1800 square feet, one story with loft and full basement, very well insulated, electric resistance boiler with in-floor radiant heat. No gas. Just installed solar panels. My thought was to put the mini-split in the loft with the air spilling out into the great room. Primary reason is for the few daytime hot days (gets quite cool at night), and secondary reason, to provide some heat at lower electricity cost. After I've lived with the solar for awhile, I'll see how much of my grid-supplied electricity they replace, perhaps it's not worth making this addition. I use about 23,000 kWh/year.

From your description and location, I'm guessing you can have heating days as much as eight months of the year? My gut is it's worth looking into more for the heat savings than the cooling, with cooling a side benefit, although depends on the cost of your electricity. Placement may be tricky - does the loft also tend to stay warm in winter? If so, the trade-off is that you'd get more heat benefit and savings from placing it elsewhere, but may not get the cooling benefit you'd like. Although with baseboards you may be able to control the room-by-room usage well.

Note, the extra efficiency of heat pump would not be much of a benefit on your coldest days (if any), depending on temperature and the heat pump performance in cold weather, but quite possible that's only a couple weeks a year.

Note/question, does the sq footage include the basement or not? Was kind of assuming not. Anyway doubt it needs any cooling.

Your situation is a unique one I'll give you that. I think given your situation I would do a toe in the water test and put the mini split in an area that that will garnish the most use.

Meaning somewhere within your home in which you spend the most time. Make sure that what ever kind of mini split you decide on that it's an inverter heat pump, obviously heat pump but also inverter. If you get any significant snow fall you want to be sure to place the out door unit some where the snow doesn't drift to. Otherwise you'll have to continually clear snow around it for performance in winter.

The inverter feature not only gets it to higher efficiency but also allows for better performance. Sizing is less of an issue, because it will only deliver what is necessary. (It will ramp up an down)

One other benefit in going the mini split option is you can just focus on the one area in your home that you use the most.

With that said, if you had air ducts already in place that would change the equation as to what I would recommend. So I am not in favor of just band aiding every problem with mini splits if you know what I mean.

For what they offer, (spot heating and cooling) they can be a solution in some cases.

Thank you, Austin and Armoured, for your thoughtful answers.

1. the square footage does not include the basement, which is the same size. The basement remains relatively cool in hot weather and relatively warm in the winter. It has the radiant heating tubes imbedded in the concrete floor.

2. No ducts, first (main) floor also has heating tubes between the sub- and finished floor. Each of the two floors has 4 zones, with individual thermostats. Each floor has one, internet connected thermostat for the main area and three non-internet connected for the side rooms. I can moniter the main thermostats and, right now, with an outside temperature of 22°, the basement setpoint is 70° and the actual temperature is 72°. On the main floor, the actual temperature (and setpoint) are both 70°. There is a lot of solar insolation (if that's the right word for heat gain from the sun) on the main floor, but since it is early morning right now, I doubt that is helping much.

3. Last summer, I contacted a local refrigeration contractor and asked for a quote for a Fujitsu cold climate heat pump. He hasn't gotten back to me, but guessed that it would be about $7000. His business is primarily commercial, and he is very busy and I haven't pressed him on the quote.

I'm going to wait to see how my electric bill is affected by my new solar panels before I make any decisions, but am just trying to get some advice now. One thing, however, both my wife and I really like silence and would rather not introduce any more noises, other than the refrigerator and the heat recovery ventilation system.

I apologise as I missed the point that you have in-floor radiant, entirely my fault. Do I understand correctly that the upper floor/loft does not have in-floor? Is it heated separately at all?

So I revise my thoughts somewhat - consider for now just the minisplit for cooling and a bit of supplemental heat if smaller and more reasonably priced ($7k sounds way up there in cost, likely quite high end). The key for heat pump is it will be much more efficient when outside temps moderate, you have to look at specs. If I'm right that you have relatively long heating season, it will be quite efficient for a lot of those 'supplemental heat' days; many work pretty efficiently down to about freezing, specs have COP figures against outdoor temps.

In other parts of the world where hydronic is more common and electricity expensive (eg. Europe/UK), it would be relatively common to consider an air-to-water heat pump to replace your resistance boiler. But that would be a pretty big step, and lots of technical issues such as is your hydronic system 'sized right' for low temperature, and be costly. You'd really want someone on the install side who's comfortable with these systems.

I'd definitely do more research and see how your solar system is working for you before jumping to anything like that.

Thank you, armoured. The loft does not have any heat, it only covers about 1/2 of the area of the main floor and is not occupied (you need a ladder to get up there). I have looked into an air-to-water heat pump but haven't come up with anything yet - still looking. I've also considered a ground-sourced heat pump (geothermal) but the capitol cost is quite high. My electric rates are fairly low ($.09/kWh) and I do have the solar (in fact, I'm not sure the solar has a good ROI because of the low rates).

We live in NH and put a mini-split in the apartment attached to our house. It has been there for about 3 years. Due to the metering configuration and other changes to the property it is hard to say exactly how much $$$ we are saving, but there is no question we are saving money compared to cost of electric baseboard heat and window AC. My sense is that general comfort wise it is much better also, especially in the summer. We are now looking at putting one in the heated barn space as well. That is a bit more complicated because although we don’t really need cooling, it might be useful sometimes.

Ah, another slight revision as I didn't realise your loft was not living space, or at least not used much at all.

I think this actually simplifies esp if your hot weather is humid - just put the minisplit wherever you spend the most time and want the most comfort. Which is basically austin's suggestion. (I think I heard that the inverter ones are a bit better at lowering humidity? Not sure)

It'll get some use in heat pump mode, and be a useful supplement for tweaks to temperature (slight issue with radiant floors is they're pretty slow to ramp up heat or cool down, so you can get overshooting if operators are impatient). It may help some with bringing down heating costs but don't count on it and if it does, it's a bonus. If it does save you money you can use it more.

Obviously would be nice if it can handle heat down to very low temperatures, but for this use I wouldn't overpay - it's supplemental heat, not your main heat source. You'll still get decent efficiency on cool but less-than-Arctic days when you'll be heating, and I guess you have a lot of those days.

You can also wait and see, of course, or consider lower-cost solutions like ceiling fans, reducing sun, or venting/opening windows at appropriate times.

I assume that if I set the thermostat for the heat pump higher than the main thermostat, the heat pump will run continuously and if it can't keep up, then the resistance boiler will start up.

yes, it can work that way. Mini split HP depending on the model can deliver heat down into the negative temps.... especially an inverter based model.

So I think the weather would have to be really dire for the need of the boiler.

I think the bigger problem would be in areas of the home outside the reach of the mini split HP. It's really a *spot* heating and cooling appliance.

Yes, those areas have separate thermostats for heating. I would direct the output from the mini-split to the great room, which includes the kitchen. We don't have the bedroom thermostat even turned up, and it's cool enough at night, in the summer; the bathroom is its own zone.

It may take some experimentation to find the right balance for comfort in heating seasons - mainly you probably don't want to run the heat pump so much that the floor goes completely cold, and as it's (probably?) a low temperature system, it would take some time to get back up to comfortable level. Will depend a bit on your lifestyle and preferences and what you're used to and the like - likely not a big difference but you may notice sometimes. That might mean for example turning the minisplit heat to a lower temperature periodically (at night? or possibly the opposite), or programming your hydronic system to pump warm water to that room every few hours, or things like that. But you'll likely be doing tweaks like this anyway, just something to keep in mind. Or maybe you won't notice this aspect at all.

It takes so long for the floor to heat up that we don't really change it much. We used to set it for 50° when we were away. When leaving to get there (it's our vacation home), I would turn it up to 70 and by the time we got there, 6 hours later, it might be up to 60°, but usually that was at night and we'd be going to bed anyway!

Adding supplemental heat like this may change how your existing system feels in that room. What I'm referring to is that - depending how your existing set-up works - if you have a heat pump on all the time or at a slightly too high temperature, that in-floor circuit may get shut off or not circulate water through at all. The room will still be warm enough but the floor feel colder and the heat of the room feel different. (Especially if you prefer to go barefoot) Or, this may not bother you at all.

There could be a couple different ways of dealing with this depending on set-up. It may not consume much energy but still be important for comfort, esp if it's the main room you use most of the time and how you use it.

One other advantage, would probably help in bringing temps back up in events where the place cools down.

My real hope (for heating) is that I can heat the house to above freezing while we are away, without using my resistance heat (much)!

I don't know style of the place or the floor plan, what your budget is, what your level of need for A/C is, but what I think I would do is consider getting an air source heat pump. And run exposed metal ducting (which can be very stylish) in the interior of the home.

No different than a mini-split technology wise but different hardware wise. And cheaper. An air source "heat pump" has almost the same hardware as central air but is different because it can run forwards and backwards. In summer, it takes the heat out of the interior air in the home and dumps it outside. In winter, it runs in the opposite direction and takes "heat" out of the exterior air (which there is until things get very, very cold) and dumps it inside. Same technology too as a ground source (geothermal) heat pump but it uses air instead getting or giving up heat into the ground with pumped liquid in pipes.

Google "exposed residential ductwork" for a sense of what it might look like. It can be quite stylish. You've probably seen exposed ducts in retrofitted public spaces (like restaurants) in older buildings.

At 9 cents per kwh, I join you in wondering why solar makes sense. Unless it was quite cheap. But 23,000 kwh per year for a place used occasionally is a lot of electricity.

23,000kW at 9¢/kW = $2,070. This is a country home? I’ll guess that 75% or ~$1,550 per year is almost all heating. Installing properly designed air-to-water heat pump I’ll guess might save about $900 per year on electricity.

I would install 1-air-to-water heat pump that would heat your in-floor radiant, provide hydronic supplemental forced air heating to quickly change temperature, provide hydronic air-conditioning, domestic hot water (DHW) and integrated zoning.

Your existing electric hydronic boiler that’s already there would be integrated into the system to provide back up for extreme cold weather or compressor lock out. This is a cold climate system that will provide useful, efficient heating down to -5˚F (below 0˚F) with a COP 1.48 and a COP 1.98 at outdoor air temperature of +15˚F with a hot water temperature at 120˚F.

Premium system but less than geothermal, no ductwork, no drilling ground loops, controls and zoning that are top tier.

I would look at this and compare cost differentials to all other options.

Link to complete specs:

Nordic Air-To-Water Heat Pump

See page 25 for schematic of integration of in-floor (heating), fan coils (heating & air-conditioning), Domestic Hot Water (DHW).

IMPO

SR

@mtvhike, not sure I understand exactly how you plan to run it while away although I get the general idea, but I think it should be more efficient / use less electricity to keep the house at a reduced temperature while away. I believe it's accurate to say that the efficiency (coefficient of performance) is a function of both the outside air temperature AND the temperature 'output' - or put differently, the temperature differential, or how much it needs to raise the temperature between the two. So for example if it's zero celsius outdoors and your target temperature indoors is 15c instead of 24c (sorry I think in celsius only), it will run more efficiently/with a higher COP. (Note a simplification as unit will heat air to some higher temp to hit/maintain the room target). This is above and beyond the usual savings of maintaining a lower temperature and different from how resistance heat works. (Note another simplification here is that there are temperature ranges in which given units work best)

So in those cases where away, setting the temp for the floors lower than the heat pump, and both lower than usual, should save money.

I still wouldn't rush into it or splurge on an expensive unit as your electricity rates are not that high and your solar is pretty recent. From your numbers your electricity bill is perhaps $2500 a year (before solar), and I've no idea how your tariffs work with the solar fed into grid. A high-cost heat pump might take a long time to pay off in those conditions and your place is relatively new.

My gut would still be to look at moderately priced units to help with summer temps / cooling / humidity and any heat benefit/savings you get is a bonus.

@fsq4cw, what is the ballpark cost or price range of systems like you're describing? (My biggest concern with that would be finding reasonably local and competent installer/dealer...)

@fsq4cw, although this was built as a "vacation home", my wife is there ~90% of the time, and we occasionally leave for visits to the grandchildren in the winter, hence my concern about heat unattended. I agree that "A high-cost heat pump might take a long time to pay off in those conditions and your place is relatively new", so that's why I'm not doing anything now about this, just gathering information.

We have two story house with 1940 radiator heath and new gas boiler, combined with a ducted A/C system installed in the conditioned attic. The characteristics of well designed hydronic is much different than a forced air system, so I'm not sure how your mini-slit will fit in when you have it running with existing heating system.

Hydronic provides smooth, unobtrusive heat that just seems to be where its needed, but any system with a fan creates air movement (obviously) and the air movement changes the balance of the heat. For example, we don't get significant amount of air moving from one floor to the next, so the 2nd floor is only about 1 degree different than the 1st. If we had forced air I don't think we could keep that same balance. Since you have only one floor, maybe this is a moot point.

Bruce

Bruce, hot air rises, cool air falls. If you have an open stairway and a temperature difference between the floors, no fan is needed for air movement between the floors.

I have both a 1 story and a 2 story home with ducted heating and cooling. Getting balanced temperatures between rooms and floors (for existing systems) is the same task and can be helped by tweaking the openings and louver directions of each outlet. Not a big deal in my experience.

If you decide to go your original route, be aware that DIY minisplit systems with full warranties are now available from Mitsubishi. They range in price from about $1500 to about $2500 depending on size. If you are mechanically inclined you should be able to install one of these yourself. They have pre-charged components (outside condenser, inside evaporator/fan unit and refrigerant lines). HVAC installers charge outrageous prices to put in a minisplit.

Re: amoured

It’s difficult to give a ballpark figure for numerous reasons but it would be in the range of a geothermal heat pump minus the drilling and ground loops, there would be a lot of specialized plumbing involved. Heating contractors that specialize in hot water and in-floor radiant systems should be able to install and service this equipment.

I would contact the manufacturer for a recommended installer and at least get an estimate for comparison sake and to learn more about energy efficiency & system integration.

Re: mtvhike

I understand that you’re just gathering information and in the end everyone does what’s best for them. The biggest concern now would be that there are no burst pipes during prolonged power outages and the home being unattended – especially the in-floor radiant!

IMPO

SR

@fsq4cw - how about a ballpark price just for the air-to-water heat pump? Just out of my own curiosity.

Note that due to @mtvhike having a fairly large solar array and the specifics of his tariff scheme, I'm not sure that any significant capital cost to reduce his electricity usage will pay off well. This shows one of the oddities about specifics of solar tariffs.

As I understand the way it works is this: his base tariff for electricity is 9.5 cents/kwh. Any solar he generates offsets the electricity he consumes (calculated annually?). BUT: any solar generated in excess of consumption is only paid at the wholesale rate, which is dramatically lower than the retail rate (call it 1 cent / kwh). Right now - without full year data - he expects to consume somewhat more than his panels generate.

The upshot is that his savings from reduced consumption are limited (You could analyse this differently but I find it easier to just take what he already has as the base case, I think the conclusions are the same). Let's say, for example, he consumes 4,000 kwh annually more than he generates, for a bit less than $400 (netted out annually). If he saves more energy than that, every 'extra' kwh saved only gets him 1c / kwh. At least unless or until he adds some significant extra load, like buying an EV or putting an aluminium smelter in his garage or something.

So: I'd say it makes sense to consider a minisplit primarily for the air conditioning comfort and if it also offsets some of his straight resistance heating, great, there's some modest savings there (within a 'limit' of however much his consumption currently exceeds his solar generation). Spending much additional for the 1c/kwh savings just won't make much sense financially.

This is pretty simplistic as right now he has historical data for consumption and the solar array is relatively new so annual generation only estimated, and of course from year to year both may vary. And at any rate, a good argument to not rush into anything with high upfront expenses or to prioritise other projects for the time being.

Thank you, @armoured, I think you have my situation correct. Regarding historic consumption data, I have been recording my consumption regularly since the house was built at the end of 2015, and on Jan 2 of this year, when my meter was changed, the total was 95,250 kWh, giving about 24,000 kWh/year. I'm now recording it from the new meter, but don't have enough time to be significant.

Let us know what you decide and how it goes. Interesting case.

Now, in April, 2021, I'm living here full time and want to reopen this discussion. Last year, the solar array produced 16.6 MWh of energy, the lowest month (January) was 250kWh and the highest (May) was 2200 kWh). This array has been covering about 3/4 of our electric costs.

I'm going to re-contact the Fujitsu dealer I mentioned above - he may be the only installer within 25 mi, although I'm sure there are many farther away.