ᐅ Heat pump is not compatible with a hydronic (water-heated) fireplace.

Created on: 20 Oct 2023 18:54

Hello everyone,
I hope someone here can help us because our heating engineer doesn’t seem to be able to…
Key data
New build 2022 with 250 m² (2,691 ft²)
Heat pump Viessmann Vitocal 200-A
Multifunction storage tank MFS1000S
Water-heated stove with heating capacity on the water side of 8 kW
Photovoltaic system
Electric heating element

I am attaching the schematic of the storage tank layout.
The stove was initially connected under “old” and now it has been modified.
Everything works fine as long as the underfloor heating is off.
When the underfloor heating is on, our hot water temperature doesn’t exceed about 55°C (131°F).
When the underfloor heating is off, the temperature ranges between 65–75°C (149–167°F), either via the electric heating element during sunshine or by stoking the stove.
Now, however, this energy seems to be wasted. It can’t be that the underfloor heating requires 70°C (158°F) supply temperature to achieve a flow temperature of 35°C (95°F), right? The return water is only slightly cooler. So the water should only need to be heated by a maximum of 5°C (9°F).
We assume either a) something is still connected incorrectly, or b) the cold water from the underfloor heating causes such circulation that the hot water cools down within minutes.
What can be done about this?
Has anyone experienced similar problems?
Normally, we should almost run without electric power, but currently the heat pump switches on almost every hour to heat hot water…

Technical schematic of a heating system with flow/return chimney new/old and pumps.

Jessica38821 Oct 2023 08:42

Buchsbaum schrieb:

I’m not an expert, so I’ll try to explain it in simple terms. Please don’t take it too seriously. I don’t have much knowledge about heat pumps.

You have a buffer tank with an electric heating element, a supply and return line for the fireplace, and a supply and return line for the heat pump.

You draw your domestic hot water from part of the buffer tank. The hot water temperature should be at least 55 degrees Celsius, preferably 65 degrees Celsius (131°F, preferably 149°F).
When do you run the fireplace? Surely not when the outside temperature is 20 degrees Celsius (68°F). What happens when, like today, there is no sun and the photovoltaic output is almost zero?

In theory, it all sounds good. I would have installed the domestic hot water system and the heat pump separately. Technically, your domestic hot water system works like a tankless water heater. So today, your hot water is generated electrically. I assume the heat pump tries to produce hot water as well.

I think there are control issues here, priority switching, and so on.

Your buffer tank consists of three parts: the heating water storage, the solar heat exchanger, and the domestic hot water heat exchanger.
It is surprising that your domestic hot water heat exchanger only has a volume of 56 liters (15 gallons). The solar heat exchanger has a capacity of just 17 liters (4.5 gallons). You have two electric heating elements installed, each with 9 kW. So, if your domestic hot water is heated on demand, the electricity meter runs up to 18 kW. You must have a cheap electricity provider, or you don’t shower often.

The fireplace heats only the heating circuit. The heat pump supply will always maintain the water at about 35 degrees Celsius (95°F) supply temperature. And you have 250 square meters (2,690 square feet) of living space. The water will cool down by more than just 5 degrees Celsius (9°F). When I heat with a 50-degree Celsius (122°F) supply, about 20 degrees Celsius (68°F) return arrives at the oil boiler. The heat in the house has to come from somewhere, and underfloor heating is just a large heat exchanger.

8 kW heating power from the fireplace is very little, even when heating water.

At least you were clever enough to have a fireplace installed. Many don’t have one. When I come home freezing from skiing in winter, I want to sit by a warm fireplace or at least a hot radiator, not in a minimally heated house. People get older. I’m old enough to decide for myself how I heat. I don’t need green moralists telling me to save the world climate with my heating system while they fly somewhere for 50 euros. No thanks.

My responses are between yours, sorry I’m still learning how this works here..

RotorMotor21 Oct 2023 08:46

@Jessica388 what exactly is your problem now?
Are the heating costs too high? Is the shower water too cold, or are the rooms not warming up or too warm?
Or do the numbers just feel off?

And a small tip, Buchsbaum really has no idea what he's talking about.

Jessica38821 Oct 2023 08:48

RotorMotor schrieb:

Can you imagine it more easily if you just take 5x 5 degrees from the thermal storage?
So basically, with each full cycle through the screed, 5 degrees are lost in the storage.
Of course, this is not a stepwise but a continuous process, but heat is constantly transferred from the storage to the screed.

1 ton of water storage to 30 tons of screed.
That means roughly (simplified), if I cool the storage by 30 degrees, the screed only warms up by one degree.

Thank you for the clear explanation. I understand that.

So you mean everything is working correctly in our case? And despite burning 10 m3 (353 cubic feet) of wood over the season, we should accept a winter heating electricity consumption averaging 65 kW per day? Is that normal?
I don’t know…

WilderSueden21 Oct 2023 08:55

#10

This is certainly an excessively high consumption. In general, combining this with a fireplace in a new build is rather questionable (due to excessively high heat output). Having a water-based system makes things more complicated, as now two heat generators operate at opposite ends of the spectrum. The problem is most likely due to the control system.

Jessica38821 Oct 2023 09:03

#11

WilderSueden schrieb:

This consumption is definitely too high. In general, combining a fireplace with a new build is rather questionable (due to excessively high heat output). Making it water-heated doesn’t make things easier, since there are now two heat sources operating at opposite ends of the spectrum. The issue is most likely related to the control system.

Do you have any idea what I should be looking for to find the problem? As I said, we don’t want a second winter with such enormous electricity costs, and our heating installer probably doesn’t have much expertise but insists that everything is fine.

RotorMotor21 Oct 2023 09:07

#12

Jessica388 schrieb:

Thanks for the detailed explanation. I understand now.
So you’re saying everything is working correctly for us?

Based on the temperatures and other details you’ve shared so far, everything seems to be as expected.

Jessica388 schrieb:

And despite firing the stove with 10 m³ (350 ft³) of wood over the season, we’re supposed to settle for an average daily heating electricity consumption of 65 kWh in winter? Is that normal?
I don’t know...

You probably mean 65 kWh, right?
That would actually be way too much per day, especially considering the relatively high temperatures we still have.

I simply suspect that your electric heating element (heating rod) is running.
Most likely too many room thermostats are closed, causing an overall supply temperature that’s too high, which is not good for the heat pump.
So, the usual steps: turn off the electric heating element, lower the balance point significantly, perform hydraulic and thermal balancing, increase the ERR, etc.

Also, it might be worth taking another look at your house itself.
A new build that consumes 10 m³ (350 ft³) of wood doesn’t seem very airtight to me. ;-)
How do you ventilate? Do you sleep with an open window, for example?

ᐅ Heat pump is not compatible with a hydronic (water-heated) fireplace.

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