Heating/Cooling for SIP Home?
I am planning to build a 2,000 sq. ft., slab-on-grade SIP home in East Lansing, MI. The cooling load is between 20-25 BTU and heating load 15-20 BTU. My builder is recommending a radiant heat floor, ERV, and ductless minisplit(s). I like the idea of radiant heat flooring (efficiency, comfort), but if I am cooling with multiple mini-splits (ductless or ducted) that have heating capabilities, isn’t the radiant heat floor redundant? Why go through the cost and effort for radiant floor heat?
If I really wanted the radiant heat floor, what would be the preferred cooling system?
Looking for recommendations on the optimal HVAC system for such a set up considering cost, efficiency, and comfort. I have limited space (no basement, no attic although could have drop down ceiling with 3-4′ above laundry room).
Any and all advice is welcome.
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Builders love a radiant floor. It's totally redundant (but not more reliable in a power outage), expensive, and a boiler is not efficient compared to a heat pump. You can improve comfort cheaply using other methods. You could also go solar and try to zero out our energy costs.
I'd do ducted heat pump (minisplit or otherwise) if you can figure out places to run the ducts. If that's not possible throughout the whole house, try to at less duct the bedrooms, with a ductless minisplit to cover the largest, open rooms. Don't put a ductless minisplit in each bedroom.
If you're wedded to the radiant floor idea, a ducted system is still the best way to cool.
Thanks. Do you have any recommendations for electric ducted heat pumps?
Mitsubishi and Fujitsu make cold-climate models.
I have this one and it's great - The efficiency numbers look low, but that's at max capacity, which isn't needed for my house.
http://mylinkdrive.com/viewPdf?srcUrl=http://enter.mehvac.com.s3.amazonaws.com/DAMRoot/Original/10006\M_SUBMITTAL_SVZ-KP24NA_SUZ-KA24NAHZ_en.pdf
However, your load is so low and is skewed more towards cooling, so you will have plenty of options. I'd focus instead on installer instead of brand - modulating compressors and modulating fan are key features to insist on, just like the ductless options.
Filtering Northeast Energy Efficiency Partnerships list by your size requirements and centrally ducted will probably give you hundreds of options.
https://neep.org/heating-electrification/ccashp-specification-product-list
+1 on a ducted heat pump solution. Cold climate hyper heat units can easily cover your heating load and provide cooling.
Floor heat does have a place in a high performance house but you don't need to heat the whole space.
The best is to use a bit of electric floor heat areas where there will be a lot of foot traffic and near usual cold spots.
This means a bit of resistance floor heat mat in your entrance mudroom area, any tiled kitchen space and your bathrooms. It also doesn't hurt to put in a narrow strip by any large windows or any areas of the house where you'll be sitting for extended periods near windows. The bulk of the heating would still be done by the heat pump, this bit of resistance heat is just there to take the edge off. Much less to install than a full hydronic floor heat setup.
Important detail when installing resistance mats is to mount the floor temperature sensor in a small pex pipe (5/16" is great for this) instead of directly embedding it in the floor. This way if the sensor ever breaks, you can can just pull it out and replace it without having to cut up the floor.
Thanks. I hadn't thought of a partial radiant heat floor. I wonder though how much I'd really be saving (i.e. installation costs) by limiting the extent of electric floor heat.
As soon as you say hydronic heat, you are adding about $15k in just equipment, pumps, valves and fittings. Never mind the install cost of the pex pipe and the extra cost of the insulated floor panels.
For electric, you are only looking at the cost of the wire grid, cables and thermostat. Install is also much quicker plus there is minimal commissioning needed (resistance check and isolation check) beside programming the thermostat. This is also done by an electrician who tend to be more competent than your typical HVAC installer.
If you are doing hardwood and tile, install the heat mats above the slab, less chance of it getting damaged. There are heated wire systems sold that to wire mesh for embedding into a slab if the slab will be your finished floor.
As for the heat pump and ERV, without a basement, you need to design your place to include chases for ducts. This generally means either I-joists or trusses for floors in a two story structure or going with plenum roof trusses with a single story build.
Why ducted rather than multi-zone ductless. Isn't the latter less costly to install and more efficient?
Not really more efficient or cheaper - multi-zone ductless (vs. many single zone ductless) in particular can be rather inefficient. Especially for "smaller" (meaning lower load rooms) in new builds, you'll get to the point where you're adding a head for like 4 figures (plus running refrigerant lines, electric power and a condensate line) versus cheaply extending a duct. Ducted systems also can allow for more convenient filter changes.
Thanks Paul. There is apparently no real consensus on these issues. I've read and been told that you lose 20-30% efficiency when you are moving air around in ducts when compared to the ductless mini-splits. I've also been told it is cheaper to put in mulitple, single-zone minisplits (each with their own exterior heat pump) than a multi-head system with a common exterior heat pump. I am sure much of it depends on the unique configuration of my home however.
I think 20-30% is probably not a fair comparison because:
1. This is a new house, not an existing house. Existing houses, especially ones retrofitted to include AC, will have more leakage.
2. This house seems to be built with energy use in mind. There are plenty of poor duct installations (sometimes they're installed outdoors, in unconditioned attics, poorly/uninsulated, poorly air sealed and/or poorly designed). Controlling for these factors at this stage probably eliminates any efficiency hit.
If a consensus exists, it's probably that multi-zone minisplits are considered a last resort (heat pump wise) for a new house.
>"There is apparently no real consensus on these issues. I've read and been told that you lose 20-30% efficiency when you are moving air around in ducts when compared to the ductless mini-splits. "
Simply not true, if the ducts are sized correctly and installed within conditioned space, not up in an attic above the insulation.
What IS true is that a "ductless head for every room" micro-zoning with a multi-zone compressor leads to gross oversizing, much lower efficiency than advertises, and lower COMFORT too, since even the smallest ductless heads will be oversized for the rooms, and low-load can often become overheated/overcooled simply by the bypass refrigerant bypass refrigerant flowing through the unit whenever other zones/heads are being used. While the head might be nominally "off", in order to keep the refrigerants & lubricants properly mixed all zones get refrigerant flow whenever ANY head is operating. In summer this can lead to condensation inside the cabinet of the "OFF" or idling head, which then spits water into the room on startup.
Multi-zone compressors also typically have a much lower turn-down ratio than single zone compressors, leading to short-cycling of the compressor when the system (or individual heads) are oversized for the loads.
To come up with a "right" solution requires a room by room Manual-J type load calculation using AGGRESSIVE input assumptions (per the Manual) on R-values /U-factors, air leakage, etc. and this is even more important when specifying heat pumps. A pretty good quick & easy to use load tool targeted a HVAC installers installing ductless systems is the BetterBuiltNW freebie:
https://betterbuiltnw.com/hvac-sizing-tool
That tool has appropriately aggressive defaults, and was developed by a consortium of electric utilities promoting heat pump solutions after seeing poor results at the LUDICROUS oversizing factors used by the typical average HVAC contractors accustomed to selling fossil-fired solutions.
Assuming your load numbers are correct a 2-ton or 2.5 ton (depending on your 99% outside design temp) mid-static Fujistu compact ducted solution would work:
https://ashp.neep.org/#!/product/25350
https://ashp.neep.org/#!/product/25351
The 99% outside design temp for Lansing is +3F, (see: https://higherlogicdownload.s3.amazonaws.com/ACCA/c6b38bda-2e04-4f93-bd51-7a80525ad936/UploadedImages/Outdoor-Design-Conditions-1.pdf )so if the heating load is <20K the 2-tonner is really the ticket.
The minimum output @ +47 of the 2.5 tonner is 9200 BTU/hr, which would be well OVER the actual load @ +47F if the load @ +3F is <20K. With the 2-tonner's 5400 BTU/hr @ 47F it would still be modulating most of the time during the shoulder seasons, delivering higher comfort (and somewhat higher as-used efficiency.)
It's common to install a resistance heater "toaster" between the duct cassette and the plenum to cover the shortfall during Polar Vortex disturbance events where the outside temps would hit well into negative double-digits, but don't oversize those either if you want that heat to be comfortable. For best comfort you want the duty cycle to be nearly 100% , continuous operation when the temps are at or below the 99% outside design temp.
Fujitsu & Mitsubishi aren't the only players here. I like the Fujitsus and the Midea (who makes Carrier's mini-splits) compact duct cassettes more than most of the others since they can be mounted in an upflow orientation rather than horizonal. For a slab on grade house it means they can be installed in a mere ~10 square foot utility cabinet/closet, with conveent access for service & filter swaps, etc. The cover shot on this blog piece is a 1.5 ton Fujitsu mounted vertically, the shot taken before the cabinet/closet was built around it:
https://www.greenbuildingadvisor.com/article/getting-the-right-minisplit
https://d4c5gb8slvq7w.cloudfront.net/eyJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjcwMH19LCJidWNrZXQiOiJncmVlbmJ1aWxkaW5nYWR2aXNvci5zMy50YXVudG9uY2xvdWQuY29tIiwia2V5IjoiYXBwXC91cGxvYWRzXC8yMDE5XC8wOFwvMDIwOTIzNDlcL0R1Y3RlZC1taW5pc3BsaS0xLURhbmEtRG9yc2V0dC03MDB4OTMzLmpwZyJ9
A Midea/Carrier unit that might work is rated at 1.5 tons cooling, but over-delivers it's "rated" capacity:
https://ashp.neep.org/#!/product/36669
https://ashp.neep.org/#!/product/36683
I have a SIP construction house with 200sqft+ of triple pane windows and approximately the same heat loads as above located in northern Maine. My experience is that the heat pumps will be more than sufficient without any radiant. The only areas I notice any temperature difference in my house is in the bathrooms on the tile floors where a simple bath mat does the job for me.
I should note that I have 4" of XPS insulation below my slab which helps it stay warm. Otherwise, I'm sure the concrete floor would be cold.
Thanks all for the great advice and information. More to think about which is welcome.
Being an HVAC newby but really interested in learning more, it seems an ideal system would be a combined radiant heat and cooling system, geothermal heat pump with dew point switches and sensors, and a whole house dehumidifier. Lawrence Berkley Lab and others estimate that radiant cooling may be 25-30% more energy efficient than forced air cooling (https://cbe.berkeley.edu/research/radiant-systems-research/ and https://www.hpacmag.com/features/radiant-cooling/). Seems, however, that this is not quite ready for residential installations, and I doubt I'd find anyone locally with the knowledge to do the work.
Radiant heating and cooling is a fascinating subject to learn about, check out the Caleffi Idronics series if you haven’t already. I find skyscrapers using ice as thermal storage during both heating and cooling seasons particularly ingenious. It’s not so much that it’s not technically ready for residential, it’s just that so many of hydronics benefits present themselves on large, complex buildings and when you go smaller and simpler the benefits start to vanish, especially as homes are constructed with lower heat loads and solar installation costs decrease annually. You get to the point when you can add a few thousand dollars worth of solar to save as much as probably $30-60k of geothermal and hydronic equipment would get you.
Daniel,
In your climate, good part of your cooling load is humidity removal. Generally the most efficient way to remove humidity is with an air coil but even high SHR units will provide a fair bit of sensible cooling. After this bit of cooling, in most cases there is not that much sensible cooling left over, making the chilled slab almost redundant. Might as well go for a slightly larger coil sized to handle both cooling loads.
Also keep in mind that getting the efficiency out of a radiant system takes a lot of knowledge and setup time. As an engineer, I don't mind fussing with getting just the right flows and temperatures through all the zones in my house to get the most out of the system. No HVAC installer will spend the time to get this right, this generally means you never hit the efficiency numbers from the data sheets. Never mind the extra maintaince and troubleshooting costs down the road.
For high efficiency, low load residential setup, there is no ROI for many of these systems. You are just adding complexity and extra maintaince for not much benefit.
It is fun to read and think about these types of systems though.