This is our fourth winter in our upstate SC home. The house and basement is ICF, with R-60 cellulose in the attic. We have 1400 sq ft under HVAC. We just came out of a very unusual (for us) two week cold snap, with  nights being in the teens, and days barely getting above freezing. For the first time ever, the Mitsubishi mini-split struggled to keep the house comfortable. We condition the whole house with a 12K BTU unit, with a separate 9K unit in the master suite for additional capacity when needed.

We keep the thermostat at 66 F. in the winter, but the house dropped to 62 F. Running a ceiling fan in reverse helped, bringing down the warm air at the ceiling. Raising the thermostat to 68, and then 72 F. did nothing. I was concerned when I came home from work on a sunny but cold day, and the house was still only 62 F. Turning on the second unit quickly rectified the problem. My first thought was that the main unit needed service, and wasn't performing properly. But now I'm wondering if the thermal mass in the ICF walls got colder than usual due to the extended cold weather. Our ground outside froze 6" deep, which is very rare. Could the concrete core have gotten cold enough to work against us?

I doubt that the concrete was working against you, it just wasn't providing any significant benefit (other than reduced air infiltration) when outside temps stay near or below freezing for several days. You are attempting to heat a 1400 sq ft home with only a 12k BTU (1 ton) heat pump. Most conventional constructed homes of that size in my area (Arkansas) would have at least a 2 ton heat pump and even then the heat strips would be kicking in periodically in the teens when the heat pump goes into defrost mode. Your heat load is likely at or slightly above 12k BTU when the outside temperature is in the teens plus the heat pump likely doesn't put out 12k BTU when outside temperature is in the teens. Your heating capacity vs. load was probably pretty closely matched during the day, but maybe not enough excess capacity to make up for the loss during the night.

By the way, how does that mini-split defrost the outside coil? A typical split heat pump with electric heat strips will run the heat pump in cooling mode with the outside fan off to heat up the outside coil at the same time it is running the heat strips to maintain heating supply temperature inside. Have you ever seen frost build up on the outside coil? Have you taken supply air temperature vs. return air temperature measurements and checked to see if you are getting around 20 deg F rise across the coil? Are you getting the proper amount of airflow across the coil (filter and coil clean)? Have you checked/cleaned your outside unit coils? If everything is clean and you know the rated air flow from the head you can calculate how many BTUs are being supplied given the differential temperature across the head.

It looks like you may be in for some more sub-freezing temperatures early next week so you have an opportunity to take some measurements and verify your mini-split is performing up to spec at the lower temperatures.

According to the program I used to calculate HVAC, the demand is 11K BTU at 21 F- our 99% winter low. AT 0, that jumps up to 18K, which is part of why we have two units. We were well below 21 F at night for quite a while, and I expected it to get cold at night. The sunny day surprised me, though, as usually the house rebounds well during the day.

I don't know how they defrost. I do know that the coils, inside and out are clean, as are the filters. The outdoor unit is wall mounted, 3' off the ground, so no leaves, dirt, or snow. I'll try to take some temperature readings.

Like arkie6 was saying, the thermal mass is not working against you. Since it's sandwiched between insulation, the worst case scenario if it's cold enough long enough is that it will be at steady state and will perform like it performs when we conduct testing. I come from an area(Quebec, Canada) where we just had an entire week at around -30C(-22F) and the middle of the concrete of an ICF house we monitor was still at 8C(46F). My first thought was exactly the same as arkie6, your heat pump is not in it's optimal temperature range of operation. But it's also true that 12k BTU seems low, maybe you did that calculation using cooling needs? The Delta T for cooling is lower so you will need less BTU. A typical double pane window will consume around 20-25 but/sq.ft at peak and an ICF wall in your climate should be around 3 BTU, maybe you could try with those numbers to see if you have a big enough system.

Thanks

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FE & FH series "hyper heating" 1-ton Mitsubishiis have ~16,000 BTU/hr of capacity even at +5F. The GE series 1-ton only puts out 11,900 BTU/hr max @ 17F, and the GL series 12,000 BTU/hr @ +17F.

They all defrost by reversing the flow, taking heat back out of the house and putting it into the exterior coil. That has an impact on the total capacity, but usually not a serious impact until it's nearing rime-icing fog conditions outside. Most of the time during cold snaps the amount of moisture in the air is pretty low, which makes the time between defrost cycles much longer.

How do the units know when to defrost? On a typical conventional split heat pump, defrost is based on temperature and a timer. It doesn't know what the moisture level is or if there is actual frost on the coil.

This is a very interesting thread. In over ten years of working with ICF I have never seen a house that had less HVAC than needed -- usually the problem is the other way around. Many of the ICF houses here in Texas (ceiled attic - semi-conditioned) operate at 900-1200 square per ton (12K BTU). Of course it's all more complicated than that but... Stick construction with BAT insulation usually requires about 400-500 square feet per ton Spray foam insulation between the studs about 600-700 square feet per ton We have ICF houses that operate at 1400 square feet per ton of HVAC but we don't have serious cold snaps.

Posted By arkie6 on 15 Jan 2018 06:05 PM
How do the units know when to defrost? On a typical conventional split heat pump, defrost is based on temperature and a timer. It doesn't know what the moisture level is or if there is actual frost on the coil.

Mini-splits are variable refrigerant flow units with sensor feedback to make it all work well and efficiently under a wide range of load conditions, unlike the much dumber controls on 1 & 2 speed standard split systems.

Frost on the coil reduces air flow, reducing the in-to-out temperature difference on the refrigerant. The exact means by which this is measured and used probably varies from manufacturer to manufacturer, model to model, but it's not rocket science to make such a control that can work reliably without a huge cost adder, or even NO cost adder (primarily a software algorithm utilizing sensors that are already there.)

The weather has moderated, and everything is back to normal. The extended cold is very rare here. Even when it gets down into the single digits, the days usually rebound into the 40's. Our normal winter weather involves wide temperature swings, as we are near the mountains. As for HVAC calculations, I used HVACCalc. Our cooling load is 9K BTU, and we've never had a cooling problem, even when it is triple digits outside. Our house is very efficient; when we've had large get-togethers with 10-12 people, we have to open windows to cool the house down- even in winter. I've never lived in a house that overheated because of the people! I guess the problem was simply that the weather was below the design temperature for such a long period. Our normal low this time of year is 32; the normal high is 52. Cold snaps normally only last a couple days.

I don't understand what significant benefit the concrete would be providing when the temperature is above freezing if it wasn't working against when below freezing. It seems like more or all [God forbid] of the insulation on the outside of the walls would do the most to limit both heat loss and gain through the walls. Another benefit of exposed thermal mass that might have benefitted the OP is it's ability to moderate temperature swings whether sudden periods of cold or hot weather or like they experience when entertaining.

Another consideration is exposure and color of exterior walls. A dark colored wall with large exposure to sun will absorb heat and transfer through the EPS to the concrete (even on cold days). However, this will work against you if you are in a predominantly cooling climate.

Even in our area (Canadian Prairies), lighter color exteriors are recommended for efficiency. The increase of efficiency in winter will be offset by over heating in summer (winter benefit provided through +/-7 hrs sunlight, while you will be fighting against +/-17hrs of more intense sun during cooling months). If the concrete is gaining heat during cooling months, having fully exposed interior concrete would be counterproductive. Of course, forms offering additional exterior concrete can offset/eliminate these issues.

One project in the Northern Canadian Prairies (even longer days during summer) used about 5" EPS outside the concrete with interior layer intact. Dark green tin was used for exterior finish. Even through the added insulation, concrete core was reaching around 50C (122F). Builder/designers commented that better venting behind the tin and a lighter color would have been preferable, in hindsight.

Shows that function/performance of the wall involves more than just R-value and airtightness. Buildings function as a system, not just a sum of parts.

Yes, experience is an asset - as is common sense. EPS is less than r-4 per inch at summer temps. It's not really a high performance foam insulation so furring out and venting behind the siding and allowing for air space(s) between layers of insulation or the thermal mass itself would reduce or eliminate conduction. Obviously, the highest performance won't be achieved by the easiest method. It shouldn't be surprising that it takes longer and costs more to do things right - nuthin's for nuthin. Imagine that same build with only 2" of foam on the outside....

Our house is a fairly dark green. Although we are in a pretty warm climate, we have trees in all directions, so that in the summer, there's little sun hitting the siding for long periods. We also have large overhangs on the roof, so the sun doesn't wash the walls in the summer. The garage shields the western side of the house from what little sun comes through the trees. Our siding (fiber cement) is installed over firring strips for a rain screen and thermal break/ventilation. Cooling is never an issue; even the 9K BTU mini in the master suite will keep the whole house pretty comfortable. Most of the time, our house is very slow to change temperature. In fall and spring, we go for months at a time without using the HVAC at all. It was the sustained cold that seemed to have the most impact.

Jdebree,

Slightly off topic, but I'm hoping to build an ICF house just a little north of you in Jackson county NC; hopefully within 12-18 months. Can you provide any info on the ICF contractor / supplier you used, and your experiences with them?

Thanks for any info

JC

My house was about 95% DIY, but I did use a local contractor to advise me and actually fill the forms. I also rented bracing from him. I'm not sure what his status is these days, as I haven't been able to contact him for a couple years. I used Fox Blocks, which I think were shipped in from Charlotte. I found the Fox Blocks to be an excellent product. Everything fit together as it should, and the walls came out very accurate.

I found something possibly of interest to the original discussion. Last summer, I was working on finishing the basement. I removed the basement ceiling insulation (R-19 batts), and discovered that the HVAC lines were wet with condensation, and the insulation was damp and slightly moldy. I was a bit alarmed, and left that joist bay open for the time being. Now, I'm doing a suspended ceiling, so I added another layer of insulation over the refrigerant lines, as the original insulation was rather thin, and obviously not doing a very good job. It occurred to me that the previous winters, the refrigerant lines were in an insulated space, not directly exposed to the relatively cool basement. I don't know if that would make a difference, but it couldn't help. Now the lines are double-insulated, and the R-19 batts have been replaced. Time will tell (maybe) if this will help.

Jdebree- thanks for the info; I'm hoping to do something similar myself.