Air source heat pump vs single room MVHR to combat condensation & mould

[Timmyk86]

Hi all,

We live in a Victorian semi. Double glazed but otherwise poorly insulated. Centrally heated with a modern loft conversion.

We get pretty bad condensation on the middle floor bedroom windows and on the bathroom walls (despite an extractor fan) and this tends to grow mould. Humidity can get pretty high in the house.

We are expecting our first child and I’d like to sort the humidity / condensation / mould issue out. There’s not really any space for a PIV system, and I actually had one before the loft was converted and didn’t think it made a huge difference.

We could install an ASHP to the middle floor bedrooms & the loft - expensive, but I thought it might help keep the air and surfaces a constant temp, plus remove humidity. Alternatively I could put single room MVHR units in the two middle floor bedrooms and hope they sort the humidity / condensation issues.

Would appreciate any thoughts on which would be best, or even if there are any other options we could look at? We do the usual things of not drying clothes indoors, opening windows regularly etc.

Thanks!
T

 

[ericmark]

I would love to know why this house is so much drier to my last house, looking at 45% now and was around 65% in last house. So some thoughts.

1) The shower door seals at the bottom in this house, last house it was over the bath, so chimney effect allowed moisture into the whole bathroom.
2) We now have a chimney, and have a portable AC venting into the chimney, this also when running, fills bottles with condensate, so must dry the room.
3) No gas cooking, and a cooker hood above the cooker.
4) House is larger, and has doors to downstairs rooms, last house was open plan.
5) We have a cat flap. Also, three sets of patio doors, and three doors to outside.
6) Central heating is oil fired.

But in real terms, I do wonder about the cavity wall insulation, could it have been faulty in the last house? Also, the extension roof had loads of fibre glass and so any leak could be masked as it absorbed it?

If I look at the childhood house, it had single glazed windows, with channels at the bottom of the windows and a hole leading outside, so it was a very good dehumidifier, also cold, but ventilation with two coke fires running in winter, resulted in parents having high backed easy chairs to protect them from the draft.

The question has to be, is there a leak of some sort causing high humidity? Or gas appliances without a flue?

 

[Timmyk86]

Thanks for the reply!

I don’t think there’s anything specific like a leak and the only gas appliance is the boiler for which the flue goes directly out through the roof. So I don’t think it’s that!

I could just run a dehumidifier as you do your air con unit- but I’m looking for something a bit more ‘permanent’- hence asking about a heat pump or MVHR unit

 

[flameport]

We could install an ASHP to the middle floor bedrooms & the loft

Several unrelated items being confused here.

An air source heat pump is a heating device. It won't fix condensation any more than a gas boiler, electric radiator or any other source of heat would.
PIV units are a fan which shoves air from outside into the property continuously. Often used in rented properties as a sticking plaster method of covering up failings by those living there.
MVHR devices are extractors which recover heat from the extracted air and put it back into the room. For removing moisture laden air it does exactly the same as any other extractor, but removes less heat from the building.

Condensation occurs because the air contains moisture, and there are surfaces at a temperature below the dew point.
To fix this the options are:
1 - increase the air temperature, as warmer air can contain a lot more moisture
2 - increase the temperature of surfaces above the dew point
3 - decrease the amount of moisture in the air.

1 is done by using heating more often. Doesn't matter whether that's the gas central heating, electric radiators, heat pumps or other things.
The only devices to avoid here are those which burn fuel and emit the result into the air, such as a gas hob. Although they emit heat, they also emit large quantities of water vapour,

2 is partially achieved by doing 1, and more completely by adding thermal insulation to the walls/ceilings/floors so they remain at a higher temperature for longer.

3 is done by reducing the moisture emitting sources inside the building and removing moisture by other methods such as ventilation and dehumidification.
However once the obvious fails of drying washing on radiators are removed, the main source of moisture laden air is people breathing which is why proper ventilation is always required.

Usually the solution is some combination of all three.

 

[endecotp]

We get pretty bad condensation on the middle floor bedroom windows and on the bathroom walls (despite an extractor fan) and this tends to grow mould. Humidity can get pretty high in the house.

You might find it useful to actually quantify how high the humidity is. Get a couple of cheap LCD thermometer / humidity meters from eBay and put them in the worst rooms. Let us know how humid it actually is.

I'd say that getting bad condensation on a double-glazed bedroom window in June suggests that you're putting far more moisture into the air than is normal - as if you're hanging up wet laundry at the end of the bed, never opening the windows, or something.

 

[foxhole]

Having an extractor fan doesn’t guarantee it’s effective at removing humidity, many fit feeble 4” fans in poor positions which are not very efficient.What do you have? Have any vents been blocked?

 

[Timmyk86]

Several unrelated items being confused here.

An air source heat pump is a heating device. It won't fix condensation any more than a gas boiler, electric radiator or any other source of heat would.
PIV units are a fan which shoves air from outside into the property continuously. Often used in rented properties as a sticking plaster method of covering up failings by those living there.
MVHR devices are extractors which recover heat from the extracted air and put it back into the room. For removing moisture laden air it does exactly the same as any other extractor, but removes less heat from the building.

Condensation occurs because the air contains moisture, and there are surfaces at a temperature below the dew point.
To fix this the options are:
1 - increase the air temperature, as warmer air can contain a lot more moisture
2 - increase the temperature of surfaces above the dew point
3 - decrease the amount of moisture in the air.

1 is done by using heating more often. Doesn't matter whether that's the gas central heating, electric radiators, heat pumps or other things.
The only devices to avoid here are those which burn fuel and emit the result into the air, such as a gas hob. Although they emit heat, they also emit large quantities of water vapour,

2 is partially achieved by doing 1, and more completely by adding thermal insulation to the walls/ceilings/floors so they remain at a higher temperature for longer.

3 is done by reducing the moisture emitting sources inside the building and removing moisture by other methods such as ventilation and dehumidification.
However once the obvious fails of drying washing on radiators are removed, the main source of moisture laden air is people breathing which is why proper ventilation is always required.

Usually the solution is some combination of all three.

What an awesome reply. Thank you.

Re the ASHP- my thought was these heat pumps/air conditioners have dehumidify settings as well as heat and cool, so they might remove some of the humidity from the rooms and reduce the condensation we get. Also providing a more consistent warmer air temp might avoid having some variation in surface temperatures.

I don’t think we have particularly moisture emitting habits. What do you think would be the best way of improving ventilation but not losing all our heat at the same time?

 

[Timmyk86]

You might find it useful to actually quantify how high the humidity is. Get a couple of cheap LCD thermometer / humidity meters from eBay and put them in the worst rooms. Let us know how humid it actually is.

I'd say that getting bad condensation on a double-glazed bedroom window in June suggests that you're putting far more moisture into the air than is normal - as if you're hanging up wet laundry at the end of the bed, never opening the windows, or something.

Thanks. I actually have a tado system and will keep an eye on humidity levels. House is totally fine in summer - winter is the problem. And mainly the windows/bathroom walls (solid brick)

 

[foxhole]

Thanks. I actually have a tado system and will keep an eye on humidity levels. House is totally fine in summer - winter is the problem. And mainly the windows/bathroom walls (solid brick)

Would suggest ventilation problems.

 

[Timmyk86]

Would suggest ventilation problems.

In which case those single room MVHR systems should work pretty well right?

 

[flameport]

Re the ASHP- my thought was these heat pumps/air conditioners have dehumidify settings as well as heat and cool,

They do, but not while heating. The dehumidification occurs when cooling, where water in the air condenses onto the cold parts of the indoor unit.

The only difference between cooling and dehumidifying is the amount of cooling power - dehumidify is a lower setting so that water condenses but the air is not noticeably cooled, while cooling will be at higher power and both dehumidify and cool.
With excessively humid air, the cooling function will be significantly less effective.

What do you think would be the best way of improving ventilation but not losing all our heat at the same time?

MVHR, preferably for the whole building, although installing those as a retrofit is not trivial unless it's something like a bungalow where all of the ducting can do in the loft space above the rooms.

 

[Timmyk86]

They do, but not while heating. The dehumidification occurs when cooling, where water in the air condenses onto the cold parts of the indoor unit.

The only difference between cooling and dehumidifying is the amount of cooling power - dehumidify is a lower setting so that water condenses but the air is not noticeably cooled, while cooling will be at higher power and both dehumidify and cool.
With excessively humid air, the cooling function will be significantly less effective.


MVHR, preferably for the whole building, although installing those as a retrofit is not trivial unless it's something like a bungalow where all of the ducting can do in the loft space above the rooms.

Awesome. Thanks. In which case I’m probably best off using single room ones for the most affected rooms in the house - both middle floor bedrooms. I reckon it would be almost impossible to retrofit the entire house

 

[JohnD]

....and on the bathroom walls (despite an extractor fan) and this tends to grow mould. Humidity can get pretty high in the house....

please show us a photo of your extractor, and, on the outside, its vent. When is it used? Do you find it noisy?

 

[JohnD]

Are the fireplaces blocked up? Are the chimneys ventilated? Is the cellar damp?

 

[Timmyk86]

Are the fireplaces blocked up? Are the chimneys ventilated? Is the cellar damp?

Fireplace is now a wood burning stove with a flexible flue up to the chimney.

Extractors x 3 - one over the hob to the outdoors, one over the main shower directly through a vent tile in the roof and the last in loft en-suite through a mushroom vent on the roof. I’ll try to find photos. Middle floor bedrooms are the main issue though.

I’m going to get a new extractor for the main bathroom either way