Drying out a new house - dehumidifiers vs heating & MVHR

[Conor]

We've just had 200m2 of liquid screed poured and the entire house painted... so it's as damp as a November morning. Keen to get it dried out for second fixes and move in end of Jan.

 

We currently have the windows closed and four dehumidifiers running, currently removing about 20l a day. Temp of about 15c and relative humidity of 80-85%. Was 100% when we just had windows open.

 

We've no electric at the minute so can't get the UFH up and running for at least a month. 

 

Would it be better to get some space heating going and commission the MVHR (via temp power supply)? How much moisture can they remove @600m3/hr if there's a delta of about 10c?

Edited December 13, 2021 by Conor

[Bitpipe]

MVHR airflow is low and will only extract from your wet rooms (baths, kitchen, WCs etc) but may help. Your dehumidifiers will be much more effective though.

[joe90]

When ours was that wet and the outside RH was high I used de humidifiers (you could get a genny) when it was windy I opened the windows and doors, I ran the UFH with immersion later at a very low temp.

[MJNewton]

Definitely get the MVHR going as the heat recovery will lower the RH of the incoming air and help with evaporation. 

[SteamyTea]

To evaporate water takes a lot of energy, 2260 kJ/kg.

To put that into perspective, liquid water take 4.18 kJ/kg.K and air 1 kJ/kg.K. 

So basically you need to heat, dehumidify and ventilate.

Ideally you would dehumidify the incoming air as that would be more efficiently as you can easily extract moisture laden air with only a fan. But that is not really realistic unless you have designed in air-conditioning.

 You say you cannot get the UFH working because of lack of power.

How are you running the dehumidifiers? Is it a temporary supply?

You could plumb in a Willis heater to the UFH (or use immersion heater in the buffer tank). Assuming that the majority of the free water is in the slab. You may need a 5 kW generator and they can be a bit expensive.

The MVHR is not going to do a lot, even when shifting 750 kg of air an hour. The trouble is that at 15⁰C and 85% RH, there is only 11g of water per m³ of air. Or 6.6 kg per hour if you can remove all of it.

But if the replacement air is at say 70% once warmed to 15⁰C, then it will always have 9g/m³ in it, so only removing 0.8 kg/hour.

 

Heat the slab, large fan to outside, ditch the dehumidifiers, mug of tea and a good book. It back and enjoy.

Edited December 13, 2021 by SteamyTea

[saveasteading]

1 hour ago, Conor said:

200m2 of liquid screed poured

 

How thick, and what is under it?

There should not be much 'free' water anyway, as most of it should react chemically within the screed.

 

You may find that what appears sopping immediately after the pour, improves dramatically in a few days.

Most to all second fix materials will not be affected by short-term dampness.

 

[Conor]

44 minutes ago, MJNewton said:

Definitely get the MVHR going as the heat recovery will lower the RH of the incoming air and help with evaporation. 

 

That's the key thing. Outside RH is always 90% or more here. So will be a matter of seeing if the MVHR incoming air can get RH below that of inside the house.

 

18 minutes ago, SteamyTea said:

To evaporate water takes a lot of energy, 2260 kJ/kg.

To put that into perspective, liquid water take 4.18 kJ/kg.K and air 1 kJ/kg.K. 

So basically you need to heat, dehumidify and ventilate.

Ideally you would dehumidify the incoming air as that would be more efficiently as you can easily extract moisture laden air with only a fan. But that is not really realistic unless you have designed in air-conditioning.

 You say you cannot get the UFH working because of lack of power.

How are you running the dehumidifiers? Is it a temporary supply?

You could plumb in a Willis heater to the UFH (or use immersion heater in the buffer tank). Assuming that the majority of the free water is in the slab. You may need a 5 kW generator and they can be a bit expensive.

The MVHR is not going to do a lot, even when shifting 750 kg of air an hour. The trouble is that at 15⁰C and 85% RH, there is only 11g of water per m³ of air. Or 6.6 kg per hour if you can remove all of it.

But if the replacement air is at say 70% once warmed to 15⁰C, then it will always have 9g/m³ in it, so only removing 0.8 kg/hour.

 

Heat the slab, large fan to outside, ditch the dehumidifiers, mug of tea and a good book. It back and enjoy.

 

We have a temporary site supply so can run 4x 13amp extension leads... that's fine for a 500w dehumidifier but i dont fancy running a 3kW immersion with that! UFH will be a priority over christmas and hopefully be ready for when we get main power in early Jan.

 

11 minutes ago, saveasteading said:

 

How thick, and what is under it?

There should not be much 'free' water anyway, as most of it should react chemically within the screed.

 

You may find that what appears sopping immediately after the pour, improves dramatically in a few days.

Most to all second fix materials will not be affected by short-term dampness.

 

 

50mm liquid on top of suspended precast slabs... the house was open to the elements for a long time and I know there is a lot of moisture trapped in all the concrete.

 

Second fixes that are the issue are timber... joiner wont fit the skirting as the walls at the floor are too damp, kitchen company can't fit the unit gables either. Same for the wooden floor. We've moving in end of January so wan't get get as much of this stuff done as possible. It's been a long 2 years.

 

[SteamyTea]

10 minutes ago, Conor said:

Outside RH is always 90% or more here

That is at outside temperatures.  You may be easily adding 10 K to that.  Also why I suggested that if you can, dehumidify the incoming air.  That improves the partial vapour pressure ratio.

11 minutes ago, Conor said:

We have a temporary site supply so can run 4x 13amp extension leads

52 A, only need 13 A for a heater. Just remember that a normal plug and socket can only take 10A continuously.

 

[saveasteading]

16 minutes ago, Conor said:

50mm liquid on top of suspended precast slabs.

So that is just the 50mm of screed, as the precast slabs will have absorbed very little water.

That isn't a lot of free water.

 

If it feels that damp in the space, then it must be evaporating.

 

But I note your very tight time-frame, and so some expense is justified. 

I agree with some of the above, but not all.

If not already using heaters and dehumidifiers, first open all windows and doors to get a draught through and clear the currently sopping conditions.

then:

 

1. close all the doors and windows, so you are not drying the world. Site instruction to all: never leave the outside door open.

2. close the doors to the area with the wet screed, to contain  the water for removal.

2. An electric fan heater, probably commercial quality,  so that the whole space heats up and the air is moved around, 24/7

3. Powerful Dehumidifier, with big collector tank so that it doesn't need emptying in the night.

 

A week of that and you should be sorted, and it should noticeably change from sauna to dry atmosphere.

I would not want the screed to dry too quickly under and in front of the heater, so best raise it.

 

After a few days, put some plastic down on the floor for a few hours and see if it gets sopping wet underneath.

 

It should be possible to do some sums on how many litres you are looking to capture. If you keep this room closed then you are not drying out new air from elsewhere.

 

I cant see why bringing in replacement air is necessary: either use natural drying or this forced method, but not mixed.

 

And you will know this BUT just in case the plant-hire company suggests it:  Don't use gas heaters as they create water.

 

 

 

 

 

 

 

[tonyshouse]

I have always opened windows and use a blow it through approach. 

[saveasteading]

1 hour ago, tonyshouse said:

I have always opened windows and use a blow it through approach. 

Me too, except once when under pressure (big floor too, 800m2) and I learnt not panic, as it dries vey much more quickly than people want you to think (so they can hire you machines or charge for a floor seal, or delay their work)

But in this case I can see that the OP needs to try everything.

[Iceverge]

Don't forget that outside air at 10deg contains about half as much water at 100%RH than internal air at 20deg and 100% RH. 

 

Using a humidity meter as soon as internal RH gets too high say >80% at a sensible temp >15deg, you could open all the windows and doors briefly and let all the internal air blow through. The incoming air will be quickly heated by the heat in the structure if you close the windows again. It will be able to soak up more moisture. 

 

We used a mixture of fan heaters, the MVHR and intermittent window opening to dry our house out. 

 

The biggest annoyance was trades arriving at 9am and leaving the door open until 4pm. The temperature drops and the RH increases to >90% and all drying stops. 

 

 

[Conor]

Temp has been steady at 18c and humidity at 85% for a couple days now. It's 10c outside and 80% humidity... so I've opened every door and window for a couple of hours to get the air changed.

 

In other news... I'm chuffed that five small dehumidifiers (300/500W) and a 1kW fan heater are enough to get the house to 18c!!!

[Bitpipe]

11 minutes ago, Conor said:

In other news... I'm chuffed that five small dehumidifiers (300/500W) and a 1kW fan heater are enough to get the house to 18c!!!

 

Yes the dehumidifiers kick out quite a bit of heat. When we were in the caravan for 18mo we started using the gas fire but the floor was damp and cold each morning which was due to the water vapour generated by combustion condensing out overnight on the floor.

 

We switched to running a small dehumidifier which kept the inside dry and warm.

 

When the house was up we also had a major humidity issue when laying the resin floors (in June)  - needed to keep windows closed to minimise dust but moisture was condensing our and 'raining' on the curing floor causing a bumpy finish.

 

Running some big dehumidifiers for a few days ahead of a rerun made a huge difference.

[saveasteading]

10 hours ago, Iceverge said:

trades arriving at 9am and leaving the door open until 4pm

This is one of the  rules among building operatives. 

The client knows nothing.

The professionals know nothing.

I the tradesperson am the most important person here and everything must suit me.

 

How much water are you collecting in the buckets?

[Faz]

Trades arriving at 9am?  wtf?  Warm weather atm - kick everything open and leave it that way and it will sort itself out.

[saveasteading]

Tell me if I am getting this wrong. SteamyTea: red pen out please.

 

Warm the air and it absorbs more moisture, which all must effectively come from the floor slab in a sealed room.

Then chuck the saturated air out at 18 degrees and bring in outside air at 5 (?)C.

The colder air has half/third of the moisture in it, leaving capacity to repeat daily.

For a 50m3 room this will 'only'  be a pint of water each cycle, but is in addition to what is taken out by the machines.

 

I don't know how much water is surplus within the screed after the chemical reaction has used most of it. Say 3% volume. So for the same 20m2 room and 50mm screed there is 1m3 of screed and so 30 litres of surplus water. Doesn't sound too onerous.

I don't think we ever aim to get all the water out.

 

Daily removal: 0.5litres by opening the windows, say 2 litres in the machines. 2.5litres/day for 12 days.  ???

 

 

 

[SteamyTea]

40 minutes ago, saveasteading said:

Warm the air and it absorbs more moisture

Almost, you really need to thoroughly warm the object that has excess water in it.  Not just the top surface.

40 minutes ago, saveasteading said:

which all must effectively come from the floor slab in a sealed room.

And any walls, ceiling, cold windows that can cause condensation or have excess moisture in them, and it is easier to condense out, in the dehumidifier, moisture that is already in the air as it is the first to pass though the cooling side of the unit.

40 minutes ago, saveasteading said:

Then chuck the saturated air out at 18 degrees and bring in outside air at 5 (?)C.

Chuck out the dryer air, why I think it is best to dry the air as it comes into the room. Alt+248 creates °.

40 minutes ago, saveasteading said:

The colder air has half/third of the moisture in it, leaving capacity to repeat daily.

Roughly right on the RH scale, same mass of water on the AH scale. RH is a ratio that varies with temperature, AH is, well actual amounts.

40 minutes ago, saveasteading said:

So for the same 20m2 room and 50mm screed there is 1m3 of screed and so 30 litres of surplus water. Doesn't sound too onerous.

Plus are moisture being removed from the air that is introduced into the room.

40 minutes ago, saveasteading said:

I don't think we ever aim to get all the water out.

True, just be thankful it is not a lime mix, would have the dehumidifiers working for decades.

 

Bit of a better explanation a here.

https://en.wikipedia.org/wiki/Vapor_pressure

Edited December 14, 2021 by SteamyTea

[Iceverge]

3 hours ago, saveasteading said:

How much water are you collecting in the buckets?

 

No buckets. 

 

It was over a year ago. We were in no panic to dry out this happy to take our time as apparently it's better for avoiding concrete cracks. 

 

For a good while we just had the windows ajar 24/7. For experiment I did try leaving the windows closed overnight before the MVHR was running. The result in the morning was everything was saturated.