Kuuma VF 100 Honeywell Zone Question

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Birdsell36

New Member
Mar 28, 2023
5
Connecticut
Hello,

I am about to order a Vapor Fire 100 this week and am still finalizing my install plan for my situation. I am going to common the VF with an oil fired forced air furnace. I currently have two supply trunks coming out of the plenum of my existing furnace, zone 1 and zone 2. These have powered zone dampers that are normally open and are Honeywell units. I have a third zone that is not in use or hooked up to the Honeywell control unit. I know that I will need two backdraft dampers in each of these supply trucks to avoid back feeding the oil fired furnace when the VF is running. I am wondering if I can avoid the additional work using the below strategy.

My question is if I can trick the system to use these existing dampers as backdraft dampers. I would hook up a 3rd thermostat to zone 3. When the VF is running I can set this thermostat to 100 degrees which would constantly call for heat and would close zone 1 and zone 2 (they would then act as backdraft dampers). When the fire dies down and zone 1 or zone 2 need to call for heat the damper would open even though zone 3 was still calling for heat.

If this is stupid please just tell me. Thanks!
 

brenndatomu

Minister of Fire
Aug 21, 2013
8,117
NE Ohio
It sounds like a reasonable rough draft plan to me. But if you want to discuss things with someone that is a forced air zoning guru, talk to Jerry at Retrozone, they are damper central, and know this stuff inside and out. He will probably recommend Belimo damper motors on any newly purchased dampers, I have a couple and I can highly recommend them! They are only a bit more $, a lot of bang for the buck...a one and done purchase.
Congrats on the VF, solid purchase decision.
 
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RockyMtnGriz

Burning Hunk
Apr 19, 2019
219
SW Montana
If you're feeding your air from your Kuuma to the house side of these dampers, it should work. Though, I would figure on the Kuuma running 24 hours a day. It may not have the fan constantly on, but it'll probably be at least cycling on periodically - they take about 4-6 hours to cool down enough to go quiet once the "burn" is over. So, if your zone 1 or 2 is calling for heat during that time, the Kuuma is going to backfeed the oil funace if it's blower isn't on. If you contemplate being able to run both furnaces simultaneously, it's going to need to be more complicated, because the oil heater will backfeed the Kuuma when it's blower is cycled off.

At least for me, the Kuuma is a 24-7, slow and steady wins the race kind of affair. While I have a gas furnace sitting right next to mine, the complexity of making them dance together means that I only ever run the Kuuma. Not that it can't be done, I just don't see it as worth it for me since I work from home, don't go away in the winter, and prefer to burn the wood I have in my near silent Kuuma, to heating with the gas. Most of the time I can run my Kuuma on 12 hour loading cycles, so it's not that hard, but I'm home.

Depending on your situation, you might just want to plan on tricking the furnace into closing those two dampers, and plan on using the Kuuma exclusively. If you plan to switch to oil at some point, you're going to need some kind of damper in the Kuuma duct to prevent recirculation. It could be manual, for either/or, like I have, or if you want them to work together, then it gets more complicated.

Whatever you do, the Kuuma needs to be able to breathe whenever it's hot, so the duct damper needs to be open without power. If using a power close damper to prevent backfeeding when alternating in the oil furnace, you need to figure a way for that damper to open if the Kuuma fan cycles on. and the backpressure with the oil blower on will need to be low enough for the Kuuma blower to move air at the same time.

That's another reason why I run one or the other, but not both. The outllets I have open to efficiently move heat around with the almost always on but small Kuuma blower are much fewer than what is appropriate for the much larger, but mostly off conventional gas furnace blower. If you were really lucky you might be able to figure out a way to zone that and make it work, but my house wouldn't work that way. If I was building from scratch for a wood furnace, I would probably give it its own set of heat ducts.
 

RockyMtnGriz

Burning Hunk
Apr 19, 2019
219
SW Montana
What I wrote above might be a bit misleading. I should clarify:

For me anyway, when it's cold, and I'm running at higher outputs, the Kuuma blower is almost always on. But, when the weather is warmer, running at min output and less than full loads, the Kuuma spends a lot of time heating with the blower off, convecting hot air like a gravity furnace.
So, stove on = blower on is not something you can design around.
 

Birdsell36

New Member
Mar 28, 2023
5
Connecticut
What I wrote above might be a bit misleading. I should clarify:

For me anyway, when it's cold, and I'm running at higher outputs, the Kuuma blower is almost always on. But, when the weather is warmer, running at min output and less than full loads, the Kuuma spends a lot of time heating with the blower off, convecting hot air like a gravity furnace.
So, stove on = blower on is not something you can design around.
Thank you both for the replies, it got me thinking a little more clearly. I can prevent backfeeding the Kuuma by using gravity dampers in the round duct that I will have coming off of the plenum before going into the supply trunk of the house. This will allow for the Kuuma to breathe with the fan off as you mentioned in your post. My solution to prevent backfeeding the oil furnace is flawed. When zone 1 or 2 reaches the required temp and is no longer calling for heat they will close. The furnace will think that zone 3 is open as it will always be calling for heat. The blower will come on to disperse residual heat from the oil furnace but with zone 1 and zone 2 closed there will be no where for the air to flow. This will likely destroy my blower.

To your point I can probably still use this method as backdraft dampers if I set zone 1 and zone 2 to like 30 degrees so that they never call for heat and remain closed.

I either need to think this through further, use the Kuuma independently, or potentially hire an hvac guy to common them appropriately.
 

brenndatomu

Minister of Fire
Aug 21, 2013
8,117
NE Ohio
2 things to keep in mind, you want to have the ducts wide open whenever the Kuuma is "on", so that it can gravity flow when the furnace is has a fire burning, that way if the power fails the furnace doesn't overheat itself, and your ductwork (clearance to combustibles applies here!)...yes the Kuuma intake damper will close on power fail, and the fire slowly die down, but depending on where the load is in its phase of the burn, the furnace sometimes can operate on pilot air (closed damper) for an hour or two straight...that heat has to go somewhere, and gravity dampers will block gravity flow unless blocked open. What I did was to install a power closed, spring open damper, that way its guaranteed open with power off...that, and an emergency heat dump door in the plenum. It has a fusible link that melts at ~200* and it lets a 12"x12" door fall open to dump heat to the basement.
Good luck finding a HVAC guy that has a clue when it comes to wood furnaces...they are rare.
 

RockyMtnGriz

Burning Hunk
Apr 19, 2019
219
SW Montana
I either need to think this through further, use the Kuuma independently, or potentially hire an hvac guy to common them appropriately.
Well, it sounds like you're in the "zone" now ==c

I second Brenndatomu and suggest you not think about gravity dampers. Power close, spring open are what you want.

And, yeah, I think it's going to be difficult to find a HVAC person to handle this, and if you do, I'd advise you to have the operating details figured out for yourself anyway, to keep someone from creating something in your basement that could go full meltdown on you when something they didn't allow for happens.

With a little more thought I'm sure you can work it through. It might be simpler to not use the controller you have for the dampers and move the control of the dampers to operate off the fan power circuits. Maybe that is informed by thermostat input, maybe not. Though I have a thermostat in the house attached to the Kuuma, I've found it irrelevant and leave it full cold, except rarely I'll turn it to full hot on an arctic day just to make sure the fan stays on high. Generally, you would want the dampers open unless the opposite unit's blower is on, and also open when that unit's blower is on. DPDT relays are inexpensive and the bomb for us not-so-digital old guys. The younger generation would probably use digital controller to do the logic. Whichever you're comfortable with...

If just running one or the other is a possibility depending on your lifestyle, you might protect your options for the future but start simple and see how it goes for a winter. The Kuuma is going to heat your house differently than a conventional setup. I wouldn't plan on having the kind of "point and shoot" heat you can get from a conventional system - think slow and steady. You might find that you have less need, or less use for zone controls, etc,, which might allow you to make simpler changes to optimize your system.