ᐅ Offer validation for air source heat pump in a 155 m² semi-detached house built in 1982

Hello,

We have a 155m2 (1670 sq ft) semi-detached house built in 1982. The insulation is solid sand-lime brick with a cavity, and the windows will be replaced next year.
Our 14-year-old Solvis Max Gas system’s buffer tank has rusted through. It still works, but the repair is estimated to cost €12,000. Therefore, we are now installing 10 kW peak photovoltaic panels on the roof and removing the solar hot water collectors.
Our annual gas consumption was 19,000 kWh. The hot water is maintained at 48°C (118°F), and we have underfloor heating throughout. There is also a thermal storage connected to an open fireplace, which we would like to keep (+ possibly install a new insert in 2-3 years). We are receiving 40% funding support.
We also require a system separation and would like to replace the heating circuit manifolds to allow proper individual room temperature control.
We have received two offers so far:

1. Vaillant Arotherm plus 105/6 heat pump
- 300 l (79 gal) buffer tank for domestic hot water and fireplace integration with a drinking water station
- 200 l (53 gal) buffer tank for underfloor heating
- System separation
- New heating circuit manifolds (quoted at around €5,000, the current manifolds date from 1982)
- Photovoltaic-powered electric heating rod as backup
--> €33,700, 4-6 months delivery time

2. Daikin Arotherm 3 H HT with 18 kW and bivalent indoor unit with 500 l (132 gal) stainless steel tank
- No system separation required due to stainless steel tank
- Backup heater
- 18 kW offered as no on-site inspection has taken place yet
€38,500

From my research, the Vaillant unit has better COP values (about 0.5 higher), but it requires two buffer tanks in the basement, and the heat pump is larger and mounted next to the front door.
The Daikin might be sized smaller in the future and could therefore be somewhat cheaper.

What would you recommend? Should 10 kW capacity be sufficient, especially since the windows will be replaced as well?
Does it make sense to renew the heating circuit valves for individual room control?

I don’t want to request further offers, as nobody seems to have time anyway…
Thank you very much for your advice!

Best regards,
Christian

Bakero99 schrieb:

What would you recommend to me?

First of all, have a heating load calculation done, taking the new windows into account. Everything else is just guesswork.

Currently, 19,000 kWh of gas costs around €5,000 per year. The question now is how your underfloor heating is designed. The house is "old," so the heating circuits were probably installed with wide spacing, meaning you need a high flow temperature. This also means high electricity consumption for the heat pump. The COP over the year might be around 3, which equals roughly 6,700 kWh of electricity—that’s about €3,000. So overall, switching is a good idea (of course, this is a very rough estimate).

However, your statement that your house is well insulated is unfortunately incorrect. By today’s standards, it would be considered a "disaster." Can you improve insulation beyond the windows? I’m thinking of insulating the basement ceiling (if you have a basement) or insulating the roof or the top floor ceiling. This is the most sensible first step. It ultimately results in lower energy consumption, lower flow temperature, and thus a smaller heat pump with reduced initial and ongoing costs.

P.S. Depending on your budget, you might also consider renovating the underfloor heating. It is "old" as well and almost certainly not optimized for use with a heat pump. This is usually well supported by subsidies.

Hello,
What was the maximum supply temperature recently?

More information about the house and the current heating system was useful. I don’t think the house requires a heating load of more than 9 kW.
It is also not advisable to keep the hydraulic buffer tank. It complicates both the hydraulics and the control system.
If you then add a 200-liter (53-gallon) buffer storage tank for the underfloor heating, it really makes me cringe.

Daniel-Sp schrieb:

Hello,
What was the highest flow temperature recently?

Hello, the flow temperatures have always stayed below 40°C (104°F). The underfloor heating pipes are embedded in the screed with tiles on top. The roof has minimal insulation but features a beautiful open timber frame, which I plan to keep as is. We also had energy consultants here who didn’t find any major issues regarding the basement insulation. I also enjoy burning my 2-3 meters of wood per year and appreciate the fire. Of course, the wood heats the air just like the water does, but maybe someday there will be a more efficient water-heating stove.

As I said, the old heating system is worn out, and I don’t want to install another overpriced Solvis system. The solution with two buffer tanks isn’t bad: one runs at 35°C (95°F) for the underfloor heating, and the other at the top of the tank at 50°C (122°F) for domestic hot water.

Daikin uses a stratified tank, which worked well with the Solvis system too.

The question is which system is better and whether individual room controls with separate smart home temperature sensors would be helpful. The problem so far has always been that the Solvis sensor was located at the coldest spot in the house, while the rest of the house—for example, due to sunlight—was at 27°C (81°F)... I’d appreciate further feedback!

Bakero99 schrieb:

We also had the energy consultants inspect, and they didn’t find any significant issues regarding the basement insulation.

Is the basement heated or unheated? Insulating the basement ceiling of an unheated and otherwise uninsulated basement can quickly save about 10% on heating costs – it pays off within a few years if you do it yourself.

Bakero99 schrieb:

Hello, the supply temperatures were always below 40 degrees Celsius (104°F).

Then go for an affordable and simple heat pump without water jacket complications or layering storage tanks.

Bakero99 schrieb:

The solution with two buffer tanks isn’t bad; one runs at 35 degrees Celsius (95°F) for underfloor heating, and the other at the top of the tank at 50 degrees Celsius (122°F) for domestic hot water.

Actually, it isn’t.

Bakero99 schrieb:

The question is which system is better, and whether individual room control with separate smart home temperature sensors is worthwhile.

It does bring something—namely costs. Ideally, an underfloor heating system is set up with proper hydraulic and thermal balancing and then runs efficiently on its own. Due to the system’s thermal inertia, you can’t really control it dynamically anyway.