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I have a portable heat pump/air conditioner that I want to return under warranty due to some thermostat and compressor issues. The only other unit available I can replace it with has slightly less heating and cooling capacity. My current unit is 4.1KW cooling/3.8KW heating, the replacement would be 3.5KW for both heating and cooling. What effect will I notice in the reduced capacity? Longer running to reach the desired temperature? 3.5KW should still be adequate to heat and cool my roughly 18sqm room but I'm unsure of what effect I will notice exactly in the reduced capacity.

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It depends heavily

Because ultimately, you have a room with heating/cooling loads, which you're trying to meet with the heat pump. 18 sq. m. is approximately 200 sq. ft., and I've generally seen 30 BTU/hr/sq. ft. as a rule of thumb for rough sizing. With this guideline, you'd need 6 kBTU/hr, or 1.75kW. Because your heat pump is going to be twice this, it's unlikely you'll notice an impact.

Why it depends

The problem with 30 BTU/sq. ft. is that is makes quite a few assumptions about your HVAC loads. If your insulation is especially poor, you will need more HVAC power. If you live in a milder climate (ex. 20-30 °C all year) you might not need any HVAC capability. There are too many variables for us to definitively say how you will be impacted.

How to determine the impact

If you found your old unit was running constantly, that's a sign that it's incapable of keeping up with your heating and cooling requirements. If, however, it was nearly always off (or, if it used an inverter, it was running at very low power) then it was likely significantly oversized. If you don't remember, try looking back at previous electrical bills. Unless all your appliances are also electric, or you drive an EV, you should be able to tell approximately how much power your old unit used. For a ~4kW unit, you should expect 1-2kW of electrical draw while it's operating (at full power).

  • It does heat the room quite quickly so possibly slightly over sized but on the other hand it can struggle with the cooling in summer, probably due to sheer amount of heat we experience in summer. Over sized in this case is probably not a bad thing... Your reply would indicate that considering the room size 3.5kw vs 4KW is not significantly going to be noticed. I think that helps. – Kol12 May 31 '18 at 23:59
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    @Kol12, with AC, too small won't provide enough cooling, but too big is also a problem. Your comfort depends partially on dehumidification. An oversized unit will run a short time to get the air temperature to the setting and then shut off, so it isn't as effective at reducing the humidity. The air can feel clammy. – fixer1234 Jun 1 '18 at 3:18
  • @Kol12 While fixer's comment is accurate, you may also have a dehumidification mode on your air handler which would help with this, helping to mitigate the problem – Hari Ganti Jun 1 '18 at 17:44
  • I agree with @hair Ganti , there should not be an issue with that small of a room. I would rather have a "two large" unit because it will do the job of getting the room cool that's what I pay for not barely cool, I don't mind a little higher power bill and that is the only negative in my opinion. – Ed Beal Jun 2 '18 at 13:06
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The only way to tell if the capacity is correct is to do a heat load calculation. If a new smaller unit works then that means that your original unit was oversized. A properly sized unit will run almost continuously during a hot day and run less on a cooler day. If it turns out that the new unit is too small then if you set it for say 74F on a really hot day it may run continuously and the temperature inside would rise until the BTUs entering the house equals the BTUs being removed. The hotter you keep your home the fewer BTUs you will need.

  • I think 3.5-4 KW is suitable for the room size I have. It may be slightly too high but I don't think side effects of that would be major... As Hari Ganti pointed out it's not likely there will be any difference noticed as 3.5KW is still more than enough capacity for the room size... – Kol12 Jun 2 '18 at 12:58
  • Based on the specs of each unit can anybody determine whether one is better than the other? – Kol12 Jun 2 '18 at 12:59
  • @Kol12 Depends on what you mean by better? If one unit has WiFi and the other has smart thermostat integration, which is better? It's a bit more complicated than just looking at specs. – Hari Ganti Jun 4 '18 at 16:49
  • @Hari Ganti So nothing in the spec pages I linked to stands out? They're both similar I guess. Basically I have had a lot of trouble with the Olimpia Splendid Ellisse in the form of strange thermostat behavior and bad compressor noise. The compressor noise on the current Ellisse I have seems to getting worse, loud grinding and even whining noises that sound like a bearing noise. At this stage I wonder if I'm better off taking a chance and replacing this with the Dimplex unit but I don't know much about how they perform. Since this is the second Ellisse I've had it seems there's not much choice – Kol12 Jun 6 '18 at 11:55
  • @Kol12 Having never used a portable unit like the ones linked, I can't offer much advice about them specifically. Nothing really stands out in the spec sheets either – Hari Ganti Jun 6 '18 at 16:29

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