Condensation and 3G windows

[SteamyTea]

1 hour ago, Russdl said:

One other thing that really surprised me, that I’m sure our resident scientists will be able to explain with lots of confusing equations:

 

Running the dehumidifier in a room aside from drying it, also warms the room. There is little to no heat from the dehumidifier, is this temperature rise ‘latent heat’ or something to do with ‘dry/saturated adiabatic lapse rates’? Or none of the above?

Some rise in air temperature will come from the latent heat of fusion, gas to liquid, of the water.

Most will probably come from the electrical input of the dehumidifier.  If it is drawing 1 kW (or whatever), it is the same as putting in a 1 kW fan heater.

That is why I laugh when I see an portable A/C unit in the back of a shop.  All it is doing is heating the place up.

Edited January 10, 2022 by SteamyTea

[Russdl]

23 minutes ago, SteamyTea said:

latent heat

Of course it is. If I’d stopped to think about it a bit longer I would have got there. The dehumidifier is rated at 300W but that won’t produce 300W of heat energy into a room will it? However the temperature of any room it’s in that has high humidity starts to tick up quite rapidly, more rapidly than a heater would (that’s a guess, I’ve not done any experiments) so it must be converting the water vapour  to liquid water that is releasing all that lovely latent heat. It is quite noticeable. 
 

We dry clothes in the utility room, it’s never been as successful as I was expecting it to be, we put one of those 300W plug in heaters in there, made the room warm but didn’t speed up the drying process much. With the dehumidifier in there it is quite startling how quickly the clothes dry and also how warm the room gets. 
 

And, for what’s it worth it’s a good looking dehumidifier!

Edited January 10, 2022 by Russdl

[SteamyTea]

1 minute ago, Russdl said:

The dehumidifier is rated at 300W but that won’t produce 300W of heat energy into a room will it

Yes it will, apart from what is lost via noise.

2 minutes ago, Russdl said:

However the temperature of any room it’s in that has high humidity starts to tick up quite rapidly,

A room does not hold much air, by mass, in it, and it only takes 1 kJ.kg^-1.K^-1 to heat air up.

A 300 W heater can heat 10 kg of air (about 8 m³) by 10 K up in 333 seconds (5.5 minutes).

7 minutes ago, Russdl said:

so it must be converting the water vapour  to liquid water that is releasing all that lovely latent heat. It is quite noticeable. 

You can work it out as you know the room size and the RH levels. Use this calculator to work out how much (little really) water is mixed with the air.

Latent heat of fusion for water is 2264 kJ.kg^-1.

[AliG]

I just checked, we have three MVHR units and hey are showing RH of 37, 45 and 46%.

 

I used to run them on auto boost and it seemed to me that they ran unnecessarily high to try and reduce the RH. I switched them all to just run on the lowest speed as I cannot really feel the difference in RH.

 

When I did all of this I was seeing figures in the 50-55 range, so it may be that it takes a long time for humidity to get down to the dried out range. The only time I ever saw condensation inside our windows was when they laid the screed.

 

The pool has RH in the 60-62 range. When it is cold outside we get condensation on the door lock on the french doors which is a small cold break and gets cold, but have never had it on the windows in there. The temperature never drops below 21.

 

I wonder if your house is to passivhaus standard, has the heating hardly ever been on since you moved in. We run the heating at 21-22 and it comes on whenever the outside temperature is below around 12C. I am guessing that running at 20C with a better insulated house your heating has rarely been on.

 

My guess is that this lack of running the heating would cause a passivhaus maybe to take considerably longer to dry out than a more aggressively heated house.

 

 

 

[Radian]

30 minutes ago, SteamyTea said:

You can work it out as you know the room size and the RH levels. Use this calculator to work out how much (little really) water is mixed with the air.

 

Not sure what calculator you're referring to but from tables of Saturated Vapor Pressure, Density for Water vapor we get gm/m³ for 100% RH at different temperatures so multiply the lookup by room volume and take the RH percentage as currently indicated. I just did this for a 68m³ room at 20°C (17.3gm/m) @ 50%RH so there's supposed to be 588ml of water that could be condensed out. In reality with a dehumidifier running 24/7 I have to empty around 2L a day. Does that mean the air is being changed 3 or 4 times a day?

[SteamyTea]

7 minutes ago, Radian said:

Not sure what calculator you're referring to

Whoops  forgot to post link in.

Any would probably do though.

8 minutes ago, Radian said:

Does that mean the air is being changed 3 or 4 times a day?

Possibly. Could be an easy way to check air leakiness.

[Radian]

3 minutes ago, SteamyTea said:

Possibly. Could be an easy way to check air leakiness.

 

That's exactly what I was wondering. Knowing outdoor and indoor RH and quantity of water condensed could be all that's needed. ?

[Russdl]

1 hour ago, SteamyTea said:

Yes it will, apart from what is lost via noise.

Shows what I know then. 
 

1 hour ago, SteamyTea said:

You can work it out as you know the room size and the RH levels. Use this calculator to work out how much (little really) water is mixed with the air.

Latent heat of fusion for water is 2264 kJ.kg^-1.

And I got homework! 
 

Too busy messing about with a patio at the moment so I’ll have to look into that later. 
 

51 minutes ago, AliG said:

I wonder if your house is to passivhaus standard, has the heating hardly ever been on since you moved in.

It is to passiv standard, and the heating hasn’t been on much. In fact when it has been on it doesn’t seem to do much - but that’s for another thread.
 

Hopefully it is just that the house is ‘wet’ and the little dehumidifier can rectify that over time. My big concern is if I’ve porked it all up by getting an enthalpy unit and it’s that which is causing me the issue. 

Edited January 10, 2022 by Russdl

[SteamyTea]

4 minutes ago, Russdl said:

My big concern is if I’ve porked it all up by getting an enthalpy unit and it’s that which is causing me the issue. 

I don't think in the UK it will make much difference. Not as if we high temperatures, with associated high amount of AH. We basically have a very stable climate. RH and Temperature is in a small window compared to most parts of Northern Europe.

 

More a case of don't you wet systems when building a house.

Edited January 10, 2022 by SteamyTea

[LA3222]

3 minutes ago, Russdl said:

little dehumidifier

Not sure of your house size but if it is big, having a little one is no use to man nor beast. My 280m2 build was rampant with humidity Dec 2020, I was practically mopping the windows down daily. It took about three weeks of running a massive dehumidifier to get it controllable down to around 70%. Fast forward a year later and its knocking around 45% with the MVHR running at 35%.

[Russdl]

3 hours ago, LA3222 said:

Not sure of your house size but if it is big, having a little one is no use to man nor beast. My 280m2 build was rampant with humidity Dec 2020, I was practically mopping the windows down daily. It took about three weeks of running a massive dehumidifier to get it controllable down to around 70%. Fast forward a year later and its knocking around 45% with the MVHR running at 35%

 

Now that's interesting. It is a puny little thing and I do wonder if I'm trying to dry out the planet litre by litre.

 

I got the main bedroom down to 55%RH this afternoon running the dehumidifier for about 3-4 hours, taking out about 1 litre of water. 90 mins later it was back up to 63%.

 

The MVHR is bringing in just under 100m3 an hour of air that apparently has an RH of 97%. The room is around 45m3.

 

So what's happening? Are these figures indicative of me trying to dry the planet one roomful at a time or do they indicate the house is still wet? Or is it not possible to discern anything?

 

 

[AliG]

6 minutes ago, Russdl said:

The MVHR is bringing in just under 100m3 an hour of air that apparently has an RH of 97%.

I turned off the humidistat on my MVHR as the RH goes up when it rains, which happens plenty here, so the MVHR would run harder whenever it rained.

 

I am by no means an expert on MVHR, but if the RH outside rises well above the inside level could you not get into a vicious circle where the system runs harder to try and reduce the RH, but instead actually makes it worse as it brings in more air from outside? 

 

 

[LA3222]

21 minutes ago, Russdl said:

 

Now that's interesting. It is a puny little thing and I do wonder if I'm trying to dry out the planet litre by litre.

 

I got the main bedroom down to 55%RH this afternoon running the dehumidifier for about 3-4 hours, taking out about 1 litre of water. 90 mins later it was back up to 63%.

 

The MVHR is bringing in just under 100m3 an hour of air that apparently has an RH of 97%. The room is around 45m3.

 

So what's happening? Are these figures indicative of me trying to dry the planet one roomful at a time or do they indicate the house is still wet? Or is it not possible to discern anything?

 

 

Get yourself to a plant hire place and get a proper dehumidifier. You are going to be at this for months if you continue.

 

I got a big one in from plant hire, was about 30quid a week iirc. I was emptying that every 12 hours and it held 20 odd litres I think.

 

This went on for a few weeks. Taking 6l a day out piecemeal is going to be endless!

[AliG]

I agree. Hundreds of litres are used in building a house. With the lack of heating it will take forever to dry out. A small dehumidifier will take months.

[Russdl]

1 hour ago, LA3222 said:

Taking 6l a day out piecemeal is going to be endless!

 

I agree, especially if the 6 litres I take out is being replaced by the MVHR unit as fast as I remove it from the room.

 

2 hours ago, AliG said:

if the RH outside rises well above the inside level could you not get into a vicious circle where the system runs harder to try and reduce the RH, but instead actually makes it worse as it brings in more air from outside? 

 

That's what I'm wondering. 

 

~~~~~~

 

The last wet trade in the house was over a year ago and whilst last summer was a bit rubbish, it still got plenty hot enough for us to have windows open frequently and MVHR on 'party mode' to keep things cool. The more I think about it, the more I think that it can't be that the house is still drying out and the problem must lie with the MVHR or...

 

...a water leak somewhere in here. No evidence of that though.

 

Further info:

 

The main bedroom was the first room in the house to be skimmed, and that was close to 2 years ago.

The main bedroom always has the highest RH in the house.

There are two doors between the main bedroom and the en-suite, both normally pulled to, if not closed.

The supply duct to the main bedroom is the shortest of all supply ducts at 4.4m. All other supply ducts are two or more times longer.

 

Does any of that make any difference?

 

 

 

 

[LA3222]

If you are convinced it is the MVHR - turn it off?‍♂️ 

 

Then monitor what happens to the RH.

[Russdl]

38 minutes ago, LA3222 said:

If you are convinced it is the MVHR - turn it off?‍♂️

 

Not quite convinced yet, but certainly leaning in that direction. It may just be that we've been running it too fast or it may be a bit more complicated than that. 

 

Experiments continue, but not turning it off just yet.

[Adsibob]

1 hour ago, Russdl said:

 

Not quite convinced yet, but certainly leaning in that direction. It may just be that we've been running it too fast or it may be a bit more complicated than that. 

 

Experiments continue, but not turning it off just yet.

I wonder if there is a fault with your MVHR machine, as increased ventilation should really remove moisture from the air, not add it.

[Russdl]

4 minutes ago, Adsibob said:

increased ventilation should really remove moisture from the air, not add it.

 

Theoretically, but if it's more humid outside than inside, and I have an enthalpy unit does the opposite happen? B*ggered if I know so we'll see what tomorrow brings.

[Adsibob]

47 minutes ago, Russdl said:

 

Theoretically, but if it's more humid outside than inside, and I have an enthalpy unit does the opposite happen? B*ggered if I know so we'll see what tomorrow brings.

I'm not sure it works like that. Take these examples from the NCBI website:

 

• When outdoor air at a temperature of −8 °C and 100% relative humidity, which is usually very high in winter (point A in Figure 2), is brought to an indoor temperature of 20 °C, its relative humidity decreases to 15%. Thus, cold outdoor air used in ventilation is effective in carrying moisture from indoors.
• When air at a temperature of 15 °C and an absolute humidity of 5.5 g of water in 1 kg of dry air (corresponding to a relative humidity of 50%) is heated to a temperature of 18 °C, its relative humidity decreases to 40%. Thus, heating can help to prevent high relative humidity.
• When air at a temperature of 20 °C and a relative humidity of 58% (point B in Figure 2) is cooled to 15 °C, its relative humidity increases to 75%; when the temperature is decreased to 11 °C, the relative humidity reaches 100%. Water condenses on surfaces at such temperatures. Thus, when the temperature in houses decreases locally (e.g. at window panes, unheated sections of the house or poorly insulated walls), the relative humidity rises and accelerates microbial growth.

I wonder if you have cold spots in your heating in the locality of where your humidity is

[Russdl]

@Adsibob thanks for that link.

 

7 hours ago, Adsibob said:

I wonder if you have cold spots in your heating in the locality of where your humidity is

 

The main culprit for condensation is the main bedroom window and there is no heating up there. It's a large window at 1900 x 1500 and I'm sure there is no issue with the window or it's installation.

 

Overnight I've had the MVHR on a much lower setting than of late (moving circa 50m3/hr, for the last few weeks its been moving 200m3/hr pretty much 24/7). This morning there is no condensation anywhere but it was a much milder night last night with the temperature not falling below 9°. The RH is high in the bedroom again at 70%, up from 62% last night 

 

We didn't have any heating on downstairs overnight and its actually the warmest it's been downstairs for a while (by half a degree or more) at 20.6°.

 

In summary, well, I don't know, but here's what I do know:

 

Condensation forms on the inside of the windows when it's cold outside.

Running the MVHR at a higher rate for weeks has not prevented condensation forming.

The house is built to passive standards and the windows are good quality and installed correctly.

The house must be dry from any construction moisture (it's had a year or more to do so).

The RH is generally high in the house, the highest RH is in the rooms with the shortest duct runs to the MVHR terminal (coincidence?)

The MVHR has an enthalpy heat exchanger.

The MVHR hasn't been properly commissioned, it's at factory settings with the suppliers recommended restrictors for each supply/extract duct.

 

 

 

 

[Roundtuit]

21 hours ago, Russdl said:

We dry clothes in the utility room, it’s never been as successful as I was expecting it to be

 

Somethings wrong somewhere.  We can dry a machine full of clothes easily overnight. A bit of a long shot, but any chance some of your ducting has been accidentally connected to the wrong plenum and it recirculating extracted air?

[Russdl]

1 minute ago, Roundtuit said:

Somethings wrong somewhere.  We can dry a machine full of clothes easily overnight.

 

That's what we were expecting, it's never achieved that.

 

1 minute ago, Roundtuit said:

A bit of a long shot, but any chance some of your ducting has been accidentally connected to the wrong plenum and it recirculating extracted air?

 

Not really a long shot, something's clearly not quite right. I've checked it and checked it. I'll be checking it again today.

 

 

[Russdl]

1 minute ago, Russdl said:

recirculating extracted air?

 

Actually, there's no way that can happen unless something is knackered inside the MVHR unit...

[Adsibob]

Maybe take a marble and put it in each duct at the MVHR room, one duct at a time, and see where the marble goes (assuming the ducts all go down from the MVHR room (which applies to most of my ducts, but may not apply to yours) just to make sure the ducts are going where you think they are.

For the ones that go upwards, i guess you put the marble at the other end, or throw up a tennis ball?

Alternatively, a very long pipe cleaner.