Switching to low temperature?

[Garald]

[Preface: work is about to start in my major renovation project. I am trying to do everything possible to *bring down* costs at this stage. At the same time, I should consider the following question, because it is now or never.]

 

As some people here know, I've bought most of a small house from 1930 (Paris area), and I plan to switch over to a high-temperature heat-pump. The rationale was not having to change the current radiators. Now the contractor (who does have heating as his specialty) is insisting that we do need to do work on the conduits, move some radiators, etc. Hence, I am asking myself whether it wouldn't be better to switch over to low temperature.

 

The architect says that the net cost ( = price of switching the network to low temperature, minus a certain amount coming from the fact that low-temperature heat pumps are lower-cost) would be about 5k eur. (I doubt she's taking into account that, if the network is high-temperature, a smaller hot-water heater is needed; for some reason she has put a 300L heater in the plans.)

 

Is it worth it?

 

The architect seems to be taken aback mostly by the fact that it would be more work for her (but it's not as if I am not paying her enough), and that low-temperature heat pumps take a lot of wall space (I've got only about 1500 books currently, but libraries grow).

 

(For further reference: the previous owners had no problem heating the place with their old gas heater set on "medium" (about 50deg?), even though the place had essentially no insulation. We are installing plenty of insulation, obviously, as some very long threads here attest; thanks to everybody.)

 

(Another factor in favor of low-temperature: the city heat network might reach our block some day (it's 100m away, currently) and joining it might then become possible (though the town says that our co-op is too small for that on its own). That network works only with low-temperature systems.)

 

 

[Garald]

PS. Here is the high-temperature heat pump we would be installing:

 

https://www.amzair.eu/nos-pompes-a-chaleur/pompe-a-chaleur-renov-ht/

 

Not cheap and not ultra-efficient, but it's compact and indoors (important so as not to annoy the neighbors; the courtyard is shared).

Edited September 11, 2022 by Garald

[JohnMo]

What's your heat load, as you seem to be insulating the house well

[Garald]

How do I compute my heat load?

 

The place (a duplex/triplex) is 184m^2 counting the attic (low but inhabitable and heated) and the garage (unheated). I am very satisfied with the job the architect has done so far on the insulation. For those who are following the saga - we are doing 18cm of Biofib (celullose-based insulation: hemp/linen/cotton) on most surfaces, and 11cm of combined Biofib+reflective insulation on the (northern) courtyard wall (where we don't have much space, and where do we do not need much soundproofing). You can see some of the plans at

 

[SteamyTea]

Try this heat loss calculator. It is somewhere in the thread.

 

https://forum.buildhub.org.uk/topic/439-fabric-and-ventilation-heat-loss-calculator/

[ReedRichards]

A monobloc air-to-water heat pump that can be installed indoors?  How does that work?

 

It's fundamental to the physics of heat pumps that a high temperature heat pump will be less efficient and so more expensive to run than one that operates at a lower temperature (all else being equal).  My one regret over getting a heat pump is that the target water temperature (50 C for my system) is too high and I should have made it a bit lower with even bigger radiators.  

[Garald]

6 minutes ago, ReedRichards said:

A monobloc air-to-water heat pump that can be installed indoors?  How does that work?

 

It will be installed in what is now a disused, broken outhouse, which I have just bought from the co-op. There will be an opening onto the courtyard.

 

(Proper sound insulation all around.)

 

7 minutes ago, ReedRichards said:

It's fundamental to the physics of heat pumps that a high temperature heat pump will be less efficient and so more expensive to run than one that operates at a lower temperature (all else being equal).

 

I know - that's why I am asking.

[ReedRichards]

1 hour ago, Garald said:

 

I know - that's why I am asking.

 If you know why a high temperature heat pump is less desirable  then I don't see why you are asking the question.  You will have to pay an extra 5k but you will eventually recoup that investment through running cost savings.  The only reason against would be if you could invest that 5k and generate enough income from the investment to pay the extra running costs.  That seems unlikely unless your expenditure on heating is very small.  And you would have to persuade your contractor to leave things as they are and hope that that works.

[Garald]

On 11/09/2022 at 21:38, SteamyTea said:

Try this heat loss calculator. It is somewhere in the thread.

 

https://forum.buildhub.org.uk/topic/439-fabric-and-ventilation-heat-loss-calculator/

 

Inputting the following *very* rough values (please check for sanity; remember this is a duplex/triplex (part of ground floor, 1st floor (roughly 8m x 12m), attic))

 

MVHR efficiency =	85%
Internal wall area =	200.00	m²
Internal roof area =	80.00	m²
Internal floor area =	76.00	m²
Total wall and roof heat loss area =	280.00	m²
Floor fixed heat loss =	89	W
House total volume =	400	m³
Heat capacity of air =	1210	J/m³.K
Heat capacity of air in house =	409283	J/m³.K

[Garald]

... and inputting also the temperature data for Paris for Wikipedia (an underestimate: it's averages for 1990-2020, and of course temperatures are rising), I get the following. Conclusion?

 

Total daily heat loss power for average OAT (W) =	808	739	507	286	77	-108	-242	-236	2	292	605	785
Total daily heat loss power for minimum OAT (W) =	1064	1058	924	791	605	425	309	321	495	675	890	1029
Monthly heat energy input (kWh) for average OAT =	601	496	377	206	58	-78	-180	-176	1	217	436	584
Monthly heat energy input (kWh) for minimum OAT =	791	711	688	570	450	306	230	239	356	502	641	766

Edited September 12, 2022 by Garald

[Garald]

All right - new input from the architect!

 

TL;DR - the net cost of switching to low-temperature would be about 5700 eur. (14 radiators (the number we now have?) at 650 eur = 9100 eur, but we'd be saving 4100 on the heat pump and water tank (the low-temperature monobloc is cheaper by that much than the high-temperature monobloc from the same company); she estimates the cost of fiddling around with other things would be 700 or 800 eur.)

 

I suspect this may be an overestimate: do we really need 14 radiators? Recall this is a place that will be about 122m^2 loi Carrez (inhabitable space you can stand on), plus ca. 50m^2 non-Carrez attic space, plus 10m^2 unheated space (garage + heat-pump closet);, plus staircase taking high ceilings in the ground floor and first floor into account, we are talking about some 400m^3.

 

Inconveniences of low-temperature: need larger radiators that won't fit under the window sills; will have to sacrifice some wall space that could be used for books.

Further pluses of low-temperature: the low-temperature heat-pump we are looking should permit cooling (the architect is checking whether it is compatible with the positive-pressure ventilation system she is installing).

 

Opinions (on low- vs high-temperature at this price, and on the number of radiators)?

 

Here is a link to the heat pump she is proposing:

https://www.amzair.eu/wp-content/uploads/dlm_uploads/2015/04/pac-amzair-optimduo-dossier-technique-20211125.pdf

 

 

Edited September 12, 2022 by Garald

[JohnMo]

Not sure if I am missing something, but from what I can see the worst case heat input is January, where at min OAT you need 1kW at your highest internal temperature.

 

Some perspective - A small electric panel heater would heat the whole house.

 

So no idea why 14 radiators would be required, or why you are even discussing a HT HP.

 

If you want cooling, radiators don't work.  You would need fan coil units.

 

Low temp heat pump, a small fan coil in each room, heating and cooling fixed.

 

 

 

 

[Garald]

Just now, JohnMo said:

Not sure if I am missing something, but from what I can see the worst case heat input is January, where at min OAT you need 1kW at your highest internal temperature.

 

Some perspective - A small electric panel heater would heat the whole house.

 

Then I must be inputting something wrong. Paris is a bit warmer than the south of England, but it's not like it's a tropical island either.

 

When I lived in 23m^2, I had two small electric panel heaters set on medium-low and found that was quite enough, but friends sometimes said I didn't use enough heating for their taste. Also, I had a friend who lived in 40 m² in an attic - his place was a temple of the winds - it was not just uninsulated, it leaked air. He was cold in winter, even after he installed electric heaters with fans.

 

Of course I will be in a different situation - I'm doing a very thorough insulation job - but it's definitely not a heating-optional climate. The coldest place I've been to where anybody thought a small electric panel heater would heat an entire house is Santiago de Chile, and that's (a) significantly warmer, (b) peopled by Spartans.

 

[JohnMo]

I would check the your areas and u values, and window and door sizes.

[Garald]

Let me post detailed plans on a new thread. The R values will be >= 3.8 (+ the brick walls' intrinsic R) everywhere. Double-glazing with good thermal performance; I'm replacing the big library windows to better double-glazing (mainly to increase sonic performance). Outside steel doors; I'm replacing the indoor doors.

[Garald]

Here is the new thread:

 

 

[JohnMo]

Looking at you new thread, the areas don't match the areas you have in the heating spreadsheet.  The heating spread, should be the total, floor looks ok, but roof and walls seems very small.  Maybe post the heating spreadsheet on the other thread.

[Garald]

Right, I was suspecting that - I must have made a mistake. Let me find the sheet.