Size Heat Pump?

Hi Everyone,

I am new to the forum and I have been designing my Heat Pump System. Ideally to the best SCOP possible.

I have a house with underfloor heating only and have calculated the flow rates of my various rooms along with heat loss calculations based on the MCS spreadsheet.

I have a total of 14l/m for all my ufh loops combined (longest loop requiring 1.3l/m). I have a heat loss of 5.7kwh and plan to have a dT 5C if possible. This leaves me with an approximate flow rate of 840 l/h.

I have been looking at a Vaillant AroTherm Plus 7kw as a potential ASHP which has a minimum flow rate of 540l/h and maximum of 1205 l/h.

Is this under sized to achieve the best performance based on flow rate or what do you think???

What is the design (outdoor) temperature?

Has the heatloss been measured?

At a 35C flow temperature the output of these units exceeds the nameplate. You’re almost at the 5 kW unit. The 7 kW unit would be more than sufficient. Datasheet and excerpts:

kl-06-e2-verze-01-01062021-2108745.pdf (6.0 MB)

kurzanleitung-arotherm-plus-2105-2146405.pdf (5.4 MB)

@markocoheat Thanks for the response!

The outdoor design temperature is -12 C in Denmark (Cold!!!)

And the heat loss according to the MCS excel spreadsheet is 5,7kw based on my calculations.

One last question… what does rps stand for on the graph??? Something to do with the pump I am guessing?

What air change rates do you use in the MCS spreadsheet?

rps is compressor frequency/speed.

I do have a question with regards to the MCS spreadsheet…

Where do I find the following for the Vaillant AroTherm Plus 7Kw:
Output @ ODT/DFT (kw)
Output @ 0°C/HW (kw)


@marko Air change rates are set to 0,5 for standard rooms and 1,0 for web rooms (Bathrooms, kitchen and utility)…

I also filled in the following heat loss calculation based on gas consumption
Another Heat Pump Spreadsheet: Beyond the Rule of Thumb | Protons for Breakfast.

This recommended a heat pump size of 4,8kw but I guess it is better to be slightly larger than on the small side???

The disadvantage with the gas consumption spreadsheet calculator is it uses Aberdeen as a location rather than Copenhagen which is quite different as you can see from the data I filled in below.

From the datasheets above.

At -12C (design temperature) and 35C (design space heat flow temperature) the max output of the heat pump is…?

I think the second asks what the output at 0C is at hot water preparation temperatures. Not actually sure to be truthful but don’t think it relevant because your space heat load is the dominant one.

Air change rates look ok. 7 kW unit appears safer choice and it doesn’t have that much higher a minimum output.

It’s probably worth looking for real performance figures on the HG cylinder Vs the Unistor.

Do you have MVHR (heat recovery ventilator) in the property? (your ventilation loads must be high compared with fabric loads)

Hi @marko,

Thanks a lot for all the help and advice! At present we don’t have An MHRV system but we do plan to install one and some of the ducting is already installed. We do how ever have a few large rooms with vaulted ceilings (4.5m) in some rooms which I am sure impacts the heat loss calculations.

Based on my current calculations we will need a flow rate of 916 l/h. I am planning to try and use the ASHP’s pump so that I don’t have to add a buffer tank and multiple pumps which tend to impact the efficiency of an ASHP design.

My longest underfloor heating loop is about 90m with 16mm pex. Which gives a flow of 1.6 l/m.

The total flow for the entire house is 15.28 l/m or 916 l/h. I can see that the AroTherm 7kw has a maximum flow rate of 1205 l/h.

Do you think that my flow rate is too high to optimise the ASHP SCOP? Based on the graphs you sent (If I understand it correctly) the maximum SCOP would be 2.5 at -12C

I would love to hear any thoughts on this?

Thanks so much for all the input it has been great to hear other people’s thoughts especially with your experience. :slight_smile: