Heat pump sizing and availability

[EatenByLimestone]

It’s time to start looking at heat pumps. My house is fairly well insulated and my family generally likes it warm as we’re outside most of the time during the summer. Each floor of my house has about 11000 btu of poorly sealed window units. The only one that gets any serious use is the 6000 btu unit in my kitchen window.

So, as I start looking at these things, should I look into 2 smaller units that would run for longer periods, maybe never shutting down, or should I look into a larger unit that has multiple zones? It would be nice to run multiple zones, but I’d worry that a larger unit would short cycle and break often. Do they even make 24000btu multi zone units?

I’d probably use it most often for cooling, electricity is stupid expensive in NYS, however I can see where it would get some shoulder season use.

 

[peakbagger]

Generally the smaller units in the 12K nominal range have the best efficiency. In reality a 12 K unit will put out far more heat than nameplate. The multizone units are less efficient and most prone to maintenance issues. IMHO, stick with single head units. Since they are much quieter and more efficient than window units, a smaller rated minisplit will replace window units of a higher rating.

Note that for very large houses and commercial systems, there is technology called variable refrigerant flow that can be even more efficient but it tends to be way overkill for most homes and even small businesses.

 

[EatenByLimestone]

Ok, so multi zone sounds like it’s out.

Is there a practical max length to refrigerant lines? The house is a cape, so it’s not like the lengths would be super long, but I’d like to place the units on a side where snow won’t slide off the roof onto them.

 

[peakbagger]

They typically come charged for 25 feet line length, they can run longer but the installer needs to bring a scale and add more refrigerant to compensate for the longer lines. If the tubing is substantailly shorter, they need to remove refrigerant or coil up the extra line so they are laying flat (if they are coiled vertically there could be issues with refrigerant lubicant pooling in the low spot). Not an issue if you hire a contractor but, only certified techs can buy and handle the refrigerant so its no longer a potential DIY job for most.

You can cut the hole in the wall, hang the indoor and outdoor unit and purchase and run the line set cover (something like these https://www.supplyhouse.com/Line-Set-Covers-17880000) and an unfused local disconnect) https://www.homedepot.com/p/Halex-60-Amp-Non-Fusible-Metallic-AC-Disconnect-HNF60R/205785642) The HVAC tech does the local wiring to the disconnect but an electrician is usually needed to run from the breaker panel to the disconnect. If you are comfortable with DIY wiring, if a disconnect appears on the wall next to the mini split, you can usually save some bucks on the install as they do not have have an electrician come out to run the wires through the house from the panel.

 

[EbS-P]

What’s your he budget? Top shelf or mid price or as cheap as one can get?

Equally important is finding a good person or company to install. And this only really matters if you start having issues. How much heat do you want/need? 12k btu units can run off 20A 120V anything bigger needs 240v.

You name it, it’s available right now for ductless mini splits.

You are no longer stuck with the typical high wall unit. 4 way ceiling cassettes are available as are low wall units.

Avoid condensate pumps. They are an absolute last resort.

It’s not like the multi zone are more prone to failure. Let’s say you have have a 3 zone 24k system, 8,8,8 or 6,6,12k btus. Each inside coil could have a failure rate but you only have one outside coils that could fail vs 3. I agree single zone is simpler. I’m torn. Efficiency at the low end really is over rated. Most electricity will be consumed closer to full output (if sized correctly) where the efficiency is less.

Short cycling is really not a thing unless you miss size the units. They always start compressor at low speed and ramp up as needed.

Figure out the tax credit situation.

 

[peakbagger]

For folks with low income, either employed or retired, there are going to a major incentives for minisplits rolling out in the next year.

If you want to optimize for cooling, a cassette type unit or high wall type unit is the best as cold air sinks. For heating there is low mount wall unit (like a hotel room) that is better for heating as heat rises. With the standard high wall mounts, there is quite a bit of difference between the ceiling temp and the floor temp especially when bringing the room up to temp. I tend to run the thermostate on the heat pump a few degrees higher than when running a baseboard.

 

[incinerator]

For folks with low income, either employed or retired, there are going to a major incentives for minisplits rolling out in the next year.

If you want to optimize for cooling, a cassette type unit or high wall type unit is the best as cold air sinks. For heating there is low mount wall unit (like a hotel room) that is better for heating as heat rises. With the standard high wall mounts, there is quite a bit of difference between the ceiling temp and the floor temp especially when bringing the room up to temp. I tend to run the thermostate on the heat pump a few degrees higher than when running a baseboard.

Right now if you buy models that qualify you get 30% back (including installation costs) up to $2000. I made sure the last couple i bought qualify. I think the 240v models might be a smidge more efficient overall--only issue i've run into is limited space so i had to add a subpanel and move stuff in there.

 

[EatenByLimestone]

I'm pretty sure it'd only/mostly be used for cooling, but who knows. Winter temps here are generally something like 30h/15l. That can be covered by the minisplit. We get a lot colder than that, but generally 30/15 is accurate.

My main reason for not really thinking about it for heating is per btu, my electric 6x more expensive than natural gas. Thats mostly due to not using much electricity. My base fee really ups the cost per kwh since its not spread out by a lot of kwh. I should recalculate the prices. Since covid my wife has had a home office and we use quite a bit more electricity. Total usage is still pretty low though.

 

[peakbagger]

An off the wall suggestion is if you have room on a garage roof for some solar panels is go with a specialty mini split that is run off solar panels, if the sun is out you need cooling if its not, you need far less. Its DIY install that uses precharged lines. You do not need a roof full of panels since its only running the mini split off of it. The trade off is its lower efficiency than the high end brands (but still far better than a window shaker. They recommend 1000 to 1600 watts of panels. Since most panels are around 5 panels. https://signaturesolar.com/eg4-hybrid-ac-dc-solar-air-conditioner-1-ton/. You can DIY 3 of these for the price of one professionally installed unit if you use used solar panels (which are cheap)

They reportedly do not require a utility interconnect (but definitely a question for your local building inspector). The one trick part is the rapid shutdown requirements for solar panels if on occupied structures, if you use a garage roof or shed or a ground or pole mount, it does not apply but if its on the house, you would need panel mounted optimizers equipped with RSD.

 

[Prometeo]

If you use mainly for heating, the ideal is to put the external engine on the south side, and internal split directly on the same wall, internal, in this way you have zero travel losses and the yield is much higher, sizing depends on needs and available electrical power whether ducted if is better depends on whether several doors are often closed, or not.

 

[EbS-P]

An off the wall suggestion is if you have room on a garage roof for some solar panels is go with a specialty mini split that is run off solar panels, if the sun is out you need cooling if its not, you need far less. Its DIY install that uses precharged lines. You do not need a roof full of panels since its only running the mini split off of it. The trade off is its lower efficiency than the high end brands (but still far better than a window shaker. They recommend 1000 to 1600 watts of panels. Since most panels are around 5 panels. https://signaturesolar.com/eg4-hybrid-ac-dc-solar-air-conditioner-1-ton/. You can DIY 3 of these for the price of one professionally installed unit if you use used solar panels (which are cheap)

They reportedly do not require a utility interconnect (but definitely a question for your local building inspector). The one trick part is the rapid shutdown requirements for solar panels if on occupied structures, if you use a garage roof or shed or a ground or pole mount, it does not apply but if its on the house, you would need panel mounted optimizers equipped with RSD.

This is very tempting but…. Assume on the hottest days I use 30 kWh. Let’s round that up to $5 and let’s say I have 25 of those days a year. 125$. Used solar panels would be paid off quickly but and here is my issue. My max cooling demand is from 5-9 pm. (And I have a big tree that casts a shadow at about 5. So I have to play that over cooling game to the extreme.

My point is max cooling load does not temporally match match solar production. So a battery might be needed and in this case it’s going to double the cost of the system. But you could get some $$$ back on it. So why not just grid tie and skip the battery. Plenty of sunny days where you don’t need heat or cooling.

 

[begreen]

I'll probably be doing a deep dive investigating an upgrade to our 17 yr old heat pump system though I am a bit concerned that as soon as the credits kick in, installers will be bumping up pricing to match.

 

[EatenByLimestone]

I'm sure they will be.