through a dead end. It currently builds some pretty high static pressure. I finally tracked down a gravity damper the correct size. I would like to sell the motorized damper if someone is interested. It's a 8x18 with a Belimo 24v motor.

 

[brenndatomu]

I would like to sell the motorized damper if someone is interested. It's a 8x18 with a Belimo 24v motor.

Use it...cut that into your HMX2 plenum and wire it up to be the heat dump if both furnaces are running...

 

[brenndatomu]

Can I wire the motorized damper to only close if there is a call for heat and the wood furnace blower is off?

Sure, wire it to the NC contacts of a relay...then the relay is wired to activate whenever the HMX2 blower is running...(so the NC contacts would then be open, keeping the damper open, until the HMX2 blower shuts off)

 

[JRHAWK9]

Sounds like you are trying to accomplish what I wanted to do years ago.

Electrical gurus....please come in

Currently my LP thermostat is set at 55° to keep it from kicking in when the wood furnace is running/still hot (so it doesn't back feed the wood furnace when the wood blower shuts off). I want to be able to set my LP thermostat higher so when/if we aren't around to load the firebox in very cold...

www.hearth.com

I see you have a power open damper.....that was a mistake. If you would ever lose power you want that damper to remain open.

 

[andym]

Sounds like you are trying to accomplish what I wanted to do years ago.

Electrical gurus....please come in

Currently my LP thermostat is set at 55° to keep it from kicking in when the wood furnace is running/still hot (so it doesn't back feed the wood furnace when the wood blower shuts off). I want to be able to set my LP thermostat higher so when/if we aren't around to load the firebox in very cold...

www.hearth.com

View attachment 270986

I see you have a power open damper.....that was a mistake. If you would ever lose power you want that damper to remain open.

I have a power closed damper by the wood furnace so I'm covered there.

How do I wire in a relay when the power wire for the wood furnace blower might be one of 4 options? That's what has me confused.

 

[andym]

Is it possible to use a relay as suggested when the motorized damper is 24v? I guess maybe you would just wire in a transformer somewhere?

 

[brenndatomu]

How do I wire in a relay when the power wire for the wood furnace blower might be one of 4 options? That's what has me confused.

Does it ever really come off low? From what I read, they almost never do.
I'm not sure how they are wired, you'd have to do some research/testing, but is there a place that you can tie in ahead of the control board/whatever that selects the speed?

 

[brenndatomu]

Is it possible to use a relay as suggested when the motorized damper is 24v?

You can get relays that operate on about any voltage, and AC or DC.
And yes, you can get a 24V transformer (if needed) pretty cheap...last one I bought was from SupplyHouse.com...less than $10 IIRC

 

[andym]

The main time it comes off low is in the first 45-60 minutes of a medium to full load. Or if the reload timer gets stuck, which has happened. Either would normally be a time when the LP is not needed.
Tapping into the power ahead of the relays may be possible, but I think it would require connecting directly to the circuit board.
The idea of using my spare damper as the bypass dump is good. I will think about that.

 

[brenndatomu]

Either would normally be a time when the LP is not needed.

Exactly...

 

[brenndatomu]

You could get into wiring in diodes, or a whole bank of relays, (relay logic) but I think there are other more realistic and less complicated solutions here.
Can that Belimo be flipped over, so that it is spring open? Some of them can...mine is that way...

 

[andym]

Can that Belimo be flipped over, so that it is spring open?

Yes. Whatever I use for the bypass dump I'm planning to (eventually) install it on a hinged plate and fasten it closed with a fusible link so I also have a EHD. I could install the motorized damper to spring open (and be my EHD) but don't like the idea of it always being powered closed.

 

[brenndatomu]

I could install the motorized damper to spring open (and be my EHD) but don't like the idea of it always being powered closed.

I'm with you...just FYI, the Belimo's have every low power draw once they are locked into their full "power on" position...

 

[andym]

Thanks for all the feedback. I'll do some checking with my multimeter while I wait for the gravity damper to arrive. I may just do some kind of barometric bypass because it's simple. I first want to investigate the possibility of the relay because that would be the cheaper option.

 

[andym]

Just double checked the circuitry on the HMX2. The blower is controlled by 4 relays that are attached to the actual circuit board. I can't see the back of it without removing it, but I'm sure you probably could solder on at the right place to 'steal' power ahead of all the relays. Don't think I'm going to try that though. Any volunteers?

Now that I've puzzled over it a bit it seems that all I need to do is follow @JRHAWK's diagram above except add 3 more relays, one for each speed. The 'juice' going to the damper motor would pass through each NC relay and could be interrupted by any of the 4.
Does that sound like a plan?

I could also just use another pid controller to do the samething based on plenum temp. Might actually be cheaper than 4 relays! This would give me a real time plenum temp display as well.

 

[JRHAWK9]

Just in case this helps you any. I have 4 relays total. One for my powered damper and three others.

My spring open/power close damper is in the duct between the Kuuma and the main house duct. It's wired as shown above to only be CLOSED when the LP/AC blower is the only one running.

You may be wondering why I have three other relays.
Without the blower switching relay and on very cold mornings when the wood furnace would be at the end of it's burn cycle and if the LP thermostat happens to call for heat (set at 68°), both furnace blowers would be running at the same time. Even though it's been a few years since this has even occurred, I want things setup correctly so it CAN. At this particular point in the burn when the LP may call for heat, the Kuuma would be ready for a re-load and therefore the speed controller would have the Kuuma blower turning very slow and the LP blower would overpower and backfeed it. This relay transfers power AWAY from the speed controller and to the low speed winding on the Kuuma blower as soon as the LP blower kicks in. I like to say with this setup (including how I have my damper wired) both the LP and wood furnaces can "dance" together without any issues at all. Everything is fully automatic and I don't have to do a thing. Even from summer/winter, I don't have to change a thing or flip any switches or dampers. It's fully automatic.

The overtemp relay is there just in case the speed controller fails. There is a secondary snap switch in the plenum set at a much higher temp (180°). If the plenum ever reaches this temp it will automatically take power away from the speed controller and power the low speed winding on the Kuuma.

Having said that, here's what my controls setup does.

Wood furnace blower motor is speed controlled via the high speed tap only when ALL of the following are true.
- Primary low limit snap temp in wood furnace plenum is met
- LP or AC blower is off
- Emergency secondary snap switch temp in wood furnace plenum NOT met

Wood furnace's low speed winding is being powered when one or the other, or both of the following are true AND the primary low limit snap temp in wood furnace plenum is met:
- LP blower is on
- Secondary emergency snap temp in wood furnace plenum is met.

I have the secondary emergency snap disk temp set at 180° (the maximum this particular snap switch goes up to).

I've had this setup now for 3 or 4 heating seasons I believe.

 

[andym]

Just in case this helps you any. I have 4 relays total. One for my powered damper and three others.

My spring open/power close damper is in the duct between the Kuuma and the main house duct. It's wired as shown above to only be CLOSED when the LP/AC blower is the only one running.

You may be wondering why I have three other relays.
Without the blower switching relay and on very cold mornings when the wood furnace would be at the end of it's burn cycle and if the LP thermostat happens to call for heat (set at 68°), both furnace blowers would be running at the same time. Even though it's been a few years since this has even occurred, I want things setup correctly so it CAN. At this particular point in the burn when the LP may call for heat, the Kuuma would be ready for a re-load and therefore the speed controller would have the Kuuma blower turning very slow and the LP blower would overpower and backfeed it. This relay transfers power AWAY from the speed controller and to the low speed winding on the Kuuma blower as soon as the LP blower kicks in. I like to say with this setup (including how I have my damper wired) both the LP and wood furnaces can "dance" together without any issues at all. Everything is fully automatic and I don't have to do a thing. Even from summer/winter, I don't have to change a thing or flip any switches or dampers. It's fully automatic.

The overtemp relay is there just in case the speed controller fails. There is a secondary snap switch in the plenum set at a much higher temp (180°). If the plenum ever reaches this temp it will automatically take power away from the speed controller and power the low speed winding on the Kuuma.

Having said that, here's what my controls setup does.

Wood furnace blower motor is speed controlled via the high speed tap only when ALL of the following are true.
- Primary low limit snap temp in wood furnace plenum is met
- LP or AC blower is off
- Emergency secondary snap switch temp in wood furnace plenum NOT met

Wood furnace's low speed winding is being powered when one or the other, or both of the following are true AND the primary low limit snap temp in wood furnace plenum is met:
- LP blower is on
- Secondary emergency snap temp in wood furnace plenum is met.

I have the secondary emergency snap disk temp set at 180° (the maximum this particular snap switch goes up to).

I've had this setup now for 3 or 4 heating seasons I believe.

View attachment 271030

View attachment 271029

So you wouldn't think I'm weird to wire the 4 relays? Lol!

 

[brenndatomu]

So you wouldn't think I'm weird to wire the 4 relays? Lol!

If you have ever seen old school equipment control panels, they were full of relays...

 

[andym]

When the blower is on speed one the #1 wire has 125v, #2 has 112v, #3 has 89v, #4 has 77v. Would those voltages be compatible with a relay?

 

[JRHAWK9]

When I was metering my speed taps when looking into speed controlling it, I had residual voltages on the other two speed taps as well. When one of my taps was given 120V, the other two would show some real goofy voltages....like you are seeing.

 

[brenndatomu]

When I was metering my speed taps when looking into speed controlling it, I had residual voltages on the other two speed taps as well. When one of my taps was given 120V, the other two would show some real goofy voltages....like you are seeing.

Yes, induced or "phantom" voltage...if you tried to hook up an actual load (not just a meter) you would see those numbers go to zero...

 

[andym]

I was positive that when I checked it out previously I had zero on all but #1. That explains it then.

 

[andym]

I started to shop for relays, but am a bit overwhelmed at all the options. I need a relay with 120 coil and an option for normally closed. Any recommendations?

 

[brenndatomu]

Everybody and their brother makes/sells relays...should be fine as long as you stay away from elcheapo junk.
I use these a lot...2 sets of contacts, they can be wired NO, or NC, (or one of each) and you can get whatever coil voltage you want too...I like this style that plugs into a seperate base like this...and these bases can be screw mounted directly, or clipped onto a DIN rail. (for easy/clean/quick mounting of multiple relays... )
(broken link removed to https://www.ebay.com/itm/Lot-of-2-Omron-LY2N-J-120V-A-C-Coil-10A-Cube-Relay-with-Socket-Base-PTF08A/392317448941?epid=2212813620&hash=item5b57f132ed:g:lD8AAOSwqgddDB3~)

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DIN Rail (mm): 35. RoHS Compliant. Due to center-to-center distance between mounting slots, rails may be 11.75" - 12" in length. Condition: New.

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