I'd be interested in hearing if a remote thermostat might help with the short cycling. My Fujitsu has this problem...I think. I don't really know what's normal. I was able to at least adjust the remote control so that it reads close to what the actual room temp is. But I'm not sure where I'd put a remote 'stat, as there isn't much of a difference in temp from floor to ceiling (measured 59 near floor, 64 at intake of unit near ceiling). Temp was set at 60. So I don't know where I'd get cold air from. Was thinking of cutting an open vent thru the floor (ceiling above unit) into the bedroom above, but the temp upstairs is also 59. I'm worried that I might be wearing out the compressor. How often should the compressor be kicking in when everything's stabilized? Thanks for any input! -michael

It's a common error to install the thing at the minimum specified distance from the ceiling. This usually works OK during the cooling season, but is problematic in the heating season. It's better to put it at least a foot below the ceiling, or even down near the floor (if it's not in the way.) Both efficiency & capacity get a modest boost when operating near the floor, with intake temps a few degrees cooler.

The amount of short-cycling depends a bit on the sizing relative to the load, as well as the mounting location issues. Short cycling doesn't add a lot of wear & tear on the compressors & blowers, but it does kick a hole in the as-used efficiency. Sized correctly the thing should be running nearly continuously whenever there's a significant heating load (say, 15-20F or more cooler outdoors than indoors.)

How did you get the remote to read the room temp rather than the set point?

Is it possible it's just too warm outside for it to need to keep running at the lowest amount?

Hi Dana...

Unfortunately, I cannot move the unit now without a lot of cost. My installer (and I) didn't know about this issue beforehand. I'm wondering if it would make that much difference anyway? There doesn't seem to be that much of a temperature gradient floor to ceiling. Would getting it 59 degree intake air make a difference over 64 degree air? Maybe it would...

What do you think of my idea of a floor vent from upstairs dropping colder air into the top of the unit?

As for sizing, my design load (done by me, so who knows how accurate it will be.... ha) is around 10,000 btus. The unit is 12,000. Maybe I'm still too big?

Thanks a ton (pun intended)
-michael

Hi Patonbike...

There's an option in the remote set-up that allows you to choose how much correction you need. I set mine for +8 degrees, and it seems about right.

As for being warm out, yes.....I'm waiting to see how things go when it gets a little colder.

Did you get your thermostat installed yet?

Thanks,-michael

A 5F difference in the intake air tempature is a large fraction of the exterior air to intake air delta-T. That's the relevant temperature difference for both efficiency and capacity.

The HSPF capacity & efficiency testing specifies 65F entering air, which is literally never found at the ceiling of a 68F room.

When you say the load is 10,000 BTU/hr at what outside and inside design temperatures?

When you say the mini-split is rated for 12,000 BTU/hr, is that it's cooling rating, or is that heating?

The 9000 BTU/hr cooling rated Fujitsu 9RLS3 is good for more than 12,000 BTU/hr @ +17F.

The 12,000 BTU/hr 12RLS3H is good for more than 16,000 BTU/hr @ +17F.

But both of those can only throttle back to 3100 BTU/hr heating @ 47F. If the room load isn't at least 3100 BTU/hr @ +47F it'll be cycling quite a bit until colder weather arrives. It's good to run the load numbers at +47F (one of the standard test temperatures in the HSPF test, where both min & max output are specified), to get a rough idea of the temp at which it SHOULD be modulating rather than cycling.

Hi Dana...

I'm not sure what you mean by "A 5F difference in the intake air temperature is a large fraction of the exterior air to intake air delta-T." You're saying (I think) that the 64 degree air at intake is too high for a 60 degree room(?) and if I raise the temp to 68 in the room, the intake will probably see 72 degree air, which, I imagine will also be too high....

Design temp used was -15; inside not sure. Whatever Rescheck uses. I also used the calculator on Builditsolar.com. I see that Wunderground uses about 66 for calculating HDD's.

Yes, you're right about the capacity. The literature always refers to the units as 12000 BTU's. I forget they're talking about cooling. Heating is as you said... 3,100 to 22,100. So with outside having been around 40-ish lately, it's modulating down to 3100.

I guess I sized it based on some of the articles I read about others who have used them as primary heat source for similar "passive houses." And I wasn't sure how tight the house would end up....didn't want to be short on capacity.

So with colder weather, I'll watch to see if it cycles less. If it doesn't change, I'll have to get colder air to the intake somehow (what do you think of my floor vent idea?) or get a remote thermostat........?

Thanks so much for your comments.
-michael

I'm not saying that the 64F degree intake air is too high, only that it affects the efficiency. The difference between the intake air and the outdoor air is what determines the efficiency and capacity.

When it's 40F outside and 60F indoors, that's a 20F difference.

When it's 40F outside and the intake air is 64F, that's a 24F difference, an increase of 25%. The higher temperature difference means the compressor has to work a bit harder to deliver the heat to the room.

Drop the unit near the floor where the intake are might be 58F in a 60F room it's a 10% lower temperature delta, and the compressor has to work a bit less.

Colder intake air won't make it cycle much less when the total load is under 3100BTU/hr. The lengths of the cycles would be measurably longer if you put dataloggers on it and correlated it to outdoor temp and solar gain, but it would be hard to tell without measuring it.

If it's short-cycling due to localized heating near the ceiling when it's running at very low speed, with less air mixing, a remote thermostat that senses the air temp somewhere other than at the intake air stream will reduce the number of cycles, but would not affect the total duty cycle when the load is under 3100 BTU/hr.

What's your ZIP code? (For outside design temp sanity checking.)

BTW: The BuildItSolar heat load calculator's U-factors are way off. A DIY spreadsheet IBR method using U-factors that you calculate yourself with a 2-D parallel path model using the best-estimate of our framing fraction would be a better starting point, and if you start subtracting appropriate amounts for plug loads and warm bodies you'll do even better. Also, most heat load calculations (even Manual-J) overestimate the load of infiltration losses by quite a bit, since it's impossible to determine the amount of heat exchanger effect occurs along the infiltration & exfiltration paths.

Dana...

I'll have to monitor energy usage the best I can as the weather turns colder. I was thinking of installing something like this in the panel (which I can also use later for monitoring hot water heater):

http://www.ebay.com/itm/131153244228?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

But it doesn't total kwh's used. Do you know of a cheap data logger that can do this, like a 230v version of a Kill-A-Watt? Or maybe I could run one leg of the MSHP thru it and multiply X 2? I'd be afraid of screwing up the phase or something..

Zip is 54956

Not sure if I'd know how to set up my own calculator. but I was getting about the same results from Rescheck...

Thanks,
-michael

Dana...

I'll have to monitor energy usage the best I can as the weather turns colder. I was thinking of installing something like this in the panel (which I can also use later for monitoring hot water heater):

http://www.ebay.com/itm/131153244228?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

But it doesn't total kwh's used. Do you know of a cheap data logger that can do this, like a 230v version of a Kill-A-Watt? Or maybe I could run one leg of the MSHP thru it and multiply X 2? I'd be afraid of screwing up the phase or something..

Zip is 54956

Not sure if I'd know how to set up my own calculator. but I was getting about the same results from Rescheck...

Thanks,
-michael

Dana...

I'll have to monitor energy usage the best I can as the weather turns colder. I was thinking of installing something like this in the panel (which I can also use later for monitoring hot water heater):

http://www.ebay.com/itm/131153244228?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

But it doesn't total kwh's used. Do you know of a cheap data logger that can do this, like a 230v version of a Kill-A-Watt? Or maybe I could run one leg of the MSHP thru it and multiply X 2? I'd be afraid of screwing up the phase or something..

Zip is 54956

Not sure if I'd know how to set up my own calculator. but I was getting about the same results from Rescheck...

Thanks,
-michael

The ASHRAE 99% outside design temp in nearby Appleton is about 0F, the 99.6% outside design temp is -6F, so it's unlikely that anybody without a thumb on the scale would be using -15F as a design temp for ZIP code 54956.

https://www.captiveaire.com/CatalogContent/Fans/Sup_MPU/doc/Winter_Summer_Design_Temps_US.pdf?v=14112015

The ACCA begs to differ and thinks -9F is the 99th percentile temperature bin for Appleton:

https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf

A difference of 5F warmer is only a 6-7% error, but a -15F error (-15F instead of 0F) is a lot. Unless you have VERY specific weather data sets for your location you shouldn't be using anything colder than -10F.


For about $30 you can buy a refurb surplus old-school watt-hour meter, if you want to sub-meter what's going into the heat pump:

http://mcsmeters.com/products/texas-cl200?variant=1101164177

You'd have to buy a compatible meter box too, typically $25-50 new or used, good-condition on your favorite web auction site, eg:

http://www.ebay.com/itm/GE-General-Electric-Meter-Socket-Type-SV60-100-amp-600-VAC-New-/161898276032

You might be able to pick up one on your local Craigslist for less (haven't looked.)

Hobbit sub-metered his Daikin/Goodman heat pump hybrid using this approach:

http://techno-fandom.org/~hobbit/hse/log/878784/345openp.jpg

http://techno-fandom.org/~hobbit/hse/log/878784/04.html (scroll down to near the bottom)

What about this thing. Not as cheap, but seems easier .  No experience using it.

http://www.ekmmetering.com/ekm-metering-products/electric-meters-kwh-meters/basic-kwh-meter-100a-120-240-volt-3-wire-60hz-ekm-25ids.html?gclid=CPTL553l48kCFc2RHwodBloIYg

Dana...

Last year (Dec thru Feb) we had 21 days at 0 or colder, with coldest night at -16. I can't believe ASHRAE uses 0. I notice their chart lists Waukesha (100 miles south of me) at -5. I think the link on Builditsolar has me at -15. And the Rescheck calcs I submitted to the inspector has it at -15. So I guess I was being cautious by using the worst-case numbers. Also, the Rescheck was much more conservative, with my design load at over 15,000 BTU's. The big variable is tightness. I don't know...have not had a blower door test yet... But it very well could be that I'm oversized. Time will tell......

Thanks for the tips on meters.

-michael

Patonbike...

Thanks for the tip. I'll do some more searching. That one's a bit pricey for me...

-michael

My mitibushi wall mount unit did the same thing. The intake air would hit 73 and drop to 69 within a few minutes and it wouldn't be cold outside . I had a thermometer in the room and the temperature didn't drop like that. I just installed a wireless thermostat ( very easy to install). It was in the low 50s and didn't kick on for a 1-1.5 hours and run for 15-20 min. Without the thermostat it would've kept cycling and be uncomfortable.
I have a Fujitsu floor mount also and it doesnt short cycle. Being near The floor is more consistent with the stock thermostat.

Toadman...

So Mitsubishi offers a wireless thermostat? Fujitsu only has wired options for getting a remote unit with a thermostat inside (the stock wireless control doesn't sense temps). It's not a big issue, but 'd have to fish a wire somewhere... The larger issue right now is the cost: about $300 for the control and the interface board. I'm thinking of dropping cold air from upstairs onto the top of the unit via a small floor vent, possibly with a small muffin fan assisting....

Thanks,
-michael

and.... can I ask what you paid for your new remote? -m

That is an interesting idea - our upstairs air temp is always a couple degrees cooler- for simplification, downstairs might be 66, upstairs 64 and above the head unit downstairs might be 68.

would be interesting to understand how this would make the head unit act.

if you didn't have a remote thermostat, it'd probably it run a lot longer with the cold air blowing at it. it might also suck the warm air from downstairs up.
it might end up making downstairs too hot, without remote thermostat.

The thermostat was $251. It was a little pricy but definitely a lot more comfortable. Its been in the 30s and running around 102 for air temperature out of the unit
I used a box fan to circulate the air before; It would run longer but also sped the compressor up doing it. The problem is the air entering the unit is not consistent and varies to much. Is yours at the end of a wall that's near a hallway?