Multiple zones dual-fuel hybrid heat pump

[Chevsky]

I'm looking at replacing my 20-year old heating & cooling system. It's 2 zones -- meaning it's two separate, complete, independent systems--one is 100KBTU, the other 80KBTU, home is 2900 sqft.

Does anyone know if it's possible to connect two hybrid dual-fuel style interior handlers to a single outside HP unit? Ultimately, I'd like to reduce our LP use as much as possible, and "comfort" is less of a concern (we have a fireplace if it comes down to that).

I've described what I'm thinking below -- if anyone has recommendation then I'm open to your suggestions. Thank you.

My Thoughts -- I'm interested in a Dual Fuel (heat-pump and LP gas) because I'm in Massachusetts. If possible, a single outside unit feeding two inside air-handler+furnace combo units, much the way they do with mini-splits. The reasoning for a single outside unit is because the second zone gets much less use than the primary zone, and it would be nice to save the cost of the second outside unit. Also, since the secondary zone gets less use, I'd consider skipping the dual-fuel on that space, making it just a simpler ducted mini-split style connected up to the existing air ducts. (There are no bathrooms--thus no plumbed water--upstairs to be damaged if we closed down the space during super cold snaps.)

Why it's laid out this way -- Currently each system has it's own interior air handler with LP furnace, and its own outside AC compressor. The layout of the house is such that there isn't a convenient way to bridge the two spaces with duct-work, so the original installer just used two separate systems. The first system is the main house, with the air handler in the basement, and the second system is a large family room and office over the garage with the furnace in an attic space and using smaller-sized units. The upstairs system's AC coolant lines run down into the basement, and then along-side the primary zone's lines to the compressors sitting outside side-by-side. Both systems, and the hot water heater, use the same single-flue chimney.

When going 96%+ efficient I'd switch to low-temp exhaust and could remove the chimney (that is, if replace the water heater too). And if I remove the chimney this would create a passage for supply/return air ducting from the secondary space's air handler to the location of the primary furnace, thus negating the need for two separate systems. But I'm not sure I'd want to do that anyway, as the duct distances would be long, and if we reclaim the chimney footprint then we'd prefer other uses... and well, perhaps the trade off is A) cost of two air-handlers, vs B) cost of chimney removal and of running extra new ducts. (I expect there are new building codes that would dictate that upon replacement I can't continue using a common chimney flue this way. But the update would mean I'm not using this chimney for the furnaces anyway, but still for the hot-water, unless it's also replaced.)

So it seems that simplest replacements would be 1) to go with two separate dual-fuel/hybrid systems (or one duel-fuel and one ducted mini-split), or 2) with an intergrated, two-zone (via refrigerant lines) system with a single outside unit, and two interior units--either both dual-fuel HP+LP hybrid, or one hybrid and one HP-only. This is my preference, rather than removing the chimney and connecting the duct-work together with single system but with two zones using air dampers, etc.

I was told by an installer that although mini-splits have this option--one outside unit feeding multiple inside units--that dual-fuel hybrid systems don't. My initial impression is, "why not? Are you sure about that?" And after some research, I don't see any "packaged" system with this option, but I also don't see any evidence that I can't just buy à la carte two hybrid air handlers, one outside unit that has multiple zone connections, and two line-sets. Is it something to do with the outside temp cutoff of the HP and switch to gas? I'm ok with both zones switching to LP regardless if one zone is calling for much less heat compared to other; or if 2nd zone is HP-only and during a cold day and the primary zone switches to LP and the 2nd zone gets no heat because it doesn't have a hybrid backup--with today's super low-temp heat pumps I could set the cut-off low so those situations will be very rare.

 

[John Gayewski]

I'm looking at replacing my 20-year old heating & cooling system. It's 2 zones -- meaning it's two separate, complete, independent systems--one is 100KBTU, the other 80KBTU, home is 2900 sqft.

Does anyone know if it's possible to connect two hybrid dual-fuel style interior handlers to a single outside HP unit? Ultimately, I'd like to reduce our LP use as much as possible, and "comfort" is less of a concern (we have a fireplace if it comes down to that).

I've described what I'm thinking below -- if anyone has recommendation then I'm open to your suggestions. Thank you.

My Thoughts -- I'm interested in a Dual Fuel (heat-pump and LP gas) because I'm in Massachusetts. If possible, a single outside unit feeding two inside air-handler+furnace combo units, much the way they do with mini-splits. The reasoning for a single outside unit is because the second zone gets much less use than the primary zone, and it would be nice to save the cost of the second outside unit. Also, since the secondary zone gets less use, I'd consider skipping the dual-fuel on that space, making it just a simpler ducted mini-split style connected up to the existing air ducts. (There are no bathrooms--thus no plumbed water--upstairs to be damaged if we closed down the space during super cold snaps.)

Why it's laid out this way -- Currently each system has it's own interior air handler with LP furnace, and its own outside AC compressor. The layout of the house is such that there isn't a convenient way to bridge the two spaces with duct-work, so the original installer just used two separate systems. The first system is the main house, with the air handler in the basement, and the second system is a large family room and office over the garage with the furnace in an attic space and using smaller-sized units. The upstairs system's AC coolant lines run down into the basement, and then along-side the primary zone's lines to the compressors sitting outside side-by-side. Both systems, and the hot water heater, use the same single-flue chimney.

When going 96%+ efficient I'd switch to low-temp exhaust and could remove the chimney (that is, if replace the water heater too). And if I remove the chimney this would create a passage for supply/return air ducting from the secondary space's air handler to the location of the primary furnace, thus negating the need for two separate systems. But I'm not sure I'd want to do that anyway, as the duct distances would be long, and if we reclaim the chimney footprint then we'd prefer other uses... and well, perhaps the trade off is A) cost of two air-handlers, vs B) cost of chimney removal and of running extra new ducts. (I expect there are new building codes that would dictate that upon replacement I can't continue using a common chimney flue this way. But the update would mean I'm not using this chimney for the furnaces anyway, but still for the hot-water, unless it's also replaced.)

So it seems that simplest replacements would be 1) to go with two separate dual-fuel/hybrid systems (or one duel-fuel and one ducted mini-split), or 2) with an intergrated, two-zone (via refrigerant lines) system with a single outside unit, and two interior units--either both dual-fuel HP+LP hyprid, or one hybrid and one HP-only. This is my preference, rather than removing the chimney and connecting the duct-work together with single system but with two zones using air dampers, etc.

I was told by an installer that although mini-splits have this option--one outside unit feeding multiple inside units--that dual-fuel hybrid systems don't. My initial impression is, "why not? Are you sure about that?" And after some research, I don't see any "packaged" system with this option, but I also don't see any evidence that I can't just buy à la carte two hybrid air handlers, one outside unit that has multiple zone connections, and two line-sets.

I think having two separate systems is better than sharing an outdoor compressor. If one breaks down you could be weeks away from relief. Especially with heat pump stuff. I think most people would rather have two systems as long as they are both effeciant you should be using the same amount of energy.

I like the "duel fuel" heat pump/furnace idea.

You said you had a fireplace but also would tear out your chimney? Are these two separate structures?

 

[Sponsor]

 

[Chevsky]

I think having two separate systems is better than sharing an outdoor compressor. If one breaks down you could be weeks away from relief. Especially with heat pump stuff. I think most people would rather have two systems as long as they are both effeciant you should be using the same amount of energy.

I like the "duel fuel" heat pump/furnace idea.

You said you had a fireplace but also would tear out your chimney? Are these two separate structures?

Yes -- Two different chimneys.
The fireplace is in a large central chimney, but the furnaces use a small single-flue chimney located nearer to the joining of the two "halves" of the house. In fact, the large fireplace chimney has a second unused flue, complete with spare LP line to the area in the basement by the chimney complete with a flue port -- it looks like the builder imagined perhaps a wood shop with gas stove. Many possibilities

 

[Fitter30]

Two systems would better because cause haven't found one other than a mini split that wouldn't have the option of dual fuel. Minis use variable refrigerant flow along with variable air flow and haven't seen one with a vertical ahu just horizontal. Contractor is more important the brand and the higher efficiency equipment takes a better trained service person. If using a mini 2nd floor use hyper heat unit works better at low temps.

 

[Chevsky]

Two systems would better because cause haven't found one other than a mini split that wouldn't have the option of dual fuel. Minis use variable refrigerant flow along with variable air flow and haven't seen one with a vertical ahu just horizontal. Contractor is more important the brand and the higher efficiency equipment takes a better trained service person. If using a mini 2nd floor use hyper heat unit works better at low temps.

Is "Hyper Heat" a Mitsubishi trademark name? Amazing lockout temp at -22F!

But that system is very expensive ($8,500 for 42,000BTU @ 15.4 seer, uninstalled), and tho I agree it would be a technically good choice if opting for non-hybrid on 2nd zone, the high cost makes it less attractive when we don't use that space much and probably don't need it for it's design temps. Hell we'll probably never see those kind of temps here ever. And since a dedicated hybrid system for this zone is overall lower install cost... Even the cheapo "packaged" HP-only units (60,000BTU @ 17 seer) is priced at $4,500.

For aesthetic reasons it would be nice to use the same brand for both systems, even if not the same model for the two. (like having matching compressors on the two cement pads outside)

I still think there are systems that do multi air handlers (zones) insides with single outside compressors--there are being advertised, and DAIKIN seems to have it (even with mix-n-match air handler types--see below--that pic is from GreenHome Institute presentation of [Mitsubish's] Intelli-Heat system). But I hear what you all are saying -- no benefit in trying to take the multi-zone, variable-refrigerant-flow concept to a full-house multizone, ducted, hybrid, non-mini-split design, when I can simplify the install and maintenance by going more mainstream. Especially when one zone is a pseudo non-living space (tho we do host over the holidays get use out of this space -- but if it were super cold and we couldn't get it up to a comfortable temp then we have other options).

I guess another argument for keeping the systems separate, at least in my case, is the low use of zone two means that we could limit our upgrade to only the first zone in the beginning. It may be that we don't really need to upgrade zone two, or at least it's a smaller investment, and we're concentrating on the major energy-consuming unit. This would also give us the benefit of learning and experiencing prior to choosing whether to do HP-only or add the hybrid option in zone 2. OTOH, there's a large opportunity cost lost in not doing both zone at the same time -- a single project vs two, done at different times means have to re-hire work (if not installing myself), complicating tax credits, multiple permits drawn from town, etc.