MVHR warming incoming air on 100% bypass ! Zehnder Q350

[FarmerN]

 

 

The display on my Zehnder Q350 shows the incoming air at 17.5 C but air entering rooms at 20 C.

Seems odd to me, is this normal or do I have a problem?

Set on 100% bypass ,cool temperature profile, Bypass on Auto.

Surely the air should be coming in at outside temperature?

5am and still 17.5 outside in a rural area.

[SteamyTea]

What temperature is the air in the room?

 

You could switch the fans off and see what happens.

[FarmerN]

Unit Currently showing

Extract air 25 C                        from rooms

Outside Air 21 C                       into unit

Supply  Air  22.5                      into rooms

 

Bypass 100%

230 CuM /Hr for a 210 Sq M bungalow

Edited 12 hours ago by FarmerN

[MikeSharp01]

1 hour ago, FarmerN said:

The display on my Zehnder Q350 shows the incoming air at 17.5 C but air entering rooms at 20 C.

It is still coming through the fabric of the house so maybe it's picking up heat from the ductwork etc?

[FarmerN]

The figures are all taken from the MVHR unit display

I presume the sensors are all in the unit within plant room. I am not aware of any remote sensors.

[SteamyTea]

46 minutes ago, MikeSharp01 said:

coming through the fabric of the house

That is a good point I had not considered.

If the main pipework is in the loft, which may be 40°C or more, then, even with insulated ductwork, it can easily heat up.

[MPH243]

I have the Q450 the sensors are all in the unit, 1 each side of the heat exchanger on both supply and extract.

 

Mine is the same 100% bypass, I can't connect to mine to show the figures but was the same as yours earlier. I think it's probably the fan warming the air. The bypass takes it over the top of the heat exchanger to miss it.

 

I think if you look this afternoon it goes into reverse and closes the bypass when outside is warmer than the house.

[Mike]

2 hours ago, MikeSharp01 said:

It is still coming through the fabric of the house so maybe it's picking up heat from the ductwork etc?

That would be my guess (plus a little picked up within the unit itself). Varying the time the air spends in the ducts by changing the fan speed might test that - slightly warmer air on a slow fan and slightly cooler on boost? I can't try it on my Zehnder CA225 as I don't have a temperature readout.

 

 

[MPH243]

The 2 sensors are about 500mm apart in the unit so it can't be house fabric.

 

Here is my present readings and not on bypass.

 

[Mike]

55 minutes ago, MPH243 said:

The 2 sensors are about 500mm apart in the unit so it can't be house fabric.

Good point

 

6 hours ago, FarmerN said:

The display on my Zehnder Q350 shows the incoming air at 17.5 C but air entering rooms at 20 C.

Actually, that's probably working as expected as 20° is the 'normal' supply air temperature in 'fixed mode', and the Q series has an adaptive bypass (it can be partly open, not just on or off)

 

However there are a couple of ways you can change that in the advanced settings - see this user manual extract :

 

Edited 7 hours ago by Mike

[FarmerN]

49 minutes ago, MPH243 said:

The 2 sensors are about 500mm apart in the unit so it can't be house fabric.

 

Here is my present readings and not on bypass.

 

Thanks for your reply.

Yes now cooling incoming air!

 

0% Bypass   Auto setting

Outdoor air temperature 27.5C

Supply Air                        26.5C

Extract air                        26.5 C

 

Almost identical to yours.

 

So apperars to be working as it should, I was just surprised to see a small degree of warming overnight.

House while warm is not yet oppresive.

[FarmerN]

Time to Shut the windows untill outdoor temperature is lower than indoor!

[Nestor]

On days like this I switch off the MVHR until late evening.

[Russdl]

As above.
 

As soon as outdoor temp exceeds desired indoor temp we shut all doors/windows and turn the MVHR down to its minimum setting. When the outdoor temp drops below indoor temp of an evening, we open all doors/windows and turn the MVHR up to max. 
 

Currently 33.7 degrees outside and 22.5 degrees inside. No active cooling but we do have external blinds which are game changers I reckon. 

[Nickfromwales]

3 hours ago, Nestor said:

On days like this I switch off the MVHR until late evening.

That’s not the healthiest option  unless windows are opened? 

[JohnMo]

We just leave the MVHR to do its thing, windows open or closed. it only changing the air in a room every 3 hours. Not worth getting hung up on it. It's ventilation at the end of the day, not cooling.

[Benpointer]

1 hour ago, JohnMo said:

We just leave the MVHR to do its thing, windows open or closed. it only changing the air in a room every 3 hours. Not worth getting hung up on it. It's ventilation at the end of the day, not cooling.

Quite.  Our Q350 is using 21W - half a kWh per day - so next to nowt.