air temperature uplift even on bypass?

[mvincentd]

During 100% bypass how much temperature increase should I expect to see between 'external air temp' and 'supply air temp' (as measured by the unit...at the unit)?

 

I'm typically seeing 'supply air temp' at 2 degrees higher than 'outdoor air temp' when bypass is 100%.

 

To be clear, what the unit is measuring as 'outdoor air temperature' can only be outdoor air that has travelled approx 5m through ducting to the point of entry to the unit.  It had been suggested to me that the house was warming the incoming air along its approx 5m journey.  While this might happen it surely cannot be the explanation why the unit identifies that 2 degree difference, given there are no sensors up any ducting outside the unit itself.  (It would seem academic to this question how well the routes to air are insulated, but for the record they are insulated to spec'.)

 

So, between fresh air entering the unit and it bypassing the heat exchanger to leave the unit as supply air, it is gaining 2 degrees.

 

As I write this the house is currently 23.4 degrees in the utility room where the mvhr unit is and 23.7 in the main living/kitchen/diner area, both according to Heatmiser wall stats.  The mvhr says;

outdoor air 19

supply air 21

extract air 24.5

exhaust air 24

bypass 100%

I've set the mvhr to its 'cool' lifestyle setting and dialled the settings default temperature down to its minimum of 18 degrees.

 

Thanks

 

  

[Jeremy Harris]

I've found that the temperature sensors used in our Genvex MVHR aren't very accurate.  There is about a 1.5°C error in the sensor in the wall mounted control box (which can luckily be calibrated out) and similar errors on the internal sensors.  I've got some pretty accurate DS18B20 sensors scattered around the place and I know these are all accurate to within about 0.2°C, when checked against an NPL calibrated mercury in glass thermometer, so I'm pretty sure that the errors are with the MVHR sensors.

[mvincentd]

Ah, the simple explanations are the best!

So according to your accurate independent sensors, you get pretty well zero uplift when new air passes through the unit on bypass?

In which case i'll ignore the readouts on the unit because they were leading me to believe the mvhr was less helpful against our overheating than open windows currently!

Thanks

[Jeremy Harris]

According to the display on our MVHR, the exhaust air always seems to be about 1°C warmer than it really is, the fresh air intake sensor reads about the right temperature and the fresh air supply sensor reads about 1.5°C too low (it always looks as if the MVHR isn't warming the fresh air by as much as it really is).   I don't know what type of sensors it uses, probably fairly cheap thermistors I think, but it does seem that they aren't well calibrated.  The same goes for the room thermostats.  We have two identical wireless thermostats, that have 0.1°C switching hysteresis, mounted close together on the hall wall (one controls heating, one cooling).  They always read differently, usually by around 0.3 to 0.4°C, despite the fact that they are within inches of each other.

[SteamyTea]

If the sensors are DHTs, they are pretty dire to be honest. The 22 is better than the 11.