ᐅ New construction KfW 50 energy standard with air-source heat pump and hydronic fireplace

Hello everyone,

we are currently building a solid brick house with 36cm (14 inch) thick walls. The main heat source for hot water and heating will be an air-source heat pump. This will be supported by a photovoltaic system.

A fireplace is planned anyway for comfort reasons. I am now considering whether we should choose a water-bearing (hydronic) fireplace to support the air-source heat pump during winter, so that the heating element doesn’t have to run constantly.

The idea is to utilize the energy from the exhaust gases and transfer it as simply as possible to hot water/heating. Does a water-bearing fireplace make sense, or would it be more economical to use a conventional fireplace and work with storage bricks in the chimney?

Thank you for your feedback.

Best regards

From an economic standpoint, a water-heated fireplace generally only makes sense if you use it regularly and extensively—meaning daily during the winter. Otherwise, the effort (costs, maintenance, technology, buffer tank, etc.) usually does not justify the benefits.

A standard fireplace with heat-storing stones or thermal mass (e.g., masonry heater or storage fireplace) is much simpler, more affordable to install, and requires less maintenance—while providing the same cozy atmosphere. Especially if you have photovoltaic panels combined with an air-source heat pump, you will rarely need additional support from wood burning, unless you specifically want to become more self-sufficient.

So, if it’s mainly about comfort and a bit of extra warmth, a regular fireplace is more than enough. However, if you have a steady supply of wood and want to heat regularly, a water-heated system can be worthwhile—but it should be paired with a properly designed system and a buffer tank.

We also had the chimney flue installed, as well as the conduits embedded in the screed between the planned location of the fireplace and the utility room. We were convinced for a long time that this would be the most efficient solution. It will NEVER pay off! In the end, we didn’t even set up the fireplace at first because we had never lived in a modern new build with a heat pump and photovoltaic system before, and we wanted to try it out for one winter. We came from a rental property with a fireplace that we used daily in winter. The only thing we miss is the charm of a fireplace fire. On the other hand, we don’t miss the dirt, the work, or the storage space needed for the wood.

After exactly one year in the new house, I would say it was the right decision not to install a water-based fireplace. Over 12 months, we only needed to buy 2,800 kWh of electricity with our photovoltaic system, 98% of which was for heating and hot water during winter. That is less than $1,000 per year. After deducting feed-in tariffs, it comes down to about $500. How could you really “save” or optimize much with a fireplace in that case? Wood costs money, the chimney sweep costs money, and maintenance of a water-based fireplace also adds expenses.

We also used the argument of becoming self-sufficient for a long time. But if that ever really happens, we will have much bigger problems than a warm living room.

Today, I would focus more on topics like bidirectional charging, compatibility between the photovoltaic system and the heating system (our idm heat pump does not communicate with the SMA Home Manager 2.0 – a major fail), and other details that actually provide real added value.

The chimney flue with everything included, as well as the preparations for the connections of the water-based fireplace, cost us around $15,000. We now see it as an “investment in the future.” If we ever sell the house, that might even be a key deciding factor for a potential buyer. But we would not do it again and would rather invest the money saved in upgrading, for example, to wood/aluminum windows.

The cost-effectiveness of water circulation can be made to look good if you want it to.

From my layperson’s perspective, the critical issue is that the efficiency of water circulation increases with the surface area in the flue. A large surface area in the flue has several disadvantages. I consider the most serious issue, especially in connection with a wood stove that already does not burn very cleanly, to be the fouling of the surface with soot. If you decide to go with water circulation, make sure to use the highest quality firewood.

I’m not sure what KfW50 refers to, but in terms of cost-effectiveness, even that is questionable. If you meant KfW40, it can be said that the additional 100,000 to 150,000 EUR premium over the building energy code standard will not be saved on heating costs during a lifetime. Electricity and gas prices would have to rise extremely high for that to pay off.

Also, photovoltaics—if purchased expensively from a specialized company and financed with a loan at 3%, 4%, or 5% interest, often combined with an oversized battery storage—hardly pay off. It usually takes 15 to 20 years to break even. If you have a flexible electricity contract and consume power smartly when it is cheapest on the market, private photovoltaics generally do not make financial sense.

Thank you for your input. I think I will let this topic rest. The cost versus benefit is not reasonable.