Sudden temperature increase across heatmiser stats?

[Andehh]

Had our system running fine for 18 months now. UFH throughout, one stat per room and Vent Axia MVHR.

 

It's a well insulated and average air tight home/poor to build hub standards (around 3), but it's cheap and warm.

 

Something I havnt seen before though, is a sudden increase in lots of room temps this morning at 5.45am we experienced.

 

MVHR was switched off due to frost mode (which I don't think I've seen before), but I'm assuming it had been off all night as the outside temps were consistent at around -3 all night throughout. The MVHR had frost mode enabled and 0% flow.

 

Heating had been on low and slow all night. All doors were closed during this time... Below/attached is an example of what happened with the sudden jump up. Most rooms have similar, those with closer vicinity to MVHR more so.

 

The wave type rise and falls, I think is the MVHR on a different frost management mode.... Ie it's not so cold it's shut down, but is managing itself by reversing the air flow occasionally, so the air movement is reflected in the stat readings??? (saw this last year as well, but only when sub zero ish cold)

 

But the sudden jump up at 5.45, I just can't explain? But I'm sure it's linked to the MVHR being off, which I've also not seen before.....?

 

Edited January 3 by Andehh

[ProDave]

First I have heard of a "frost mode" on an MVHR unit.  Ours just continues as normal regardless of outside temperature.

[Andehh]

From google/AI..... Apparently the below,

 

Vent-Axia's anti-frost airflow mode, also known as frost protection mode,

 

automatically activates when the supply air temperature is between 0° and -20°C

 

• Reduces supply airflow

  To prevent frost from forming on the heat exchanger, the supply airflow rate is reduced.
• Increases extract airflow

  To prevent frost from forming on the heat exchanger, the extract airflow rate is increased.
• Stops and restarts supply motor

  Depending on the temperature below 0°C, the supply motor can stop for 15 minutes and run for 45.
• Switches off supply fan

  If the supply air temperature is -20°C or below, the supply fan switches off.

 

[Andehh]

If it could be/might not be related, we also find when we open the door the temperatures all jump up quite a bit in the winter. I assumed/think this as the corridor is warmer then the bedrooms (central if house, no windows, no stat on it) , albeit it it isn't an obvious thing?

 

Bedrooms are 4.5m ceilings despite only being 9sqm floor as they have Mezzanines, so they are not the warmest rooms due to lack of floor surface area and high ceilings, also north facing.

Edited January 3 by Andehh

[JohnMo]

23 minutes ago, ProDave said:

Ours just continues as normal regardless of outside temperature.

Our doesn't seem to suffer even down to prolonged -9.

 

Are your thermostats in the air flow from the MVHR, so MVHR off then read room temp instead of air from MVHR.

 

UFH and that quick change of temp isn't realistic. 

[Andehh]

Yeah, they likely are in the air flow... So it could just be basic thermodynamics at work.... You can't fix everything!!