ᐅ Is the heating system properly sized?

Hello everyone,

We are about to decide which offer to choose from the heating installer!
As always, the quotes and sizing vary widely!!!

A few facts:

Annual primary energy demand targeted at 42.5 kWh/m²a (according to the latest calculation), which is 31.5% better than a new build.
The house will have about 240 m² (2,583 ft²) of heated space, including bedrooms, dressing rooms, hallways, and a basement party room (which will only be fully heated at certain times), so not all areas will be heated equally. In addition to the 240 m² (2,583 ft²), there are about 60 m² (646 ft²) of basement space.

Currently, 2 people are planned to live in the house, but up to 4 people maximum are expected 🙂

The most attractive offer is for a Vaillant flexoCOMPAKT VWF 88/4 with 8.9 kW, a 185 L (49 gallons) domestic hot water tank, and a 50 L (13 gallons) buffer tank.

On the other hand, I received an offer for a Buderus system with over 13 kW, a 390 L (103 gallons) hot water tank, and a 300 L (79 gallons) buffer tank.

We have a “normal” showering habit and rarely take baths, so very large amounts of very hot water are unlikely.

Every opinion counts, the order is to be placed on Friday.

We have the 55/4 – that is 5.3 kW and, as far as I know, suitable for 190 sqm kfw55... however, the annual primary energy demand – especially with heat pumps – is a very poor metric for sizing the heating system...
You need a heating load calculation – without it, everything is just guesswork.
I would leave out the buffer tank – or is it intended for domestic hot water?

BeHaElJa schrieb:
We have the 55/4 – that is 5.3 kW and so far it seems suitable for 190 m² kfW55... However, the annual primary energy demand – especially with heat pumps – is a very poor value for sizing the heating system... You need a heating load calculation – without that, everything is just guessing. I would skip the buffer tank – or is it for hot water?

The heating load calculation will be arranged by the Vaillant provider after the contract is awarded. The contract guarantees that if the offered system is too small, the next larger one will be installed at the same price... What about the storage tanks? Is 50 liters (13 gallons) for heating and 185 liters (49 gallons) for hot water sufficient?

I wouldn’t consider installing a heating storage tank. So far, our hot water supply has been sufficient – but we’ll see how it goes once the children start showering every day.

The heating storage tank is mandatory and is not a normal buffer tank on the supply line, but rather for the return line – to provide a buffer when all thermostats are turned off at the same time.

There are different system diagrams that illustrate this setup.

For me, 185 liters (49 gallons) would be too little – we have just under 400 liters (106 gallons). We also enjoy long, hot showers...

Interesting. Of course, we don’t have that 😀 although we also don’t have any thermostats.