Heat pump in a house from the 1930s: performance, power and DHW? – Renewable energies and installations

We want our 1936 house to be more sustainable in the near future. We are going to insulate a lot, we want to install HRV ventilation (probably decentralized) and eventually induction cooking. But the most difficult and the most important: a heat pump for heating and DHW.

I have been reading this forum for the last six months. However, I still have three things that I can’t figure out: is the output enough, what power, and what solution for DHW?


At the end of 2021 I started a journey with Thuisbaas. Very positive at first. As here I had already become much wiser, had clear ideas and indicated that I wanted to continue, I was offered a faster path without extensive advice (also for a somewhat lower amount than usual). Unfortunately the ‘technical inspection’ in early April was canceled by them at short notice due to illness/problems and it is now unclear when it will start moving again. So now I am looking for more alternatives and trying to get as complete a picture as possible of the right setup for our house. Other budgets have also been requested, but nothing has yet been received.

Basic data

Terraced house from 1936, 163m2, Arnhem region.

Gas consumption: 977m³ in August 2020 – July 2021 for heating (+ DHW 191m³) = 0.335m³ per weighted degree day (depending on less gas). It consumed 191m³ in February of that year. Unfortunately there are no specific dates of the coldest days in February 2021.

What seems important to us: the environment, the noise (in relation to the neighbours), the availability, the appearance, then comes the costs. There is a budget in itself, as long as a higher price is proportional to the added value. With a difference of €5,000, we would go for the P/E, but now it looks like more than €10,000, which we think is too much given the rather limited perks.

We prefer to outsource it. If needed, we look for someone with general plumbing and electrical experience to assist with a monobloc, but prefer only someone with knowledge of the business.

delivery system

Radiators almost exclusively: living room/hall on the ground floor, three old column radiators, a type 22 and vvw radiator of approximately 4.5m2 in the kitchen. First floor and attic room type 22. There is no radiator in the bathroom (broken), but there is an infrared panel. We’d rather not replace the living room flooring (beautiful hardwood floor that wears out quickly with little kids, which is a shame with a new one).

Estimated total capacity of the ground floor at Ta 35: 2880W.* This will always be open. We have been running on a 40ºC supply since February this year and it has been going well. We almost never warm up. We would like to have heating in the bathroom again, which in winter cools down to 12ºC, which is not enough for the owner of the house. In the long run, we want to be able to heat about 80% of the house (kids grow up). We prefer to postpone the treatment of the subject of the first floor for a few more years. In the attic, we are considering immediately finishing the walls with wall heating after the insulation of the ceiling (from the inside). The big question is if this release capacity estimate is somewhat correct and how can I calculate the water content? Since it worked quite well with Ta 40ºC, I prefer not to replace everything right away. But of course we don’t want problems with scrolling, infinite unfreezes and the like.

* Rationale: I find the water content and release difficult to determine. I can barely find directions for column radiators. So the calculations are based on rather shaky assumptions derived from “stuff on the internet”. For the column radiators (see photo): calculated with 55W per member, I arrive at 68 + 25 * 55 = 5115W for the living room; at Ta 35 that would be around 1400W. For the room 110W per member * 20 = 2200W, at Ta 35 = 603W. Radiator rear chamber 210 * 60cm, type 22 = 2833W (Ta 35 = 777W); VVW kitchen 4.5m2 * 80W/m (estimated) = 360W.

Which brand?

The head of the house works a lot with Mitsubishi. So I looked at that a lot and the brand also attracts me because of the clear and comprehensive data books and the high adjustability. I prefer the R32 models, only they seem to be much louder (eg PUHZ-SW75 YAA does 58 dB(A) at 0/35; SUZ-SWM80 VA does 62 dB(A) at 0/35).

Lately I’ve also been looking at Vaillant with a slanted eye, also because little by little I’m considering going for a monoblock (install myself with help). COP values ​​seem to be close to Mitsubishi and the units seem significantly quieter (hard to compare though). The latter is important due to sensitive neighbors.

I find the Panasonic less attractive because of the noise level and also because of the appearance (it can be quite visible). Possibly with casing, but in my opinion it is better to immediately buy a quieter brand.

What power?

I find this quite difficult.
Some estimates of the necessary power based on current gas consumption:
– We heat at 19.5°C, 8.2m³ consumed in one day with an average of -2.4°C outside: (19.5- -10)/(19.5 – -2.4) * 8, 2 = 11.05 m³ (for 24 hours at -10 ) = heat requirement 99.45kWh = HP power 4.7kW.
– With very slight night reduction (1 day tested on April 1 this year): inside 19.0°C, average outside 1.4°C, consumption 8.2m³ (same coincidentally): (19.0 – – 10)/(19.0 – 1.4) * 8.2 = 13.5 m³ (per 24 hours at -10) = heat requirement 115kWh = WP power 5.5kW.
But: the historical gas consumption does not give a good indication because on the one hand we are still going to insulate quite firmly, but on the other hand we want to heat a larger part of the house.

I have done a number of heat loss calculations via bouwenergie.be. (I don’t have enough data for Ubakus about the precise construction of the walls and the like). It takes some guesswork, but hey, that results in a heating demand of around 7.3kW to heat 80% of the house to 20C. at 7. Heating 75% of the house reduces this to 7kW. My reasoning is that if it’s really -7 or colder we’ll study/work in the living room, so 7kW should suffice (we expect to live here for a long time, so our own habits may be the criteria).
The question is if this is not too much. The same site gives if I enter the current insulation with a desired temperature of 19ºC, outside temperature of -10 and 50% of the house heated (thus the conditions in the second calculation above) a heating requirement of 7.12kW. So in practice we arrive at 5.5kW. Of course, there is a considerable margin of uncertainty, but therefore it could be that we can do it with less.

I’m leaning towards 7kW anyway. It can modulate a little less backwards, but it runs more often at part load, which is cheaper and quieter. Do you see any problem with my reasoning? Would you dare to bet on this, or would you be better off getting a professional heat estimate?

DHW tank or heat pump boiler?

The other big question. We have a Quooker downstairs so it’s just bath water. We have a bathroom, but we don’t mind not using it anymore. Low power shower, four people, never really all in a row. We also have a ‘when it’s gone, it’s gone’ setting. 200l seems enough to me. We are still considering DWTW (there is a leak in the shower that needs to be fixed, maybe it can be done in a short time).

Given that we heat with radiators and we do not have very good insulation in the living room, I think it would be better to opt for a separate heat pump boiler, so that the heating is not interrupted by DHW. In addition: the central heating boiler is now in the attic, so also in terms of pipes it seems inconvenient to use a barrel boiler. But maybe I’m wrong? A WPB could be placed in the attic instead of the central heating boiler or in the bathroom. It is difficult to estimate if the attic floor is strong enough, it should be possible in the bathroom, now there is a bathroom that is also very heavy when full.

Thanks in advance for reading this long epistle… I look forward to some ideas!

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