Outside heat pump element in the inside (well, so to speak): should this work?

[Garald]

So, should I be happy with what I have got with the worksheet and double-checked by hand? Notice that I'm not actually getting the same values by hand, at least not for the ventilation heat loss.

[Marvin]

37 minutes ago, SteamyTea said:

 

 I start from first principles and make up my own spreadsheets. They are only glorified graph paper that we used at primary school to do our sums on.

+1. and we then use ours to review the results: Theory to actual.

[Garald]

At any rate, yes, from what I see now, the Freedom Heat Pumps does add a fudge term of 0.15 to the stated value of the wall. Fair if that's what experience shows them, I suppose; does it sound sensible?  (It's a bit funny that it's a fudge additive term and not a fudge factor - it does affect things very heavily when you've worked hard to get the U all the way down to 0,23 - but perhaps that's what reality indicates.) It also adds 0.15 to windows' U value (there, it matters less).

 

Or is there a meaning to that term?

 

[Garald]

(I'll redo the calculations for the attic by hand, incidentally; there's no way to tell the Freedom Heat Pumps that you have a room that is not a box, so the spreadsheet is underestimating the value. OTOH, the architect just told me that there's more insulation in the attic than I thought. Let us see...)

[JamesPa]

32 minutes ago, Marvin said:

+1. and we then use ours to review the results: Theory to actual.

Id definitely go for reviewing the results: 

 

partly out of curiosity and also to check my workings I paid for an MCS compliant survey of my house, which resulted in an estimated power demand at -2C of 14kW.  The surveyor ignored most of the insulation upgrades I told him about.  As a result I've was advised to spec 14kW+ heat pumps

 

My own spreadsheets, taking into account the upgrades (but being fairly conservative about U value assumptions, particularly when it comes to older double glazing) comes out at 10.9kW.

 

For the last few days the heating (still gas) has been ticking along nicely (and the house comfortable) consuming, according to our smart meter, a pretty reliable 7kW (space + water heating) 

 

Now there are, admittedly, three rooms we don't heat, so perhaps the demand might go up to 9kW if we heated them, but the 14kW starting point is just nonsense.

 

At least I am now confident that an 11/12kW heat pump, when I fit it (hopefully in the first half of next year), will be sufficient (possibly even a bit to big!).

 

Based on this, if there is any way to take an actual measurement, I would do so.  Unfortunately we don't often (in South East England) get a longish period when the temperature is reasonably constant and low.  In this respect the last few days have been a bit of a godsend for confidence in my design.

 

 

[Garald]

2 minutes ago, JamesPa said:

 

 

For the last few days the heating (still gas) has been ticking along nicely (and the house comfortable) consuming, according to our smart meter, a pretty reliable 7kW (space + water heating) 

 

Now there are, admittedly, three rooms we don't heat, so perhaps the demand might go up to 9kW if we heated them, but the 14kW starting point is just nonsense.

 

At least I am now confident that an 11/12kW heat pump, when I fit it (hopefully in the first half of next year), will be sufficient (possibly even a bit to big!).

 

I'll double- and triple-check my calculations by hand. I have to make a decision that is tighter - I'm getting a predicted consumption of about 6kWh for heating alone if I heat the whole house (admittedly this doesn't take some further improvements into account - for instance, we'll be preheating the air in our PIV, and that should give efficiency gains relative to just using radiators, though the manufacturer's propaganda is a bit overly cheery and imprecise on that particular) and then there's hot water. The question is whether to install a 7kWh heat pump (which would be considerably quieter (-6dB) than a 10kWh heat pump, also from Saunier-Duval aka Vaillant). I'd lean towards "yes", except "7kWh" is clearly a bit optimistic, if you read the technical specifications closely.

 

That's what I am surprised that an installer just told me that his back-of-the-envelope calculations gave him enough confidence that he'd recommend the 7kWh model. He was telling me on the phone that people in his branch do calculations so as to cover 90% (if I remember well; it was something less than 100%) of heat loss - as if 90% were plenty. What do they use for the remaining 10%? Cats?

[SteamyTea]

4 minutes ago, Garald said:

I'm getting a predicted consumption of about 6kWh for heating alone

Is that for a day, 24 hours.

Or do you mean 6 kW.

[SteamyTea]

6 minutes ago, Garald said:

What do they use for the remaining 10%

Supplementary heating, usually electrical.

It is 90‰ (or whatever) if the time, so that leaves just shy of 100 hours a year when extra is needed, or the house is a bit cooler.

[Garald]

4 minutes ago, SteamyTea said:

Is that for a day, 24 hours.

Or do you mean 6 kW.

 

Sorry, I meant 6kW.

[Garald]

Just now, SteamyTea said:

Supplementary heating, usually electrical.

 

Sounds like a safety hazard in my hands. Perhaps I should look into the more expensive (but almost equally quiet) 8kW Mitsubishi Zubadan model, where again the nominal power is optimistic.

 

(Don't some heat-pumps have electrical built in to help the heat-pump operation above a certain level? I think Mitsubishi Zubadan does.)

[SteamyTea]

1 minute ago, Garald said:

Don't some heat-pumps have electrical built in to help the heat-pump operation above a certain level? I think Mitsubishi Zubadan does

Yes they do. Worth checking the power as it may only be 1 kW.

You also need to know how much control you have over the supplementary heater, they are often used to get the DHW up to 70⁰C. 

[Garald]

At any rate, one other way in which these calculations are useful is that they show what the actual weaknesses are, while keeping things in proportion. My architect was concerned that the large library/main room (you've seen it in pictures) would be difficult to heat if we aimed at a working temperature of 45C rather than 55C, but, since we have insulated the walls to new-building standards and are replacing all the double-glazing by better double-glazing, the heat loss there is barely about 1kW (with almost half of that coming from the ventilation). We'll be fine there.

The nice big stairwell with stained glass will be losing a bit over 4kW - we had only 10cm to work with when insulating there, but we are using a mixture of reflective insulation and cellulose, and insulating the stained glass from the outside, with a second pane. But that little corner staircase going to the attic? We didn't have space to insulate that at all, and so it loses 650kW. I better have the door leading to its lower landing be as well-insulated as possible (people in a different thread are advising me on what kind of kitty flap to install). The staircase is open at the top, but hopefully convention will be my friend this time.

 

And then I can also hope that skylights and southern-facing windows will be my friends during cold winter days...

[JamesPa]

1 hour ago, Garald said:

. The question is whether to install a 7kWh heat pump (which would be considerably quieter (-6dB) than a 10kWh heat pump, also from Saunier-Duval aka Vaillant).

If they are from the same range I am not sure I'd let the sound level difference influence the decision.   A 10kW pump running at 7kW is likely to be no more loud than a 7kW pump of a similar design running at max, and its quite likely to be quieter.    The amount of air they have to shift (at equal power) is the same so that starting point would be that they would be about the same volume.  However larger capacity pumps often have larger air heat exchangers, which for a given volume flow of air will reduce the sound volume.

 

If the manufacturers publish sound vs power curves you could check, but otherwise I would probably ignore this particular factor, unless of course it trips a regulatory issue.

 

[Garald]

14 minutes ago, JamesPa said:

If the manufacturers publish sound vs power curves you could check, but otherwise I would probably ignore this particular factor, unless of course it trips a regulatory issue.

 

 

Good point, or rather two good points (I *should* be ok if the coop agrees with me, but if some tenant chooses to complain later, they might have some ground to stand on if I get a 7kW pump rather than a 10kW one).

 

There's something else: the 7kW unit is 96.5cm tall, whereas the 10kW unit is 156.5cm tall. The smaller unit is thus easier to soundproof. (At 54dB, the sound production of the 7kW should be low enough that no soundproofing is legally required (though there might be reverberation...), but why not be safe?) Let's see https://solflex.eu/fr/hcschalldaemmgehaeuse10db/ :

 

Reducing the noise of a 7kW unit by 10dB costs 1600eur, 

whereas

reducing the noise of a 10kW unit by 10dB costs 2500eur, and, equally to the point, the soundproofing will then look like a man-sized Dalek. (Two man-sized Daleks, side, by side, really.)

 

(If anybody knows of less unslightly and bulky (and less expensive) soundproofing that is equally effective, please let me know...)

[Garald]

8 hours ago, SteamyTea said:

 

PSI (thermal bridges) could soon add up on a complicated shape.

 

By the way, as I said in another thread, the foreman and workers took the initiative and removed the moulding before installing insulation, and then managed to reattach it, thereby avoiding a thermal bridge that the architect and I didn't have a good solution to. Nice!

 

 

(The foreman says not to worry about the glue spots - he'll deal with that in a bit.)