Is an air-to-water heat pump sensible for renovation as an efficiency house monument (160% Energy Saving Ordinance)?

Hello everyone,

this is about the renovation of a semi-detached house [REH] listed as a monument for rental. According to the energy consultant (=architect for renovation), the use of an air-to-water heat pump is recommended because with a gas condensing boiler significantly more insulation effort (internal insulation 8cm instead of 4cm) and thus also reduced living space would result. The air-to-water heat pump could be placed in the basement, with the air intake/exhaust through the former coal window (basement top edge approx. 80cm above ground). Photovoltaics or solar thermal are not possible due to monument protection. So far, I have always assumed a gas boiler for the house; the gas connection is available in the street but has not yet been laid to the house (the existing system is a coal central heating system, but unused/uninhabited for 30 years).

As a rental property, it is of course also interesting that the green/red ideas of imposing the CO2 tax on the landlord would be unproblematic, and the electricity costs are billed individually with the tenant, so initially do not appear in the ancillary costs.

On paper, the air-to-water heat pump is clearly advantageous, but what does it look like here in reality? In 2017 we chose gas for the new building due to the high electricity consumption. Here it is an old building which may have 60% more primary energy demand than a new building. Can the paper figures for the air-to-water heat pump be trusted to some extent, or will the electricity consumption be a disaster?

Thanks in advance for the opinions – I had never really considered an air-to-water heat pump before and therefore have not really dealt with it yet.

Air-to-water heat pump is calculated to be good for the KFW - at the targeted efficiency level, the tenant will be happy...

There are the following questions. 1. What is the heating load? 2. Which heat distribution system is planned? Underfloor heating or radiators? 3. What supply temperature is targeted? Without knowing this, I would rather assume a high heating load, with radiators and high supply temperature. That argues against an air-water heat pump. In the case of rental, I would prefer gas over a heat pump, as it is less problematic. Alternatively, if storage space is available, pellet.

1. How high the heating load is.

It depends on whether eight or four cm of insulation is installed. Higher heating load with air-water heat pump than with gas. Madness what results come out of KfW calculations.

Hello,

the planned heat distribution is underfloor heating, so a relatively low flow temperature. The house has about 120m2.

To just achieve the Efficiency House Monument standard, the following results:
with gas and 8cm insulation (WLG 042) inside:
Primary energy (per m² of usable area): 117.43 kWh/(m²a)
Transmission heat loss Ht`: 0.468 W/(m²K)
with air-water heat pump and 4cm insulation (WLG 042) inside:
Primary energy (per m² of usable area): 85.72 kWh/(m²a)
Transmission heat loss Ht`: 0.537 W/(m²K)

What kind of energy (electricity) costs are we talking about for the air-water heat pump at temperatures around 0°C, is it 10€ per day, per week? Besides the fact that the tenant pays for the electricity, I already have concerns because of the discussion about landlords’ participation in the CO2 tax. Thanks to the subsidies, a high-quality, interior air-water heat pump would also have lower investment costs than the gas condensing boiler (since half of the insulation material can also be saved). As a side effect, the rentable area also remains larger. Of course, I don’t want constant tenant turnover due to high electricity costs, but if it’s about ±50€ per month between gas and air-water heat pump, I believe this is acceptable for a tenant. How would an additional wood stove affect usage (not the calculation)? The chimney is available and would not be needed with an air-water heat pump.

The primary energy per sqm from the KfW calculation is initially not very useful for estimating the heating load. If you have Ht and the building volume, you can at least estimate it. Google Trenchplaner and ring trench collector. The first consideration is what supply temperature you can best achieve with the underfloor heating. For this, however, you need a room-by-room heating load calculation in order to be able to calculate the underfloor heating for each room. Then you can see what supply temperature is required. Only then can you decide whether an air-to-water heat pump makes sense at all or is just a money pit.

Another point is the question of whether KfW requires ERR. If yes, the heating installer will install either a separation buffer or an overflow valve to ensure the minimum volume flow of the heat pump. Since you are renting out, you have no influence on whether the ERR is used by the tenants or not. This means you will not be able to close the OV. Open overflow valve or separation buffer are the entry to a cycling heat pump. This means increased wear and premature failure. Then the cost advantage over gas heating is gone. A mega disaster is a combination storage tank, then you can also have problems with the hot water.

I myself have an air-to-water heat pump and am very satisfied. However, I also spent a long time dealing with the design of the underfloor heating and the technology of the heat pump including operation. That is why I would never choose a heat pump in a rental property. It has too many pitfalls and demands discipline and knowledge from the operator/tenant. Therefore the question whether a pellet heating system is not still an option, otherwise gas.