ᐅ Lower Indoor Temperature After Starting Operation of Mechanical Ventilation with Heat Recovery

Hello everyone,

On Thursday, our controlled residential ventilation system (Vaillant recoVair 260 E with heat recovery) was put into operation.
We have an indoor thermometer, and since the system started, the temperature has been about 1 degree (maybe a bit more) cooler than before.
It feels noticeably fresher in all rooms (including the unheated ones).
My question is, can the heat recovery unit not warm the incoming air sufficiently? Have you experienced a similar effect?
I understand this might be due to the constant air exchange, but the consequence would be needing to heat more, which seems to contradict the energy efficiency advantage of passive houses.

Thanks for your opinions.

chrisw81 schrieb:

Did you also experience such an effect?

No, not at all. I would have pushed my heating engineer hard in that case. That's exactly what heat recovery systems are for – maybe your air exchange rate is too high, or the heat recovery isn't working properly.

chrisw81 schrieb:

My question is, can the heat recovery ventilator not warm the incoming air sufficiently?

No, of course it cannot. The heat recovery ventilator only recovers a portion of the heat. The remaining heat loss is called ventilation loss and is factored into the calculations.

chrisw81 schrieb:

Have you also experienced such an effect?

Yes, of course—the physics are the same for everyone. You simply coordinate the heating system and ventilation properly, and that’s it.

With mechanical ventilation with heat recovery, the air just feels “fresher” because it actually is, and that can feel unusual at first.

chrisw81 schrieb:

I understand that this happens because of continuous air exchange, but wouldn’t the consequence be that you have to heat more, which would contradict the energy efficiency advantage of passive houses?

No, there is no contradiction here. You don’t have to “heat more,” you only need to provide enough heating to compensate for all heat losses that occur through thermal bridges, ventilation, walls, windows, roof, etc.

chrisw81 schrieb:

Since the system started operating, the temperature is about 1 degree (maybe a bit more) lower than before. It feels noticeably “fresher” in all rooms (including unheated ones).

This effect is probably not caused by the mechanical ventilation with heat recovery itself, but rather by the unheated rooms increasing the heating demand. The heated rooms struggle to maintain temperature because they also have to compensate for the temperature loss through adjacent unheated spaces.

Hello,
if your system has a bypass for the heat recovery unit (so you can draw cool air into the house at night during summer), check if it might accidentally still be open. We had the same issue with our system once and wondered why the air coming from the ventilation ducts felt so cold.

Best regards,

Andreas

andimann schrieb:

Hello,
if your system has a bypass for the heat recovery unit (so that you can draw in cool air into the house at night during summer), check if it might still be open by mistake. We had the same situation with our system once and wondered why the air coming from the ventilation ducts felt so cold.

Best regards,

Andreas

Yes, there is such a bypass, I can check again. But it shouldn’t actually be active at the current temperatures.

Measure the temperature at the valve with an IR thermometer, preferably at the point where there is a short duct run from the mechanical ventilation with heat recovery system. Where is the mechanical ventilation with heat recovery system located?