Humidity and MVHR

[lizzie]

I am learning how to cool house....summer bypass and night venting......but I am struggling with humidity levels.  Humidity is around 37% (never higher), that is too dry for me and is giving me problems.

 

 I have done usual things such as boiling kettles, doors open with shower running etc but MVHR too efficient.  I know I can turn it off and just have windows open but the pollen count is through the roof and I have severe hay fever so not really an option.  I have asked MVHR people if there is something we can fit to allow me to control humidity better. How do others do it?

[jack]

When it's dry outside, over-ventilation is the main enemy. Can you turn the MVHR down to a lower setting? 

As an example, we're supposed to run ours at something like 180 m³/hr, but it was on for a year at 50 m³/hr with no issues at all.

 

Another possibility is pollen screens for windows. I have no idea how effective these are, but could you get at least some windows covered with these so you can open them and get some flow-through ventilation overnight and when it's not too hot?

 

Edited to add: large pot plants are also supposed to help. 

 

Edited to add: example of pollen screens: https://newblinds.co.uk/pollen-screens

[joe90]

I am yet to commission my MVHR but just looked and my RH is 67%, I don’t see how moving air around can dry it out if it’s not heated?

[SteamyTea]

We used to use Vapac units as steam generators.  They were designed as HVAC humidification units.

You may be able to find something, like a wall paper steamer, that can do the same thing.  Just stick the pipe in the ducts to the rooms, simple RH controlled switch, and job done.

 

I am not sure where you are in the UK, but my external RH has been pretty low, down to 62% on the 19/05/2018, by the time that has warmed up in the house it is probably down to 45%.

 

Edit:

Just looked at yesterdays RH numbers at between 2 and 3 PM, RH was at 50%.  No wonder I feel so good.

 

Edited May 22, 2018 by SteamyTea

[le-cerveau]

2 minutes ago, joe90 said:

I am yet to commission my MVHR but just looked and my RH is 67%, I don’t see how moving air around can dry it out if it’s not heated?

The MVHR process (heat exchange) removes moisture from the air (heating it although only slightly) (hence condensate drain) so it will dry the incoming air.  If it is dry outside anyway it will only increase this.

Helios http://www.heliosfans.co.uk/products do humidification units (select the last option under accessories) but will be expensive!

[dpmiller]

Rather than a steamer (high power consumption) try an ultrasonic unit. Maplin used to do them for fog over water features but you can get standalone too. See for example:

 

https://www.ebay.co.uk/itm/1-3L-Ultrasonic-Humidifier-Air-Room-Diffuser-Steam-Purifier-With-7-Color-Light/142721319238?hash=item213ada8146:g:TdAAAOSwk4tapWRz

 

https://www.ebay.co.uk/itm/DC-24V-Ultrasonic-Mist-Maker-Fogger-Humidifier-Water-Fountain-Pond-with/253483323394?epid=2076907171&hash=item3b04c89002:g:yRoAAOSwKvJapXmu

[jack]

If you use an ultrasonic humidifier, it helps to have a fan beside it to disperse the droplets into the air. I've used these in the past, and if you aren't careful, the area around them (floor, furniture) gets damp.

 

30 minutes ago, joe90 said:

I am yet to commission my MVHR but just looked and my RH is 67%, I don’t see how moving air around can dry it out if it’s not heated?

 

Because there are moisture sources inside the house (eg, people, cooking, showers, plants) that add moisture to the air. The more you ventilate, the more of that moisture is lost to the outside. For example, turning off the MVHR completely would result in nearly all internally generated moisture accumulating in the house over time. Obviously that's not a desirable situation, so it's a balance between fresh air and keeping moisture levels at a reasonable level.

When the weather is cold, you also get a larger drop in RH as the (often already relatively dry in winter) incoming air is heated. 

[joe90]

I live in Devon, I don’t think anything gets dry down here ?.

Edited May 22, 2018 by joe90

[SteamyTea]

I only visit Devon when the weather is dry

[A_L]

Which MVHR do you have? At least some can be optionally fitted with an enthalpy (or latent heat) heat exchanger. This allows water vapour in the exiting air to transfer to the incoming air, humidifying it. Particularly useful in cold conditions where the incoming air has little water vapour but should also help in summer.

 

Theory here:- https://www.paulheatrecovery.co.uk/components/moisture-heat-exchanger/

 

                         http://waermetauscher.paul-lueftung.de/en/product-information/enthalpy-exchangers-erv.html

 

It should also prevent loss of heat exchange efficiency when temperature falls below zero in winter, down to -6°C according to Paul.

Edited May 22, 2018 by A_L
added second link

[joe90]

My MVHR which I think is the same as proDave,s has a paper filter and no condensate drain. When I investigated this a couple of years ago I believe the paper filter allowed some air moisture through so we may not suffer from the dry air with ours!!!

[ProDave]

Yes like Joe I have a Kingspan badged, Mitsubishi Lossnay mvhr with a waxed paper heat exchange core.

 

We are not yet living in the house, thought all the clothes washing is dried inside the house, and have never had any problems.  I don't have an RH meter but can't say I feel in any way unconfortable. I spend a lot of time in the house as my office is there even though we are still living in the static caravan.

 

I would definitely turn the mvhr speed down.  Mine is only running at the very slowest speed. I have not done flow measurements and comissioned it properly, but my feeling is even when I have done that, air quality will be what sets the speed, not some measurement on a table to satisfy a building regulation number.

[joe90]

I think several people on here have said they have reduced the airflow from what BR say.

[lizzie]

Thanks all.  Its so frustrating all the effort on air tight and heat loss etc I never for one minute considered the mvhr being a problem.  One of the motivators for having this type of house was the clean air to help breathing and I have been delighted with that aspect.  Improved my quality of life no end even in a few short weeks. 

 

The very hot weather and huge solar gain (still organising blinds/awnings just have temp ones at mo) have meant that I have been running mvhr on boost to cool things.  Yesterday I tried the venting overnight and low mvhr speeds and it helped no end in getting the house cooler overnight.  It has not affected the humidity levels though they seeem pretty static at 35% and this is giving me real problems.  Such low humdity is causing nose bleeds and breathing issues. I know I am best at about 45%, I feel really stupid that I did not consider this  aspect I have monitored humidity and air quality in my homes for years and I totally forgot humidity as we got carried along on the airtight low energy quest.

 

I am drying washing indoors and boiling kettles but mvhr is very efficient at removing all that moisture!

 

I have the Frankische Profi air 250 unit.  I think there may be something that can be done by fitting an alternative part, might be to do with the heat exchanger I’m not clear. Waiting to hear back. It has a filter and condensate drain.

 

I have noted all the various suggestions and will be trying out what I can.  Think my trusty Dyson fan with a big bowl of water in front and mvhr turned to lowest setting might be the way to go until a more permanent solution can be found.

[le-cerveau]

You can get an Enthalpy heat exchanger for your unit, sell pages 19/20 of your Install/Operation guide, that would help.

[lizzie]

1 hour ago, le-cerveau said:

You can get an Enthalpy heat exchanger for your unit, sell pages 19/20 of your Install/Operation guide, that would help.

Thank you!  I think that must be what was mentioned but I have e-mailed specifically mentioning page 19 details.

[Temp]

Never measured the humidity in our house but it only seems too dry in winter (which is when humidity is lowest outdoors I think).

[SteamyTea]

I was pondering this internal RH problem last night.

Regardless of whether you have MVHR or not, if your house is hotter than the external temperature, and you are not doing silly things with hot water, then the internal RH will be lower than the external RH.

This is because internal RH tracks external RH and for any given mass of water in the air (the AH), the higher the temperature the lower the RH.

I seem to remember that there was someone over at the other place who had used lime plaster and had trouble getting it dry (not a surprise with lime plaster, but that is another story).  He played with his MVHR and it seemed to make no difference.  I offered to measure his RH levels, but he never took me up on it as he was convinced that the problem was the MVHR, so turned it off and just opened the house up.

 

One thought that has just crossed my mind, where is the RH monitor?

[Gone West]

23 hours ago, lizzie said:

How do others do it?

On my system there are four fan settings and it is recommended to run it on setting two. Setting one is for when the building isn't occupied and turns everything else off leaving the fans running very slowly. It is possible to change the fan speed for each setting so I have reduced setting two to 30% which allowed me to change the ventilation rates to that for a PH. This has increased the humidity from 37% to 45%.

[SteamyTea]

1 hour ago, PeterStarck said:

This has increased the humidity from 37% to 45%.

Has the internal temperature changed?

Also, the external RH can change quite rapidly for a seemingly small temperature change.

 

All that is needed is a lot more data

[Gone West]

2 hours ago, SteamyTea said:

Has the internal temperature changed?

 

All that is needed is a lot more data

No, temperature the same.

 

If only I had the time.

[HerbJ]

We have a PAUL Novus 450 with the Enthalpy Heat Exchanger. The HE was specifically recommended by PAUL Scotland because we have a relatively large house - 337m3, excluding garage, but only my OH and myself normally live there and  the concern was that the air would get too dry, in this situation.

 

The HE  seems to work well and we are very happy with the air quality ( we use F7  Pollen Inlet filters). Internal Temp   and RH are currently 25 C and 43%.  The fan setting is currently on 2,  the normal setting (actually the 100%, BR Setting) to give us a little bit more air flow in these hot and sunny conditions. We normally have it on 1 Low ( 60% of Normal) for most of the year .  

 

 

[lizzie]

So much info to digest here thank you all.

 

I have had a price for enpalthy heat exchanger and its very expensive. My installer is recommending that we try playing about with things to see how much better we can get humidity levels without mvhr running then we can start tweaking on mvhr settings. He thinks we should be able to get some results from doing the sort of things you are suggesting. The change of heat exchanger is deemed to be the drastic last resort..... by him not me but at about £1500 all in I am inclined to agree with him

 

I am going to print out the suggestions here and work through over the next week or so monitoring results.

 

Currently I seem to be on quite a high default fan setting. I am going to start by knocking the mvhr off for a day ot two and work it back up from there. I already felt a benefit last night from lower fan speed and windows cracked open on tilt in bedrooms we dont use and leaving doors to those rooms open for air flow.  

 

There must be an answer here somewhere, both clean air and decent humidity must be achievable. Thanks all for your input it is much appreciated.

[PeterW]

7 minutes ago, lizzie said:

the drastic last resort..... by him not me but at about £1500 all in I am inclined to agree with him

 

It’s a 90 second job and the unit is £815 at full list price.....

[PeterW]

 

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I reckon you can find them cheaper.