Under sizing heat pump

[Raisedonadeere]

I heat my garage to avoid freezing and to take chill off when I work out there. Low 40’s to mid fifties is just fine most days. Most mini splits systems cannot be set lower than low sixties. Am I right? I want to put a 1800btu minisplit in my 24x36 stick build that is well sealed and insulated. By my calculations, on majority of days during KY winter it will hold temp when set a lowest temp. On maybe 20 or 30 days temps might pull below set temp but I am fine with that. I can always supplement the heat if I really need to be warmer out there.

My question is will heat pump be harmed having only 40 or 50 degrees across the inside coil several days at a time? Sure would be nice to have a freeze free garage while avoiding cost of keeping temps in the 60’s with a bigger unit.

Also are there any brands that are designed to heat at lower temps? My only experience is with mr cool hyper heat which is great in the house.

 

[jabelding]

Just buy a unit with freeze protection mode, it will keep your garage at 42-45 deg then you can turn it up when you work out there.

 

[nadogail]

Just buy a unit with freeze protection mode, it will keep your garage at 42-45 deg then you can turn it up when you work out there.

What he said.

 

[Raisedonadeere]

Freeze protection sounds like the apt description for what I want. I hve seen that feature featured. Is that feature something buried in the specs or do I just need to start googling.

 

[yeldogt]

you need to read the specification ... they are have a range where they are designed to work. My VS carrier is not designed for those internal temps .. they are expecting a minimum return temp to the compressor.

read the operating degrees as published

 

[Bdflies]

Heat pumps aren’t very good at heating, when ourdoor temps are below about 45. At that point, auxiliary heat strips kick in. Then, you’re powering the compressor, heat strips and the air handler. Seems to me that O P would be better off with a good stand alone plug in heater, that’ll easily cycle at the desired temps.
My $0.02

 

[Raisedonadeere]

If I can find a unit that is designed to operate similar to the some Mitsubishi models that have some sort of bypass mode for the thermostat which goes down to 61 degrees or so I would look at that. Price of a Mitsubishi is not in the budget. But the main point of my original post is a bit different.

The question was will it hurt a minisplit to operate in an environment where it cannot maintain set temp because it is not sized big enough. What if I put a 1.5 ton unit that needs a 2 ton to do the job on the warmest and coldest days. If on a zero degree F day outside it takes a 2 ton unit to maintain temp then if instead there is a 1.5 ton trying to do the job it of course will not keep up. Inside temps would drop and perhaps only be able to get temps to 45 degrees. Will the 1.5 unit just sit there and happily put out its maximum with the 45 degrees or will it be overstressed somehow over time?

I have two Mr Cool Hyper heat in my home and last winter on one of the coldest days I turned one of them off just to watch what happens. The working unit reached its limit and the house temp dropped a degree or two. But I observe no signs of strain. The outside unit just keeps quietly purring like a kitten. But what if it was working against a 20 or 30 degree deficit?

If it would not harm the unit I would just have a poor mans freeze protection while enjoying 55-60 degree set temps on all but coldest days.

 

[Raisedonadeere]

you need to read the specification ... they are have a range where they are designed to work. My VS carrier is not designed for those internal temps .. they are expecting a minimum return temp to the compressor.

read the operating degrees as published

Basically you have answered my question. Figuring out what those specs mean is a bit daunting since they play so many games with what they call things. I will be doing the research.

I understand COP vs efficiency, performance curves etc, although there is a dizzying array of formats for presenting it. But the actual machine may be designed with such a narrow safety factor that it will blow up (not literally) if some particular parameter is exceeded or perhaps it has a robust safety factor that would avoid the unit being over stressed when operating not within spec.

The manufacturers avoid giving you that information for fear they would be liable to guarantee it on every unit etc.

So I was just looking for any actual experience some of you might have had where your unit operated for a long time in a space where it could not maintain temps, -- In particular when the inside temps might be in mid 30's or 40's but the unit is designed to operate lowest temp around 60. Obviously if you go into a cold room and turn the unit on it will be out of range for a while and no harm done but what if it never catches up and for a week or two during a blizzard or such it is sitting there in a 40 degree room.

 

[Jackfre]

Heat pumps aren’t very good at heating, when ourdoor temps are below about 45. At that point, auxiliary heat strips kick in. Then, you’re powering the compressor, heat strips and the air handler. Seems to me that O P would be better off with a good stand alone plug in heater, that’ll easily cycle at the desired temps.
My $0.02

This is incorrect as it pertains to mini splits. Read up on the technology. The success of mini splits has forced the unitary manuf to invest in improving the performance of their equipment. Also mini splits do not have 2nd stage elec heat. You can’t put the strips in a plastic case.

 

[Bdflies]

This is incorrect as it pertains to mini splits. Read up on the technology. The success of mini splits has forced the unitary manuf to invest in improving the performance of their equipment. Also mini splits do not have 2nd stage elec heat. You can’t put the strips in a plastic case.

In that case, I stand corrected. I’m familiar with more traditional and 'package' heat pumps, like the one in my shop. I was under the impression that mini splits had similar characteristics.
Thanks for the information.

 

[jabelding]

op, Heat pumps can run flat out heating for days on end no issue or concern for the unit, if pushed past its heating capacity only problem will be lack of heat and expensive power bill.

 

[Raisedonadeere]

op, Heat pumps can run flat out heating for days on end no issue or concern for the unit, if pushed past its heating capacity only problem will be lack of heat and expensive power bill.

Thanks, I was pretty sure that was the case but just wondered if perhaps pressures went out of range such that some overstress might be induced if say for instance you ran one of things in a cold warehouse scenario.

For the limited times when it would not keep up I would just pay the bill as the saying goes. We had a blizzard and subzero days with high winds in 1978 and everybody's utility bill went through the roof, it mattered not what you were using. We paid the bills somehow and life continued.

 

[Mogana]

What size of heat pump should i buy for a 3 bedroom flat?

 

[Raisedonadeere]

Update: talked to service techs who seemed baffled by my question but one of them did some research and got back to me. His answer, (sells Mr Cool,) from Pioneer was it helps, that is the cooler the room the easier the compressor is working all other things being equal, in a heating situation. I already knew of course that at normal room temp raising the thermostat temp causes the unit to work harder. This follows from refrigeration theory, but I also watched my pressure gages for a while after an install just to observe this happening. Pressure difference goes up with increase temp in house.

My question was to verify from user experience that some quirk of minisplits make them not like to be operated in a cold space when heating. Based on responses here I am going to go with a 18k unit in my 864 sqft garage. That is near its max area for a living space.

 

[JBudd2]

I built an 850 sq. Ft shop here in central Nebraska, in 2012. Installed a middle priced LG mini split (18 k , I think). We get below zero weather and sometimes the unit has been buried in snow for a week. Yes the shop might be in the 50’s and the compressor is just spinning it’s wheels, but so far the unit is everything I expected.

 

[Raisedonadeere]

JBudd2 I never even heard of minisplit in 2012 wow. They have been out there all this time. I think it was on GJ 3 years ago that I became aware of them and went with them on my house. Anyway I am finally pulling the trigger on a Cassette !8K Hyperheat Mr Cool. I will be lucky to get it installed before the coldest weather has passed. I should have ordered months ago. Been busy and trying not to make too many mistakes on this build cost me on this one.

 

[yeldogt]

An undersized unit will fall behind .... the question is "how far" .... this will be based on outside temp and building size.

Again ... have to look at the operating specifications. The units need minimums -- you can 't operate an AC when it's 40 out to cool a server room unless it is designed for it. Same with a heat pump -- they can't have the evap temp fall under particular levels for extended times.

The compressor (condenser coil) will begin to freeze and it will go into defrost ... if it has timed it will not protect itself .... on demand will cycle all the time.

 

[Steve W.]

I see you have pretty much made your decision, but I would like to offer my observations on what I learned in this thread.

First off, I have no experience with a mini-split (yet). I will be shopping next year to equip my shop and the one-room expansion in the house (yes, two separate units).

I understand that you want to simply set the thermostat lower than the 60 or so degrees that would be standard in a house. I learned in this thread that there are some units that offer "Freeze Protection". You are suggesting under-sizing the unit. My question would be: if you undersize the unit so it doesn't quite make it to the set temp on the thermostat, what's going to happen when you turn the thermostat UP? If the unit can't warm your shop to over 50 when the thermostat is down, it's certainly not going to be able to get it any warmer when you turn the thermostat up.

In your first post, you mention your "well sealed and insulated" stick-build is 24x36, which is just a little bigger than my 24x30, which is also well-sealed and (reasonably) insulated. I also noticed what I hope is a typo, where you are hoping to install an 1800 BTU unit. Hoping that is actually 18,000 BTU. I am also looking at an 18,000 BTU unit, but I have calculated that to be a bit of overkill, not under-sizing it at all. I have found that if I turn off the heat (currently a kerosene-fired torpedo) in my place, it takes a couple of days to get down into the low 50s. I would think that simply turning your unit off if you are not going to be there for a couple of days would be fine. It is my understanding that mini-splits are better at maintaining temperature than they are at massive heating and cooling cycles, so you might have to turn it on several hours (a day?) before you anticipate being there to do some work. On the other hand, mini-splits are supposed to be rather efficient, so why not just let it maintain its lowest temp of 60°?

.

 

[HamAndEggs]

Don't have gas in the building?

I've seen pretty poor results with the longevity of mini split units, especially in dusty environments

 

[Hobby_Man22]

Don't have gas in the building?

I've seen pretty poor results with the longevity of mini split units, especially in dusty environments

They are die hard fans of those things on sawmill creek, but that's what I heard too. They're more sensitive to dirty filters and such. Although i'm not sure what it takes to clean them. I wonder if it just means i have to go at it with my air compressor and blow out the inside of the air handler.

 

[Bill vonSteuben]

I heat my garage to avoid freezing and to take chill off when I work out there. Low 40s to mid fifties is just fine most days. Most mini splits systems cannot be set lower than low sixties. Am I right? I want to put a 1800btu minisplit in my 24x36 stick build that is well sealed and insulated. By my calculations, on majority of days during KY winter it will hold temp when set a lowest temp. On maybe 20 or 30 days temps might pull below set temp but I am fine with that. I can always supplement the heat if I really need to be warmer out there.

My question is will heat pump be harmed having only 40 or 50 degrees across the inside coil several days at a time? Sure would be nice to have a freeze free garage while avoiding cost of keeping temps in the 60s with a bigger unit.

Also are there any brands that are designed to heat at lower temps? My only experience is with mr cool hyper heat which is great in the house.

Is the intent to save $ on the install? If so, how much are you saving as opposed to a properly sized unit? If the calculations are run with your stated temp goals in place, the answer will be a correctly sized unit, no?

 

[Raisedonadeere]

Most properly sized heat pumps need aux heat for the coldest days of the year. I am fine with the drop in temp that would occur if the aux heat is not there but just wonder what the effect on the heat pump will be if running for a protracted period where the garage temps are perhaps in the 40 or 50s and the heat pump is running 100% of the time because the thermostat is calling for heat.

My needs during those times is no freezing in the garage but otherwise would not implement aux heat if the heat pump doesn't mind working day and night for a week or so with room temp in the 40's.

 

[Raisedonadeere]

Also max humidity control in hot weather is achieved with a minisplit just big enough to maintain temp. As you go larger you get less humidity removal and so the main reason for my install is to control the garage temp during the hot humid central ky summers and to have freeze protection in the winter.

 

[Yankeefarmer]

I would argue that, using current heat pump technology, a properly sized heat pump would not need aux heat during the coldest days of the year. An exception might be made during exceptionally cold spells which are colder than standard minimum design temperatures for the region, but that can affect the ability of electrical resistance and fired heating systems to maintain preferred temperatures too. With older heat pump technology, heat transfer capability dropped off more quickly with decreasing temperatures, and so aux heat strips were a necessity unless a dual-fuel system were provided. Even today, my HVAC company was unwilling to trust a heat pump-only system for heating in cold New England winters. (And there’s other parts of the country where it is colder and for longer periods of time.)

Let’s keep in mind that, in this forum, we’re talking primarily about heating our garages and workshops, not our living quarters. If my shop is going to max out at 5 to 10 degrees colder than I normally heat it for working out there because of extraordinarily cold weather, no big deal- nobody is living there.

As far as mini-splits are concerned, they can provide excellent humidity control even if a bit too big for current cooling requirements. They do this through use of a “dry” mode, where the evap coil is kept very cold and airflow is at minimum. My ducted heat pump system cannot do this, because there is insufficient communication between the compressor and air handler to modulate both for this ”dry without overcooling” effect.