MHVR and high humidity

[George]

Hello, 

 

I've finally got my Vent Axia Sentinel Kinetic up and running in the last few weeks. 

 

I've noticed in the last couple of days the house humidity has shot up to 70%. It doesn't feel humid, but multiple sensors agree. Prior to the MVHR being switched on, the humidity stayed within 55-60%.

 

I've been having a think and just wanted to check my working. Is this because...

 

1) the weather at the moment is wet but warm. So the incoming air isn't being warmed significantly by the heat exchanger, thus the already warmish, humid outside air is getting sent straight to the rooms?

2) as there's a bathroom that's not quite finished I haven't fully commissioned the system, could there be an issue caused by an unbalanced system?

 

Is this something to be concerned about? 
Should I switch off the MVHR unit (ideally no)?

 

Thanks

 

 

 

[JohnMo]

Could just be British weather, check a weather site, and look at local humidity levels, they vary day to day

[SteamyTea]

1 hour ago, George said:

I've noticed in the last couple of days the house humidity has shot up to 70%. It doesn't feel humid, but multiple sensors agree. Prior to the MVHR being switched on, the humidity stayed within 55-60%.

How you actually 'feel' is related to temperature and humidity.

Where in the country are you? 

[IanR]

I'd go with (1)

 

Local to me right now is 17°C with 85% RH. Warmed to 20°C this would give 70.5% RH

[Blendworth]

Several things to check.

 

1. Has the system been commissioning, or even part commissioning? Insufficient ventilation rates could allow humidity to build up within the building as you are relying on the MVHR to provide cross-flow ventilation over a traditional extract system with passive supply air via trickle vents. 

2. Has the system been completed in rigid ducting or flex? Flex just shouldn't be used as any movement in the ducting can affect performance and any 'traps' in the ducting can cause water to form and therefore restrict the free area of the duct. 

3. Has the condensate been connected to the drainage system, and has a trap been provided to prevent foul moist air from the foul drainage system entering the supply air? With no condensate drain, any moisture that forms on the MVHR unit could leak into the supply air side of the unit, then evaporate increasing the humidity within the building. 

4. Are there any extract points directly over a wet area, IE shower? Extracting from directly within a shower area can cause uncontrollable condensation to form within the ducting, and if no additional duct mounted condensation traps have been provided if extract directly from any wet area, this can also prevent the correct extraction from within the building. Any long vertical sections of ducting after extracting from areas of high humidity, it would be prudent to fit an a inline trap to prevent any issues, and have a dry trap. 

I have the same unit in my roof space. Its been perfect for me as the main reason I got it was to help reduce the humidity in the house. 

[George]

58 minutes ago, SteamyTea said:

How you actually 'feel' is related to temperature and humidity.

Where in the country are you? 

 

West Midlands. Yeah the internal temperature is only a few degrees above external but has been tipping it down all night.

[George]

44 minutes ago, Blendworth said:

Several things to check.

 

1. Has the system been commissioning, or even part commissioning? Insufficient ventilation rates could allow humidity to build up within the building as you are relying on the MVHR to provide cross-flow ventilation over a traditional extract system with passive supply air via trickle vents. 

2. Has the system been completed in rigid ducting or flex? Flex just shouldn't be used as any movement in the ducting can affect performance and any 'traps' in the ducting can cause water to form and therefore restrict the free area of the duct. 

3. Has the condensate been connected to the drainage system, and has a trap been provided to prevent foul moist air from the foul drainage system entering the supply air? With no condensate drain, any moisture that forms on the MVHR unit could leak into the supply air side of the unit, then evaporate increasing the humidity within the building. 

4. Are there any extract points directly over a wet area, IE shower? Extracting from directly within a shower area can cause uncontrollable condensation to form within the ducting, and if no additional duct mounted condensation traps have been provided if extract directly from any wet area, this can also prevent the correct extraction from within the building. Any long vertical sections of ducting after extracting from areas of high humidity, it would be prudent to fit an a inline trap to prevent any issues, and have a dry trap. 

I have the same unit in my roof space. Its been perfect for me as the main reason I got it was to help reduce the humidity in the house. 

 

Thanks for these suggestions.

 

1. No the system hasn't been commissioned yet. I didn't want to do that until the last bathroom was finished, but I suppose I could roughly do it then do it again later. It's just getting hold of a anemometer...

 

2. Rigid ducting manifold to box and to the atmosphere. Semi-rigid for the radials. I don;t think it's been operating long enough for significant water to accumulate, though. 

 

3. Yes, condensate drain has been attached and there is a dry/in-line trap.  Although right now I have unplugged it and put a measuring jug underneath to see if anything does actually come out! There is another drain point in the expel to atmosphere duct, but that just goes outside rather than to a sewer as it shouldn't have any significant flow. 

 

4. They're offset from the shower so the screen is in the way. Only one bathroom is operational at the moment but this is the case for both of them. I have an extract in the laundry room but have kept a dehumidifier working in there for the time being, so presumably that is sending relatively dry air up into the extract side of the system which would hopefully prevent a build up of moisture.

 

I *think* I have passed all these checks so will assume option (1) and it's weather related. It's not a very airtight house, but wanted MVHR as I needed some ventilation and the air quality was the main attraction, so the humidity will be more strongly related to external conditions than a truly airtight house. 

[Mr Punter]

I agree with (1) above.  External humidity is high now and temperatures are warm.  The MVHR cannot reduce that. 

[SteamyTea]

4 hours ago, George said:

Yeah the internal temperature is only a few degrees above external

So it is near saturation point.

When close to that it tends to feel damp. Rainfall tends not to increase RH in itself, it is the evaporation and condensing of the water vapour that does that.

[George]

Just a quick update.

 

Temperature dropped to 6C overnight and is still below 10C now. Humidity has dropped below 60% everywhere. Measuring jug filled up 500ml overnight (although some of that will be the kid's bathtime). 

 

It almost wants a 'humidity bypass' as well as the summer bypass to deal with unfavourable external weather conditions. I think next time I'll have to remember to cycle it onto low mode. 

[PeterW]

20 hours ago, George said:

No the system hasn't been commissioned yet. I didn't want to do that until the last bathroom was finished, but I suppose I could roughly do it then do it again later. It's just getting hold of a anemometer...

….the forum has two. They are not calibrated but can be used to set up a system although some BCOs don’t care ….

[George]

6 minutes ago, PeterW said:

….the forum has two. They are not calibrated but can be used to set up a system although some BCOs don’t care ….

Funnily enough I had my final BC inspection yesterday. The MHVR wasn't part of the original plans (hadn't decided on ventilation strategy at the start, so just sent them the BPC plans) and nor is the unfinished bathroom, so they're happy to sign off based on the completion of the extension.

 

How would one get hold of the forum anemometer??