Quote:
Originally Posted by opendoor
Interesting. Everything around here (coastal BC) is just sized for the heating load and the cooling capacity usually ends up way oversized. But I've never heard of (or experienced) it being an issue. <clipped for brevity>
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Ha, okay, so... there are a few things you're saying that need to consider context.
First of all, Fujitsu is fibbing a little bit about the performance of your units. Not that they're
lying, but they're not giving you the full picture. For example your nominal 18 MBH unit is capable of >25 MBH of heat output, max; that
is true. However, it's only true within the context of the outdoor ambient temperature being so warm that you wouldn't need heat in the first place.
Here's the design manual for what is likely your heat pump, or at least pretty close to:
https://connect.fujitsugeneral.com/3...9-5e04ed333de9
This is the heating capacity chart for the 18 MBH unit; see p. 12:
So, yeah, it can deliver a max 25+ MBH of heat,
at warm temperatures outside. Functionally it's pointless. Any heat pump by its very nature will be capable of this, but I need more heat in my house when it's colder outside, not when it's warmer.
They don't actually tell you the conditions under which that minimum 3.1 MBH of heating or cooling is possible.
You can see that below -10 °C (14 °F), performance drops off below the rated nominal capacity, until it craps out entirely at -20.6 °C/-5 °F. FYI, winter design temperature in Calgary is -33 °C/-27.4 °F. Summer design is only +28 °C/82.4 °F (dry bulb), which is not really all that hot in the first place.
So, in reality, for a climate like Calgary's this unit is only useful up to a point. All things being equal the heating and cooling capacities required are dictated simply by the ΔT between indoor and outdoor. Assuming an indoor of 21 °C, peak (design) cooling is nominally only a 7 °C ΔT whereas peak heating is 54 °C ΔT. The heat pump's cooling capacity will
inherently tend to be much too large for Calgary's summer conditions, because it simply doesn't get all that hot in Calgary and gets
that much colder.
Ultimately there's no point designing for the winter peak condition; it doesn't really matter because it's incapable of doing it anyway. Like I said, a great option is to simply assume a non-peak heating condition and deliberately 'undersize' the heating; you'll end up with a much smaller discrepancy in heating and cooling capacities and the unit will operate within the bounds of its "normal" operating temperature range. You'll have some secondary source of heat, sized for the peak heating condition, either way.