Heat pump mini-split estimates

I have two estimates for the installation of a minisplit heat pump on the third (top) floor of my house. Our cockloft is insulated and we’ll have new windows (though not this winter) throughout the house. That said, there’s a small alley on one side of the house (big enough to walk in, not big enough for a car) so it’s only attached on one side. The estimates came in at basically the same price only one guy is including dunnage and advocating for a “bigger” system (5-ton vs 3-ton on the roof) and more BTUs per air handler inside (18k, 18k, 12k, 9k, 6k vs 12k, 9k, 6k, 6k – one less here because he doesn’t think I need one in the tiny room that is over the front door). I’m not sure of the exact square footage but it’s less than 1500 square feet. Both systems are Mitsubishi. FWIW, there will be a different heat pump system on the lower two floors when we do that renovation. For now though, we need heat somewhere in this house and we’ll be living in the top floor while we renovate the lower floors.

I’ve heard that it’s just as bad to install a bigger system than you need be cause the thing cycles on and off too much and stresses out the system and it actually becomes LESS efficient. Anyone have any insight on which way to go?

Thanks!

winbrown

in General Discussion 2 years and 10 months ago

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lkrshacmzcy | 2 years and 10 months ago

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First guy is right. The funny part about Mini Split Installations is that people do not use a formula and just use their “experience.” While this can be appreciated, it can also be wrong.
It is like when a Boiler is replaced and they quote you the same size you currently have; always ask to justify even that. Sometimes even that is wrong but it is easier for someone to replace what is there rather than do their own math to make sure it is right.

colonialrevival | 2 years and 10 months ago

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The post above is correct and the poster knows his stuff. There really needs to be load calculation. If your contractor isn’t doing it… then the rest is just guesswork. Should you get the bigger system or the smaller system? There are numerous methods by which you can gain certainty

carolina-moscoso | 2 years and 10 months ago

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Generally speaking, each contractor should have done a rough Manual J load calc to justify their sizing recommendations and you might ask them to share that math. I always feel like our old houses here in NY need a bit of upsizing given how drafty they are but if your spaces are well insulated, I see no benefit to going with a bigger system just because. If the “tiny room over the front door” is its own defined space and has a door or entryway that otherwise closes off airflow and you will be occupying the space, then you will regret not putting an air handler in there. We really need corresponding room dimensions/intended use for each BTU recommendation in order to give any type of specific educated feedback here. Also, if you oversized the condensers, you could potentially accommodate system expansion downstairs down the road. Unless of course they will be rentals and you need to meter the electric use separately.