ᐅ Advantages and Disadvantages: Gas + Solar + Hydronic Fireplace for Domestic Hot Water?

Hello everyone,

we are planning to build a solid wood house and would like to qualify for KfW funding. Since we already have a gas connection on the property, we prefer a gas condensing boiler. However, without solar panels, we would only qualify for the funding if we increase the wall thickness or use an air-to-water heat pump. As mentioned, the gas connection is already available on the property, and I am not very keen on heat pumps mainly due to the noise (we will be building without a basement). Since we have always dreamed of having a fireplace or tiled stove, we were advised to install a hydronic fireplace to also heat the domestic hot water. This way, we would have an energy mix that seems quite appealing to me at first glance, although I cannot yet estimate the economic aspects. Does this solution make sense?

Good luck Telramund

I should add that we were recommended a water-carrying fireplace because it directs most of its output to heating the hot water; otherwise, the room would overheat too quickly.

Hello,

Telramund schrieb:
... However, without solar, we will only qualify for funding if we increase the wall thickness...

This is not a disadvantage, but rather an advantage, as it reduces the heating energy demand.

Telramund schrieb:
... Since we have always dreamed of a fireplace/tile stove, it was suggested we install a water-heated fireplace to also warm the heating and domestic hot water.

A water-heated stove is more compatible, if at all, with a gas condensing boiler than with, for example, a heat pump. However, its energy contribution is only partially recognized in the certification. This is not without reason, since after the initial enthusiasm, the usage usually declines significantly over the years due to the manual effort required.

Telramund schrieb:
... but we cannot yet assess the economic aspects. Does such a solution make sense?

As always, this depends on the specific conditions. If you have to buy firewood, the economics will suffer. The additional investment or financing costs also have an impact.
The overheating issue (without water circulation) only occurs with very low space heating demands in the room where the stove is installed, as well as with insufficient storage mass or post-heating surfaces (typical DIY store stoves). There are also combustion chambers available with low heat output.

Best regards

Hello Euro,

Thank you very much for your answers. As expected, I naturally have a few more questions and comments afterwards.

€uro schrieb:
Hello,
This is not a mistake, but rather an advantage, as the heating energy demand becomes lower.

The question, of course, is how this will affect the overall cost. However, this must be discussed with the house manufacturer to see how an appropriately adjusted wall thickness impacts additional expenses.

€uro schrieb:

A water-based fireplace fits better, if at all, with a gas condensing boiler system than, for example, with a heat pump. However, its energy contribution is only partially recognized in the official calculation. This is not entirely unjustified since, after the initial enthusiasm, use tends to decline significantly over the years due to the manual effort involved.

We currently also have a tiled stove, which we actually prefer to use rather than the heating system because the warmth is more comfortable. However, we mostly only use it when the whole family is home (e.g., weekends or holidays).

€uro schrieb:

The overheating issue (without water circuit) only occurs with very low room heat demand in the installation room and insufficient thermal mass or additional heating surfaces (typical DIY store fireplaces). There are also combustion chambers with lower heat output.

We were recently advised to choose a fireplace that sends only about one-third of its output to the room and uses the rest to heat water. What exactly does "too low room heat demand" mean? Does this imply it is better to select a larger fireplace with sufficient storage capacity that radiates heat more slowly but for a longer time? How can the interaction between the gas condensing boiler and the fireplace be optimally managed? We are planning to install underfloor heating, which has a slower response compared to radiators.

Good luck, T

Telramund schrieb:
...The question, of course, is how this will affect the price in the end. However, you need to discuss with the home builder how an accordingly adjusted wall thickness influences the additional costs.

The installation effort is almost the same; only the additional material costs have a slight impact!

Telramund schrieb:
...We were recently advised to choose a fireplace that delivers about only one-third of its output to the room and uses the rest to heat water. What exactly does too low a room heating load mean?

When sizing heating systems, the standard heating load and, for each room, the room heating load are essential. If the room heating load is unknown, you are basically guessing, estimating, or assuming. Whether this provides a reliable basis for a major investment decision is something everyone must decide for themselves.

Telramund schrieb:
...We will install underfloor heating, which is slower to respond than a radiator.

Exactly. Underfloor heating is slow to react and therefore operates less optimally with a wood stove without heat storage. This is very different with a masonry heater combined, for example, with radiator heating.

Because of conflicting requirements, you cannot have everything; priorities must be carefully weighed.

Best regards.

Okay, understood. You need to have all of this calculated by someone who knows what they're doing. Can any architect or energy consultant do this, or are there specific "certifications" that show I am dealing with a knowledgeable professional?