Auxiliary gas heat for ductless HVAC

Question

Background:

I hate ducts. I hate them passionately. Maybe the dust and gunk they collect irritates my asthma. Maybe the constant rumbling, whooshing, or whistling noise drives me up the wall and prevents me from sleeping at night. Maybe my house has lots of supporting I-beams that the ductwork needs to weave under, making my basement ceiling too low for me to stand up. Maybe ductwork killed my father. It doesn’t matter why I hate ducts -- for the purposes of this post, just take it as a fact that I do.

I do like having an air conditioned house, though, which is why I would like to install a ductless heat pump system. This would let me rip out my ductwork and still heat or cool the rooms in my house. I can also put in ceiling cassettes like these, which will please my wife, since she hates radiators and indoor evaporators almost as much as I hate ducts.

Unfortunately, I live somewhere with cold winters, so heat pumps alone won’t be enough. I need auxiliary heat for at least part of the year.

I’d rather not pay $500 per month for electric auxiliary heat in the middle of winter. The idea of a power outage in the middle of winter is also pretty scary if I’m relying on electricity for all of my heating. Instead, I want to burn natural gas or LPG as a heat source.

I could put a gas-powered radiant in-floor system in all of the rooms of my house to use only on days when it’s too cold for the ductless multi-split system to keep up. That seems silly, though. If I’ve run refrigerant tubes for a multi-split system through my walls and ceilings to put BTUs where I want them, why do I need to run a separate set of water tubes through my floors to put more BTUs where I want them?

What I want to do is to somehow use a modulating condensing furnace to add heat to the refrigerant of the ductless heating system. Then, on the coldest days, I could turn on the gas furnace in my basement to provide the heat that my ductless HVAC system would move through my house.

Essentially, the goal is to have 2 sources of heat coming into the ductless HVAC system (a condenser unit outside my house, and the gas furnace inside of it) that both fed the refrigerant loop carrying heat or cold throughout the house.

The question:

How can I use a gas furnace to ‘add heat’ to a ductless multi-split system? If anyone has any ideas, I’d love to hear them.

Answer 1

I'd look at hydronic distribution with an air-to-water heat pump (reverse cycle chiller)

The bad news is that you can't do what you're talking about -- your only bet for gas emergency heat in a ductless form factor is an (inefficient) "Empire" style wall furnace.

The good news is that modern hydronics are very flexible, and can accommodate high-efficiency heat pumping (systems like the SpacePak Solstice and Chiltrix CX34 are out there if you know where to look). Furthermore, condensing gas boilers are as good or better than any gas furnace out there when it comes to efficiency, and a hydronic system can also cover your domestic hot water needs in addition to space heating.

Answer 2

Can you? Probably. Should you? Almost certainly not

You could certainly rig up some weird gas-fired heater for your outside coils, but you're just asking for trouble. And adding some type of heat to help your coils will have all the inefficiencies of adding a second unit on top of a first.

Must better would be some type of supplementary heating. Wood-fired stove, high-efficiency fireplace, even a secondary gas wall heater would be much better bets and all would be guaranteed to work, versus something rigged up with baling wire and duct tape. Whatever you invented would also need to be easily removeable for when you want to sell your house. I certainly wouldn't buy a house with a redneck-engineered heat pump, especially one that uses a substance that could burn the house down.

Answer 3

You have been given a lot of reasons why not to try this.

'But screw the reasons! I want to try it anyway!' - OP

I don't think adding heat to the refrigerant would work; at least I don't know how to do that, and it might mess up the heat pump's operation. But moving air and water are within my realm of activity.

So I put on my jury-rig cap, and here's how I would try it: Use a hydronic heating system with desired furnace, inside the house, where it would normally go, and installed and vented according to code. Then run antifreezed water through insulated water pipes...

...to car radiators installed in suitably-designed insulated support boxes behind the outside heat pump units, so the heat pump fan draws its input air (where it would need to find at least warm air) through the hot car radiator. Possibly add an auxiliary fan behind the car radiator to reduce back pressure on the heat pump fan.

I'd try this first with a temporary setup, before sinking hard cash on the furnace installation. I'm not sure how to lay my hands on a temporary boiler that I can set up outside to get hot water for the test. So going full-on back-woods engineering, I'd try getting a small car up close to the heat pump and radiator setup, connecting extended radiator hoses between the engine and the heat pump's added radiator, then carefully monitoring the car's engine temperature while asking my wife to see if the heat pump is making heat. I'd probably have to try the test on a really cold day when the heat pump would normally rely on electric backup heat to be sure it really works.

If id does work well enough, I'd consider finding the contactor for the emergency electric heat inside the heat pump and wire it to a relay switching the 24Vac thermostat inputs for the hydronic furnace, so the heat pump turns on the hydronic heat instead of the electric heat.

Then I'd take off the jury-rig cap and see if the possible lack of efficiency of this setup and the fact that I'm the only one who knows how to troubleshoot it (meaning no service calls to competent repair folks if I'm nasty sick and it breaks down on that really cold day) might make me cool my heels a bit and see if there's a better way.

And I'd probably decide on a regular hydronic heating system, on the ground floor, either under-floor or say, "Honey, we're going to have to have some small baseboard radiators. I'll try to make them look nice...".

(And then I'd say to my inventor self, "Maybe this is the way folks can get non-electric backup heat for their heat pumps! Maybe I'm on to a big business opportunity! I'll try it anyway on one heat pump, take efficiency measurements, fuel in vs heat out, etc, but I won't rely on it to heat my home in the dead of winter yet.")

Answer 4

This is a bit of a "2 out of 3" problem. You can get any 2 but not, practically speaking, all 3.

• Ductless
• Gas emergency heat
• Reasonable price

The reason ducted systems are so popular is because they provide an easy way to distribute fully conditioned air - heated or cooled, humidified or dehumidified, filtered, etc. - throughout a house while using one central system for doing the work. When it comes to "doing the work", anything involving burning stuff benefits significantly from a central system as there are significant costs for fuel distribution, exhaust gas removal and heat exchanging. 5 small gas furnaces may not cost that much more than one large gas furnace, but the 5 exhaust vents, and in a modern high-efficiency system also 5 input air vents, will add significantly to the cost.

The end result is that gas emergency heat really works best as a central system unless you are willing to use something like an Empire furnace in each room. But that runs afoul of the "no radiator" rule.

Installing a gas system to provide hot water for radiant floor heating is quite doable. But very expensive, particularly as a retrofit.

By the way, personally I find ducts to be just fine (but maybe I'm used to them) and relatively unobtrusive compared to radiators. On the other hand, I really have not gotten used to the look mini-splits - these big things high up on the wall are out of the way but ugly. So a lot of this comes down to personal preference.

In the end, money matters, at least for most of us. I know you don't want to pay for electric resistance heat. But you need to do the math:

• Cost to stick with ductwork and install a high-efficiency central heat pump, using gas for backup heat. (Gas heat, reasonable install price, low running cost)
• Cost to install multi-zone mini-split heat pump, using electric resistance heat for backup. (Ductless, reasonable install price but high running cost)
• Cost to install multi-zone mini-split heat pump and gas radiant floor heating. (Gas heat, ductless, high install price, low running cost)

Calculate initial cost + running cost for 10 years, 15 years, 20 years. Then figure out what makes sense. Cost is not the only thing, but it is a very important factor.

Answer 5

Bad idea, because in transfering heat from gas flame to circulating fluid in a heat pump system, which Must stay coolish ( examples; about 40C for R32 or R290 ) you are sure to get poor efficiency. You don't have the option of running a superheated steam circuit like a coal power station did at 200+ C, and even those get imperfect efficiency in transfer of heat from a flame, albeit better than to coolish circulating fluid.

If you try to heat either type of refrigerant with a gas flame, your neighbours will hear it very promptly after you lit it and hear about how that happened at the inquest afterwoods.