Minisplit vs. Radiant Heat
Hello:
I am building a near-passive house 2900sf SFR climate Zone 5B with HRV and 10kW PV. Floor-plan (living is upstairs) with heating/cooling loads attached (negligible latent cooling load). Trying to decide between:
Plan A: Ductless (don’t want ductwork) mini- or multi-split (2 or 3 indoor units)
Plan B: Small scale in-floor/ceiling radiant heating AND cooling panels (no concrete slabs) using ATWHP
Questions/trade-off that come to mind:
1. How few mini-split interior units (wall/ceiling) can I get away with given my floor-plan and where should those units be located? Most doors could remain open most of the day
2. Multiple mini-splits or multi-split?
3. Hidden nature of radiant seems desirable for interior aesthetics and I can have radiant loops in more rooms than mini-split interior units
4. Would like to do large part of either install myself (exception would be vacuuming any refrigerant lines)
5. Seems much easier to install and maintain glycol pipes than refrigerant lines, especially becoming aware of and dealing with future leaks (a glycol feeder tank readily alerts you)
6. With the ATWHP I can get some DHW
7. Getting the radiant design right, spacing/length/location of loops for heating and cooling. This seems tricky
Have greatly appreciated all the expertise contributing to this site and look forward to any advice
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
In my opinion mini splits make the most sense and a heat pump water heater for dhw. I’m having radiant designed mixture of walls and floor for my house and it’s complicated. For my project it’s worth it but I don’t see what you gain for the added expense. Since you insist on cooling, radiant because that much more complicated and expensive.
Is the house two stories on slab foundation? What are the second floor framing and roof framing details? Why don’t you want a ducted mini split? With an HRV I imagine you’re okay with some ducting
Given the stretched out floor plan and lots of rooms I don’t think ductless is a good option. Why no ductwork? If you are still in the design phase I would drop the ceiling in the hall way on each floor and have one ducted mini split per floor. Would work great.
I’m not sure what you mean by cooling panels. Do you mean fan coils fed by the AWHP? I think you will find this approach very complicated. But a properly designed system will work.
Running two line sets and paying an HVAC tech to pressure test and pull vacuum would be much easier than installing all of the glycol tubing, pumps, sensors, manifolds, tanks, etc in a AWHP system. Even if you did the ductwork yourself.
Thank you for your replies.
Brandon: Lower level is crawl space with slab under garage. Plenty of room in upper-level floor/ lower-level ceiling for any ductwork. Butterfly roof is SIPs (no room there). I’m struggling to come up with a response to your last point, other than with Zehnder HRV all ducts are home runs, 3” flexible with smooth interior, seems like an easy DIY proposition to get right. Bigger ducts with joints, harder DIY install and to get details right as compared to ductless
Kyle: Can’t really drop the ceiling on the upper level because of aesthetics. No to fan coils, Cooling Panels = Radiant Cooling = running chilled glycol (above the dew point) through the same radiant loops as used for heating. Main house is 28x42 plus the bonus rooms above the garage, maybe not so stretched out, but am concerned about using only ductless
Radiant cooling will be one of the most difficult to get right and expensive hvac systems you could have. Given your response above, putting ductwork in the floor of the upper level/ ceiling of the lower level will give you the a much simpler system with even distribution. Ductless would be even simpler, but with your floor plan it’s hard to say how even your temperature distribution would be.
In 5A, radiant cooling by itself won't work. You'll need something to remove humidity, at that point, might as well use that something for cooling as well. Much simpler and less costly. With radiant cooling, without humidity removal, the best you can hope for is a cold clammy house. Imagine how an uninsulated and unconditioned basement would feel, but in the entire house.
With a place with so many rooms, the only way to get proper cooling is with ducting. At that point might as well use that ducting and equipment for heating as well. There is no point for radiant.
Since you have a crawlspace, you can easily run the ducts through there for the main floor, for the 2nd floor, go for I-joists or trusses for the floor joists and run the ducting through there.
For ducted mini splits, I usually home run the ducting, pretty close the Zahnder idea but with larger ducting. The simplest is to get long lengths of appropriately sized aluminum semi rigid ducting and a duct adapter for the mini split (ie Fujitsu Airzone) with dedicated runs to each room. Since the ducting is inside conditioned space, they don't need to be insulated so they are not as bulky as standard flex.
The mini split for the main floor could be installed in the crawlspace. The one for the 2nd floor could be mounted in the ceiling of the main floor laundry room.
A 3/4 to 1 ton hyper heat ducted unit can easily handle each floor.
Appreciate your advice Akos and Kyle. I have to confess to another “hang-up” that I failed to mention - I don’t like floor registers (dirt/dog hair always seem to end up in them). Might the following work:
Zone 3 (lower level): ducted mini-split unit #1 in mechanical room floor supplying 6 ceiling registers (MBR, 2 studies, entry, bath, laundry)
Zone 2 (upper North): ducted mini-split unit #2 in mechanical room floor supplying registers mounted low in the walls (stairway, bed 2, bed 3, bath 3)
Zone 1: ductless ceiling cassette/unit #3 in South West corner of this larger open-plan space
A mix of ducted and ductless might work - ducted for first floor and second floor bed/bathrooms. Ductless for the kitchen/living. But ducted for everything would be my choice, it’ll make life easier. Radiant is just overkill and also the equipment isn’t there yet.
Separately, you have a lot of hallway in this house. Try playing around with that some, GBA has some amazing architects that’ll help.
Thank you Paul. Following your suggestion, I’ve been looking at Mitsubishi SVZ ducted air handler(s) to go in the mechanical room. If I were to go with just one inside and one outside unit, how would you recommend it be zoned? Power dampers?
Or, would I be better off with 2 indoor units and if so with a single outdoor unit (multi-split) or with 2 outdoor units?
BTW I like Akos’ idea of home run ducting in post #5 (no concealed joints) - but just as well I have a large mechanical room for all the spaghetti!
P.S. The floor-plan is the end result of many design/site constraints
Kyle has a good point about the two units. The one caveat is that the 24k btu Mitsubishi has much better dehumidification than 2 12k units. I have the 24k Btu Mitsubishi unit: SUZ outdoor, SVZ indoor. You’ll be thrilled with how quiet it is, I’m getting a COP somewhere between 3.5-4.
Good point Paul. I have a Fujitsu 12RLFCD for my second level. I love having a separate zone to keep the bedrooms cooler, but because our cooling loads are pretty low we do struggle with humidity. I’m considering installing a stand alone dehumidifier. I would still install 2 systems, but there are trade offs.
Paul: nice to know the SVZ is quiet.
The Structure Latent Load is 567Btuh, Central Vent -557, Equipment Latent Load 10. Not exactly sure what this means in terms of dehumidification requirements, but it does seem very low (we have almost no “muggy” days in summer).
I would always go with 1:1 units if you can. Multi splits do not have the ability to modulate as much as a 1:1.
Going with one ducted unit per floor will make it easier to zone. It’s possible to zone one unit for both floors, but it’s more complicated and probably won’t perform as well.
Yes, I have the same thought about 1:1, but it’s just a hunch in my case as I don’t have much experience with heat pumps. Another hunch says 2:2 might be the sweet spot here. If so, then I might be back to a Mitsubishi MLZ ceiling ductless in South West corner of living/dining/kitchen and one ducted SVZ for the remainder. (This would entail zoning the lower floor bed/studies along with the above garage beds/baths).
Take a look at this article about the cons of radiant heat. It might sway you more firmly in the direction of a minisplit system: Radiant Heating and Cooling.
Thanks Kiley. I have read that article as well as all the other articles that I can find on radiant cooling, both on GBA and elsewhere (GBA is best). I’m lured by the siren call of an architecturally hidden, non-ducted solution. Dehumidification could be part of the micro-ducted HRV system.
I don’t think a buffer-tanked supply of hot/cold water is difficult to achieve with current ATWHP technology (e.g. Arctic). However, there just does not seem to be enough field experience to know whether the control systems can sufficiently fine-tune the glycol supplies in a low-load home.
I realize it’s not the cheapest solution, but closed glycol loops (I use them in my current house for heating) are pretty easy to install and maintain provided you have an “open” mechanical room layout and can easily get to all the component plumbing parts.
The Structure Latent Load is 567Btuh, Central Vent -557, Equipment Latent Load 10. Not exactly sure what this means in terms of dehumidification requirements, but it does seem very low (we have almost no “muggy” days in summer).
I have never lived with hydronic radiant heat floors, but I like the idea. I have always disliked ducting for forced air systems that accumulates dust and creates a draft near it while leaving dead zones in other parts of the room.
Ductless Mini-splits can fix the first problem but not the second. Having a mini-split or even multi-split with a head unit in every room seems like a) the only way to really do it right, and b) overkill from a BTU perspective if you are building a well-insulated tight house because they don't come smaller than 9000 BTUs that I've seen. I would end up with 60,000 BTUs for a 2,000sf house.
The radiant heat embedded in a wood subfloor looks deceptively simple, but if you have to use all "approved" products like WarmBoard rather than piecing it together from 12" wide plywood strips and aluminum foil, it seems pricey. What products would you use if you chose radiant heat ? What are you figuring for ballpark cost DIY or installed ? From a cost perspective, I am considering embedding the pex in a 1" layer of drypack mortar like for a shower floor. Obviously screed-ed flat instead of sloped (except possibly in laundry room with an overflow drain). Cheaper and easier to DIY than wet concrete or gypcrete.
In a low load home, going for full surface floor heat will never be warm enough to notice. In my place, the only time I notice the floor is slightly warm is on those very cold winter mornings and only by the windows where I have tighter tube spacing. The rest of the time, the floors are close enough to room temperature that it can't be felt.
In a high performance home, there is a place for floor heat, but only in limited areas. Heated tiled entrance and closet, kitchen and bathroom floors are fantastic. Floor heat is also the only way to get decent comfort if you have overhangs or bonus room above unheated garage. Since these are limited to smaller areas, you can use resistance mat and save all the cost of hydronics. In most of these cases the floor heat would be there to take the edge off the cold surface, since as the bulk of the heat would still come from the heat pump, it costs very little extra to run.
A well set up modulating heat pump in a low loss home is practically unnoticeable. The air flow and heat delivered is low enough that it can barely be felt. I can also tell you a hydronic heated house, ERV with a HEPA filter is still not dust free by any means.
When you say "heat pump" are you assuming there would be a heat pump ducted or mini-split system in addition to radiant floor ? I think the original poster is trying to decide either/or, not both.
I personally am considering radiant floor with water heated by a HPWH running 130F water through a heat exchanger to provide 80F water to the floors as the only heat source. During winter, the WH would be ducted inut and output to the outside by dampers so it didn't pull heat out of the house and exhaust cold air into the house. During the summer, the dampers would reverse so cycled the air in the house, providing a small amount of cooling. The area I plan to build rarely gets above 85F during summer peak.
I do worry about both the HRV and range makeup air being filtered sufficiently, but surely a Merv13 filter, much less HEPA will remove dust ? The smooth plastic hoses I've seen some HRVs use must at least be easier to run a cleaning rag through than HVAC flex duct.
Kirk, your plan to use a ducted heat pump water heater for space heating may run into trouble on days when it's very cold outside (I don't know what climate you are in) or just because it runs the heat pump many more hours than it was designed for.
Kirk: I share your dislikes in post #16. My hope is that in a low load home the actual volume of forced air is much reduced. I have sat in front of inverter, ductless mini-splits in cooling mode and they seem very innocuous.
Yes, I am looking for an either/or solution. Given that I need a/c (heatdomes?, the next 30 years of data for 99% temps might be very different from the last) and given my floor-plan, I think I need ductwork in 2/3 of the house. The remaining 1/3 is open-plan and I think ductless will work.
For the radiant solution, which I consider more elegant theoretically, I am considering the panels made by Messana because they are packaged along with the crucial control systems. I would install them myself. I have yet to get the system priced. I am not expecting to have a warm floor for barefoot toes (it’s more an absence of cold than a presence of heat). In the lower level the radiant panels would, in any case, be in the ceiling! On the upper level the radiant panels would be in the floor and hopefully remove some of the solar gain through the windows, landing on the tile floor, in the summer
Akos: Thank you for #17. The radiant discussion always seems to get hung-up on warm toes and large radiant (concrete) slabs in low load homes. As I have said, I’m not trying to achieve warm toes and I’m not planning on any concrete floors. What I’m trying to achieve is elimination of forced air by making radiant panels also handle cooling needs in a non-muggy climate.
In your earlier reply you said I also need some dehumidification, but based on my Manual-J results, I’m left wondering how much?
If you set aside the warm toes fantasy and avoid concrete slabs radiant hydronic heat can actually be a super efficient and super quiet way to heat a house with a tight envelope. The water temperature needed is very low, which means you won't notice warm toes, but you will get awesome COP out of the heat pump. It's expensive, and it's complicated to design and get working right, but it definitely can give great performance. Ceilings can be a better plan than floors.
For summer in 5A, you could, in theory, do dehumidification on the supply air and use chilled water in the floor or ceiling for cooling, but I would be inclined to chill and dehumidify the supply air and then use a few fan coils, either slim wall-mount units that are like minisplit heads, but less obtrusive, or more boxy units tucked into closets or the basement.
From a practical point of view, such a system is overkill and not worth the extra cost. But if part of your objective is to pioneer a superior technology, or just to have the very best system, go for it. Especially because your refrigerant can all be in a factory-sealed unit, so you are using less of it, and you are much less likely to have climate-damaging refrigerant leaks.
Thank you Charlie. Yes, one must first have a clear grasp of one’s objectives. I think this post comes closest to summing up my mindset. I’m prepared for some extra cost, but not at the expense of being that pioneer forever tinkering with a system that never really works!
Whether you end up with a system that works great or a system that never really works right probably depends on your engineering skills, or on finding an hiring an engineer who can make sure it will work right.
I would read through this link and note the cost discussion for cooling with radiant floors. This would be the deciding point for me. When you start talking about ceiling panels from Messana, you are probably talking big money. But if you want to investigate and price it out the more power to you! Just please come back and tell us what you found out.
https://www.greenbuildingadvisor.com/article/does-radiant-floor-cooling-make-sense
Sorry, I realize now that I mis-typed the climate zone, it’s 5B (corrected it in original posting)
Oh, that changes things a lot. That might make radiant cooling more practical than we thought.
I would drill down and look at detailed weather history for your location, particularly at dew point.
Thanks Charlie, I did do that, its attached
https://www.infosys.com/newsroom/features/2012/commercial-radiant-cooling-building.html
I’m back because of the incredible amount of sane and practical advice. Notwithstanding that advice, I can’t seem to let go of the idea of radiant heating and cooling. In part because of the idea of long refrigerant lines (not friendly if they leak and not DIY serviceable) or having ductwork beyond ventilation requirements. I think radiant is going to cost me more, but I’m not going for the cheapest solution with walls and windows either. I’m not expecting floors warm to the touch. I’m hoping for heating and cooling that is mostly hidden from a visual and air movement perspective.
Per ManJ, heating load is around 8Btuh/sf, cooling around 7Btuh/sf of which latent component is practically zero if I use an ERV (which gives me a credit of 567 Btuh).
My current plan (attached) is a combination of low-mass (e.g. EcoWarm) and high-mass radiant with two FCUs.
I understand the issues with high-mass in a low-load home (slow response) but am assuming that it will not be an issue at all in a moderately-heated garage and will not matter much in an entryway area (where we want polished concrete).
The first FCU is for reduced installation cost in an attached ADU and the second FCU (which I’m hoping will never be used) will be through the mechanical room wall and is a back-up cooling/dehumidifying unit for the main house.
Considering a Tekmar 406 for dew point control. The FCUs will need to be on a separate (below dew-point) glycol loop. I realize this will mean running the ATWHP (e.g. Chiltrix) buffer tank at a cooler temperature, with correspondingly lower efficiency, at such times that either of the FCU are needed.
As for engineering the system beyond ManJ, I’m relying mostly on the manufacturers to design the loops and the hot/cold supply water to the buffer and DHW tank. Beyond that, the plan is to rely on an adaptable design and installation (hence the FCU loop) so that any comfort issues can be resolved as they arise.
Any thoughts? I’m approaching build time. Thank you.
What does your ventilation ductwork look like?
There's no reason in the world to do high-mass radiant heat if you can avoid it.
" I realize this will mean running the ATWHP (e.g. Chiltrix) buffer tank at a cooler temperature, with correspondingly lower efficiency, at such times that either of the FCU are needed."
I'm doing something similar on a house I'm building right now. I haven't commissioned the system yet so I can't report on the results.
We put a FCU on every floor. If a humidistat indicates that dehumidification is needed then the radiant loops are shut off and the FCU''s are turned on. If only sensible cooling is needed it's done through the radiant loops. There's a two stage thermostat and the FCU's are the second stage, for both heating and cooling.
I would recommend a FCU on each level for humidity control.
The thing you have to be really careful about is keeping the radiant loops above your dew point in cooling mode. So I don't think your plan of adjusting the water temperature on the fly is going to work, I don't see how you can keep the cold water out of the loops. You could put a tempering valve on the loops. My system is big enough that I just have two heat pumps, one for the loops and one for the FCU's.
You should price it all out before committing. It's very expensive. Everything about it is expensive. I do like that it's easier to monkey with but that comes at a luxury price.
Thank you for the reply and cost warning. I’m still pricing it out!
“So I don't think your plan of adjusting the water temperature on the fly is going to work, I don't see how you can keep the cold water out of the loops.“
The radiant loops will have a separate circulator and separate take-offs from the buffer tank (the buffer tank has multiple take-offs, see attached). If the buffer tank is chilled below the dew point, a mixing valve will return some of the return water from the radiant loops to temper the supply to the radiant loops (as you suggest).
I do not see the problem with putting pex in a garage floor slab to keep the garage around 60F in the winter. What problems would a (separately zoned) high-mass emitter create here?
Too warm of a garage (60 F) may be sub-optimal (e.g., consider snow/ice/deicer coming in with vehicle(s), etc.). A somewhat colder garage (50 F or below?) may be a good thing in winter as to keeping all reaction rates lower, for corrosion (metals, concrete, any in-slab fittings, etc.) and/or for bio growth. I think garage temp/humidity is a discussion unto itself (along with garage winter time ventilation).
https://bestcarshelter.com/what-temperature-should-i-keep-my-garage-in-the-winter/
“I would recommend a FCU on each level for humidity control”
My climate has low Summer dew points. 95% of the time below 60F, 99% of the time below 65F. What I didn’t mention in post #30 is that I also intend to rough-in vapor-sealed pex to two additional locations in the main house (upper and lower level) connected to spare manifold locations, just in case and in accordance with your advice here.
I had looked at the originally posted floor plan and thought your study was too big and the "hers" bath too small ;-) But now there's a closet - maybe consider changing the swing of the door as I think I'd get tired of having to close the door to get to the closet.
As to 1:1 heat pumps, if you do choose heat pumps, and go above 2 (like 3 or more), I think you may regret it. I'm in zone 5 and have one ASHP tied to three ducted air handlers with short runs, comfort has been good - both summer and winter. The turn-down issue versus 1:1 is more complicated than many consider (e.g., circuits, baseline power utilization of each ASHP, the mound of ice they generate in winter, the vibration, visual/aesthetics, direction each faces, etc.).
For radiant heat, some type of air filtration system would likely be beneficial to service the entire house, along with make-up air, etc.
Looking at Zehnder ComfoTube 3” for main house. For ADU I’ll likely cobble something together with range/bathroom make-up air and a filter box (no heat recovery)
SpacePak makes air to water heat pumps and high velocity air handlers that use similar ducting to the comfotube. You could duct the fresh air from an ERV to the return of the air handler, which would have a coil connected to the air to water heat pump, this would give you AC with the same type of duct work you were going to use for ventilation only. Just a thought…
Thank you Kyle for this thought. The prevailing view says that one should keep ventilation (the V) separate from heating/cooling (the HAC). What you are proposing would create a type of CERV (see Equinox). It’s very tempting to try and get double-duty out of all that ductwork …
Arctic heat pumps also sells a high velocity air handler made by hivelocity systems. They have an equipment calculator that will help you specify equipment and give you pricing. It will give you an idea of equipment cost.
Thank you. I also notice that Zehnder now offer the ComfoPost unit, which provides the same functionality of your #33 posting