ᐅ Sizing of Hot Water Heater and Heating and Ventilation Systems—Is It Proper?

Hello everyone,

We are about to finalize the contract for heating and sanitary installations with the following conditions:

- Single-family house
- 170 m² (1830 sq ft) heated living area
- Solid construction with brick/concrete
- Underfloor heating
- Partially basement
- 4 people

We have been offered an Alpha Innotec LWD 70 HTD heating system combined with a Zehnder ComfoAir Q350 ventilation unit. Overall, I am okay with the devices, but I have some questions I’d like to discuss with the heating installer. Beforehand, I wanted to gather some opinions:

1. There is a hot water storage tank with a net volume of 180 liters (47 gallons). Is this sufficient for 4 people? The heat pump provides 7.7 kW only for hot water production (no solar or other systems).
2. It is a combined unit: Are the full 7.7 kW power for hot water and underfloor heating used alternately or only partially, but at the same time?
3. Is the heating system adequately sized?
4. Is the ventilation system adequately sized?

I would appreciate any experiences (possibly also with these devices) and opinions to prepare for the discussion.

Best regards

It is impossible to assess whether the device is sufficient without knowing the heating load.
Overall, it can be said that a particularly large heating system is not necessary to provide hot water.
If hot water is set as the priority, the underfloor heating will be "switched off" during the heating period. Because the entire screed and the rest of the building structure act as a heat store, this is hardly noticeable.
Whether the 180-liter (48-gallon) tank is enough is also uncertain. If you have a large bathtub and use the rain shower at the same time, it might be tight.
However, the plumbers usually know this...

4 people, hot water demand without major requirements (whirlpool tub, rain shower, 2 bathtubs used simultaneously...). However, make sure to have this confirmed by the plumber and not just assume "it will probably be fine."

For orientation only: After detailed inspection by the master plumber, this installation would have only been partially suitable for us:

5 people, 2 showers with a flow rate of about 9 liters per minute (2.4 gallons per minute) and expected almost simultaneous use of both showers.
Vaillant compact heat pump with a 170-liter (45-gallon) hot water tank, set to 55°C (131°F), would have provided only about 5 minutes of shower time per person based on these conditions. Vaillant states the operating range for households with up to 3-4 people. So, it might be suitable for you with 10 liters (2.6 gallons) more in the tank and 4 people.

The plumber’s suggestion: shower heads with a flow rate of 6 liters per minute (1.6 gallons per minute) and a constant hot water tank temperature set to 60°C (140°F). This was rejected.

I don’t really have a choice when it comes to storage size, do I? For the combination unit, it seems there is only one size (200 liters (53 gallons) = 180 liters (48 gallons) net).
The alternative would be another combination unit or an additional storage tank? Space would probably be tight and it would certainly be more expensive.

Yes, combination units are designed specifically for about 75 to 80% of standard families. Usually, they fit well, and especially prefab home manufacturers have virtually no work with the sizing. The underfloor heating manufacturer handles the dimensioning, which determines the heating load, then you select the next larger combination unit with or without ventilation next to it, and the manufacturer carries out the commissioning. End of story. However, if you have specific requirements or deviate from the standard, things often look difficult. And for some, having just three children is already enough to cause issues.

The size of a combination unit is naturally limited. More water cannot be accommodated in the available space.

However, an external storage tank is not much more expensive. You need to have the space for it, but a system with a small storage tank does have its advantages, as it refills significantly faster and delivers “bath temperature.”