Mini split heat pump and ac

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mkgrady

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Massachusetts
I just finished wiring an addition in Massachusetts. it is a two car garage with a big bonus room above and a one story mud room to the existing house. Before I started wiring the gc told me that the hvac contractor could not utilize the existing heat/ac system for the addition. So he asked me to install electric baseboard and wire for a large window ac.

I asked why not use a mini split heat pump/ac ductless system? He found an hvac contractor to do just that but the contractor said we would need electric baseboard as a backup in case the temp dropped too low for the system to function.

It sometimes briefly gets down to about zero degrees around here and right along the coast where the job is it is very unusual. I had heard that there are new mini split systems that work down below zero. Can anybody confirm that? If so then the baseboard electric is a waste of money.
 
The mitsubishi FH hyper heat models will work down to -15 outside temp. We don't recommend them as a primary heat source, but we are up in northern VT so it gets a little bit colder up here.

Ben
 
Heat pumps don't like the cold, there is too little heat to pump.

If they will run at all their efficiency drops rapidly once it gets below 30 F. Electric resistance heat is pretty much standard in our area. Usually it is a electric coil in the duct work but in this case it will have to be baseboard.
 
iwire said:
Heat pumps don't like the cold, there is too little heat to pump.

If they will run at all their efficiency drops rapidly once it gets below 30 F. Electric resistance heat is pretty much standard in our area. Usually it is a electric coil in the duct work but in this case it will have to be baseboard.
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That has been my understanding until I heard that there are recent changes in the technology that makes heat pumps reasonably efficient down to below zero. I don't know, maybe that is not really the case.
 
mkgrady said:
That has been my understanding until I heard that there are recent changes in the technology that makes heat pumps reasonably efficient down to below zero. I don't know, maybe that is not really the case.
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I think it may depend on what is purchased. A low temp rated one is likely a lot more money than a standard one.

I really don't know. I just know that virtually every heat pump I have wired had electric supplemental heat. Of course technology changes and I may be out of touch. :)
 
iwire said:
I think it may depend on what is purchased. A low temp rated one is likely a lot more money than a standard one.

I really don't know. I just know that virtually every heat pump I have wired had electric supplemental heat. Of course technology changes and I may be out of touch. :)
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I suspect you are right about the cost of a good one. The gc rejected the first hvac contractor based on cost. The second hvac contractor had a a better price but stipulated that baseboard electric would be required for when it got really cold. I suspect the backup electric baseboard cost more than the amount he saved.
 
since a window unit was being considered as well as combination mini-split, would a PTAC be considered? Depending on where you need the heat some baseboard for the mud room might be needed, but PTAC's can be half of what a mini-split would cost and you can get decent size resistance heat.
 
USMC1302 said:
since a window unit was being considered as well as combination mini-split, would a PTAC be considered? Depending on where you need the heat some baseboard for the mud room might be needed, but PTAC's can be half of what a mini-split would cost and you can get decent size resistance heat.
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the window ac was only considered in conjunction with electric baseboard. Neither of which I think the home owner would be happy with. The noise from a window ac unit and having to install and remove it every year would not be appreciated by the owner. And of course electric baseboard is so inefficient.

I don't know much about ptacs but I always found them to be noisy and give off a bad smell but I have only seen them in hotel rooms.

I'm not seeing a lot of consensus here but my thought was that a good mini split heat pump and ac combo would have been a good choice but I'm really just guessing.
 
USMC1302 said:
since a window unit was being considered as well as combination mini-split, would a PTAC be considered? Depending on where you need the heat some baseboard for the mud room might be needed, but PTAC's can be half of what a mini-split would cost and you can get decent size resistance heat.
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Do not forget that what counts most for the owner is the annualized operating cost, not just the installation cost. Installing baseboard as a backup/auxiliary heat source will have a one time cost but may only be called on to deliver heat a few days of the year.
 
GoldDigger said:
Do not forget that what counts most for the owner is the annualized operating cost, not just the installation cost. Installing baseboard as a backup/auxiliary heat source will have a one time cost but may only be called on to deliver heat a few days of the year.
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Should the baseboard electri tstat be set just a little lower than the heat pump Tstat? In this way it only kicks on when the heat pump can't produce?
 
mkgrady said:
Should the baseboard electri tstat be set just a little lower than the heat pump Tstat? In this way it only kicks on when the heat pump can't produce?
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Yes, unless you use a timed temperature setback for the heat pump (usually not a great idea unless it is a very smart controller that ramps up the HP setpoint slowly to avoid calling for resistance heat.)
 
mkgrady said:
Should the baseboard electri tstat be set just a little lower than the heat pump Tstat? In this way it only kicks on when the heat pump can't produce?
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That is what I would do.

Normally you would use a heat pump thermostat with auxiliary heat controls but that may not be an option with a mini-split.

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Inverter is the way to go. They utilize an AC-AC technology to change the frequency (hz) to the condenser. That way, instead of cycling on and off, they simply slow down.

I've been installing them at my son's school. Four of them so far in 6 months. 2 have been wall mounted, and 2 are roof mounted. The wall mounted units seem to do a better job for some reason.

18k btu units, minimum circuit amps of 10, max breaker 15, used in rooms that are 23x28 with 12' ceilings.

And they're doing just fine down to about 8 degrees for our coldest night so far
 
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