Still Grinding – Uninsulated basement and minisplit problems
It’s been about 1 year and 10 months since I started building my house and I’m still grinding. Doing the board and batten siding mixed with cedar is taking some time. Especially since I’m doing it mostly by myself!
We’ve moved in though with a temporary certificate of occupancy. My family and my parents (5 people total) have been living in there since the beginning of March. What surprised me is the high amount of energy we are using despite having 10 1/4″ double stud walls filled with damp-spray cellulose and open-cell spray foam in some of the hard to reach areas. I estimate that I have around R-30 in my walls and R-60 in my attic. I have LG ductless minisplits and Intus triple-pane windows. We also have a Stiebel Eltron Accelera 300 e heat pump water heater. I haven’t done a professional heat load calculation and probably too late now. Probably have to do a blower door test and see if air leakage is a major problem.
Our house is 2800 square foot with a 1600 square foot unfinished basement.
From March 9 – April 9, with average temperature of 48 degrees, we used 2,313 kwh. So far from April 9-May with average temperature of 63 degrees, we’ve used 1,205.64. That is a lot more than I was expecting. Here are some reasons I think it’s so high and I am hoping the community can offer some opinions.
1. I haven’t insulated my basement walls yet nor really air-sealed the 8 windows. Again, the basement is 1600 square feet with 8.5′ ceiling. I intend to do 2″ of closed cell spray foam. The slab is insulated with 2″ xps. I am hoping once I finish insulating and air-sealing the basement, I can cut my energy usage by 15-20%. Maybe higher?
2. I didn’t have an interior door on the first floor where the stairs lead down to the basement. I recently put one of those solid core doors in. This may only help slightly, but I felt that the 21 sq ft. opening let cold air blast up into my living area very easily. With a door, maybe this will slow it down enough to see some energy savings.
3. My lines from my condenser to my upstairs ceiling cassette minisplit units are too long. LG sets maximum lengths for single line runs and total line runs. We came in under their maximum allowed, but I felt the lengths were still too long. Pumping refrigerant 80 feet from the condenser up two floors may be costing me. Plus, the cassette units are not as efficient as the wall units. Part of the cassettes stick up into the attic. I sprayed a thick layer of cellulose over it in the attic and air sealed around the unit, but I thick in hindsight, I should have just stuck with the wall units.
4. An odd thing is happening with the cassettes where they short cycle when the single first floor wall unit head is on. When the first floor unit idles after reaching the set temperature, the cassettes behave normally. I talked to the installer and LG technicians and they are still investigating. I think some coolant is leaking from the wall unit line and being sucked up into the cassette lines. LG said some leakage between lines is normal and can’t be avoided. However, I’m thinking I might just have to separate the cassettes to a separate junction box to prevent this annoying short cycling.
5. Maybe our house is still just too dang big and we take too long of showers!
Thanks for any help you can give.
Sincerely,
Jimmy
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Replies
Jimmy,
At the very end of your comments, you mentioned domestic hot water. That's what I was thinking.
What type of water heater do you have? Do you have a drainwater heat recovery unit?
With five occupants, domestic hot water might easily require more energy than space heating.
"We also have a Stiebel Eltron Accelera 300 e heat pump water heater."
That's a 300 liter/80 gallon tank with a slow recovery time, and draws half it's heat from conditioned space, which lowers the air conditioning energy use, but increases the space heating energy use.
Drainwater heat recovery would DEFINITELY help with the long shower performance (and subsequent recovery time), with a noticable dent in the power bill, but probably less than a double-digit percentage.
Insulating & air sealing the basement should reduce space heating use by a double-digit percentage, possibly more if the rest of the house is high-R.
Thanks Martin and Dana. I'll let you know how it goes. One of the Passivhaus standards is that total energy usage including appliances cannot exceed 11.1 kwh per sq. ft. per year. If I have a 2800 square foot house, then if my total kwh usage is under 31,080 (11.1 x 2800) that is pretty good right? Or I am misinterpreting?
Dang I missed your whole point about the drainwater heat recovery. Never even looked into that. Seems like everyone has positive things to say about them. If my shower drains are far from my hot water heater or only accessible after 10 feet of piping from the drain, is it still worth it? Also, I haven't insulated my water pipes with those foam sleeves. Hopefully that will help a little.
Insulating ALL of the hot water distribution plumbing (not just the runs to the showers) to the IRC 2015 code-minimum R3 would be a VERY good start.
The drainwater heat exchanger doesn't need to be close to the water heater- at 2 gpm the water travels 50' in the first 15 seconds through half-inch plumbing, or about 40' in the first 30 seconds through 3/4" plumbing. The output of the heat exchanger needs to feed both the cold side of the shower (which would be pretty close to the drain?) as well as the input side of the water heater.
Typical showers are in the 8-minute range, so even if there is half-gallon to one gallon of water in the plumbing between the heat exchanger & water heater at the end, 90% or more of the water that flowed from the heat exchanger into the water heater still ends up in the water heater, and the fraction of the heat exchanger output that fed the shorter run to the shower has an even higher utilization rate. (There is far more heat abandoned in the plumbing between a remote water heater and the bathroom/shower.)
The heat loss from ~10' of drain plumbing is nearly meaningless, hard to even measure.
The output of the heat exchanger is tepid, never truly hot, and inside of conditioned space (even an unheated 55F basement) it doesn't need to be insulated the way the hot water distribution plumbing needs to be. The taller and fatter the heat exchanger, the warmer it will be, but it takes a pretty big 'un to have annual average output temps north of 80F at a 2 gpm shower flow but you might get there with 4" x 96" tall version.
Some of the distribution losses to the basement would get taken in by the heat pump water heater, but the sooner the basement walls the better, to ensure that a smaller fraction of the heat leaves via the foundation.
Thanks Dana. So let me get this right - if I buy one heat exchanger that has one output hole, I have to split that outlet to connect to both the nearest shower cold water line and the input of the water heater? Attached is a picture of the diagrams I am commonly seeing. The output goes directly to the input of the hot water heater.
Jimmy,
The diagram is correct (except for the misspelling of the word "and"). For more information, see this article: Drainwater Heat Recovery Can Lower Your HERS Score.
The error in the diagram is that the mixer output is going to a faucet rather than a shower head. (Most people don't rinse off for several minutes at a time under a faucet. :-) )
https://www.greenbuildingadvisor.com/sites/default/files/Power-Pipe%20US%20Basement%20Image%20of%20Installation%202013%20FV.jpg
But yes, plumb it as drawn, but if the drain & shower are closer to each other than the water heater, keep the tee close to the heat exchanger & shower. The outbound side of the heat exchanger must feed both the water heater & cold side of the shower mixer to get the maximum heat return.
Dana,
Good catch!
Below is the more accurate diagram on which the error-filled one was probably based.
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Yep- I like the shower to be dripping on my head & shoulders, not just shins & feet- YMMV! :-)
Measure hot water vs LG power usage. What is the average power usage for your area? If I have to guess I'd say the LG is operating at low efficiency.
Five showers per day at 15 gallons per shower (low flow head 1.9 gpm for 8 minutes each) is 75 gallons , which is 625 lbs of water. Assuming an incoming temp of 45F and a showerhead temp of 105F that's a 60F temperature rise. The BTU content of that temperature rise is 60F x 625lbs= 37,500 BTU, but if delivered solely by a heat pump water heater (no resistance heater to speed recovery times) at a as-used COP of 2 (due to being in a cold basement, not a 65F basement) the input BTU was 18,750 BTU, which is (/3412 BTU per kwh =) 5.5 kwh/day.
Over the course of a 30 day month that will add up to 165 kwh. Not knowing the actual flow rate, average shower length, incoming water temp, or actual as-used efficiency of the heat pump water heater in your basement reality could be as much as twice that, and if the water heater was engaging the back up resistance element a lot you could be looking at maybe 500 kwh/month, but it's more likely to be on the order of 250kwh (+/- 50kwh ) per month for showers, plus a bit more to cover all other power use. A drainwater heat recovery heat exchanger would probably cut that by more than half, since it also reduces the amount of time the resistance backup element is running. Figure it'll save on the order of 100-150kwh/month, 1200-1800kwh/year.
But you're still looking at no more than 500-600 kwh/month for water heating (all uses), so you have to figure out where the rest of that power is going. During the month that was averaging 63F the LG probably wasn't a very big factor at any operating efficiency, since that's probably within a degree or two of your heating/cooling balance point, but the month that averaged 48F it clearly was running quite a bit. You're probably using 500kwh/month for lighting & plug loads with 5 people in the house, maybe a bit more if you're drying laundry for five in an electric clothes dryer, so between hot water and other non HVAC your background use for things is probably on the order of 1000 kwh/month.
So on the the 2,313 kwh used over the 31 days from 9 March- 9 April, something like 1300 kwh of that went into the LG, the rest went toward other things. That's ~42 kwh per day. Assuming a heating/cooling balance temp of 60F the 48F average temp would be 12 HDD per day, so the LG used about 3.5 kwh per HDD, or about 12,000 input BTU/ HDD. That's actually quite a lot for a much-better-than-code, house even if it were a resistance heater with a COP of 1. For reference, my 2400' sub-code 2x4 framed antique with antique clear glass double-hungs used ~14,000 BTU/HDD for space heating- your house should be doing much better than that (even without foundation insulation).
That either means the LG isn't running anywhere near it's optimal efficiency or you're sleeping with a bunch of windows open, or your basement losses are far greater than I would ever imagine! At an average outdoor temp of 48F the LG should have a COP of 3 or better, which means it should only use of 1 - 1.2 kwh / HDD for space heating. But it's up to the LG techs to figure out why it isn't performing better, but some of it could be how you are using the system.
Are you turning the ductless heads up/down multiple times per day? ("Set and forget" is a much better strategy.)
Are the heads sized to the zone loads such that they run nearly continuously when it's 40F or colder outside, or are they cycling on/off a lot?
I'ts unlikely to be the basement since hot air rises. You just got to measure all power usages and figure out what is using the power, heck could someone be stealing power, do you have a heated outdoor pool, is the street light on your meter, etc.
Anon3,
You wrote, "It's unlikely to be the basement since hot air rises."
While it's true that hot air rises, heat flows from hot to cold (by conduction and radiation) in all directions. Dana is right than uninsulated basement walls can be responsible for significant heat loss. Moreover, Jimmy told us that he hasn't finished air sealing work in the basement. Those two facts are significant.
Even a 55F uninsulated basement is losing quite lot of heat during months when the average outdoor temperature is 45F. Installing continuous R15 (=code min for climate zone marine 4 or higher) cuts the basement wall losses by about 85-90%, even though the average basement temperature then rises a bit. Even in an otherwise code-min house an uninsulated basement with 2' of above grade exposure can sometimes account for 25% of the total heat loss, but probably not 50%.
Even with presumed high basement losses, the energy use numbers are telling us the LG is probably underperforming it's efficiency numbers, possibly by quite a lot. His house should be nearly twice as efficient as my antique, due to lower U-factors and a far more efficient shape, yet it appears to be doing worse, if the LG is is delivering even a COP of 1.2, let alone a COP of ~3 (which it should be capable of.)
Almost all homes with basements (insulated or not) have the water heater installed in the basement, which is my presumption here.
You all are great energy detectives. What I fear is that the LG units are not as efficient as I thought they would be. I have compared similar days with similar average temperatures, having all of the units on and off and the kwh usage is different on average by 50 kwh, which kind of proves Dana's point.
My concern is that maybe minisplits are not as efficient as they are marketed.
I will update you all when I finally get the basement insulated and sealed. I have to admit I have been turning the minisplits on and off a lot. Not sure how much that affects the efficiency.
Lastly, I bought a Efergy Elite 4.0 Wireless Electricity Monitor. I will hook it to the minisplit circuit and see how much it is using.
Have you investigated the plug loads? Your parents might not be as energy-conscious as you are. I know my two parents use roughly twice as much electricity every month in their all-gas house as my family of 4 does in our nearly-all-electric house.
Update - I became disciplined and left my minisplits on (e.g. I didn't turn them off and on, which is tempting in Virginia since we have wild swings in temperature). I also told my parents to do so, although they probably won't listen. When I did change the temperature on the thermostat it was by only 1 or 2 degrees.
It looked like it was doing some good for my energy usage because on the hottest day so far this month it averaged 82 degrees and I used 68.22 kwh. Maybe 1.8 kwh/HDD. What's that COP? At the hottest hour of the day, it averaged 89 degrees and I used 3.58 kwh. But then the next day, the average temperature was 77 degrees and I ended up using 88.96 kwh. We had some big spikes and I think it was my parents showering or doing the laundry. Our biggest spike was at 4 pm, when we used 6.27 kwh and the average temp was only 74 degrees.
It might be that our condenser is oversized or that the ceiling cassettes in the upstairs floor just aren't that efficient. The upstairs units don't seem to satisfy our cooling demands. Or maybe the uninsulated basement still is a major factor. The other thing that I realized is that I basically have two houses. My parents' "in-law suite" is actually it's own apartment with kitchen, master bath, and all the other amenities. They are also retired and so most of the time the building is occupied with people.
So the big lessons are: - keep those minisplits on
- don't ever buy ceiling cassettes
- oversize your indoor wall unit a little upstairs in hot climates because it can get hot up there, especially if you are trying to cool a few rooms with one centrally located unit.
Here is a screen shot of my energy usage the past two days.
For anyone that comes across this thread, I believe my energy overuse was due to my LG minisplit condenser being overcharged by 3 lbs. I came upon this discovery because some of my minisplit indoor units (ceiling cassettes) were short cycling. I ran through every possible reason with the LG technicians and my contractor, until finally someone suggested there might be too much refrigerant in the system. When the contractor took out 3 lbs of refrigerant he noticed the amperage draw on the condenser dropped from 24 amps to 12 amps when all of the units were running full blast at 64 degrees. With the units set at 75, the amps dropped to 6.
So for anyone trying to understand why their minisplit or heat pump isn't as efficient as they thought it would be, check the refrigerant.
Jimmy,
Thanks for the update. I'm blown away that the overcharge increased your amperage draw do much!
Editor's note: GBA readers who are interested in this topic may want to check out a new thread on the topic that was just created by Jimmy Nguyen. Here is the link: Overcharged minisplit condenser leads to higher electricity bills.