Condensation Problem - MVHR and Dehumidifier

[evilmrkipling]

Hello beautiful people of DIYnot forum. First post here but first of many no doubt.

My next project will be tackling the condensation problem I'm facing. I've almost completed a new bathroom and running a bath the enter room is soaked in water and I've got heavy condensation forming on the bedroom window.

The house is a 1940s PRC construction type and is known to suffer from damp. There were passive vents in the walls of the three upstairs bedrooms but I've blocked them all up now and decorated.

I was planning a MVHR system in the roof space.

Downstairs there is kitchen , living room, wc.
Upstairs 3 bedrooms and a bathroom.

With it being a concrete house its not going to be easy to install ducting to anywhere on the ground floor.

I might be able to use the chimney to get an extraction from the kitchen. Then extract from the bathroom also and supply to two of the bedrooms.
2 in 2 out

If anyone has any thoughts on how this might work best please do share them.

I am also thinking about a dehumidifier. Is there any way to incorporate a unit in the attic with the MVHR unit? Or shall I just keep it separate.

 

[dishman]

Ok, basically, you have caused the condensation problem you are seeking to fix (or at the least, a major contributor).

You need active vents in the bathroom and kitchen (preferably with humidistats). Unblock the passive vents you blocked up.

By blocking up the vents you have removed much needed ventilation from the property. Old properties were not designed to be air tight.

Read these extensive posts re:condensation posted recently. All the things you need to do are in there.

//www.diynot.com/forums/building/possible-damp-patch-on-interior-wall.387867/

//www.diynot.com/forums/building/condensation.386014/#2964199

Do not get a dehumidifier to "solve" the issue, as you are just trying to fix a problem you are creating...they should only be a temporary solution if it is a self-created condensation issue.

This is a crude analogy, but: You are basically walking round your house with a bucket full of water which is leaking. Rather than fix the leaky bucket, you are proposing to clean up the water as it leaks (dehumidifier) at great energy and expense....not a great way to do it....fix the bucket!

Look at these advice guides...


http://www.scotland.gov.uk/Publications/2005/05/10103020/30217

http://www.bolton.gov.uk/sites/DocumentCentre/Documents/Condensation and Mould.pdf

If your internal walls are cold and therefore more prone to condensation, you may want to consider some insulated plasterboard. But first follow all the advice in those links.

You may be able to vent the bathroom out through the ceiling and roof if it is much easier/cheaper then drilling through the walls.

 

[evilmrkipling]

Hi,

Thank you for your reply. I sealed up the passive vents in the bedrooms with the intention of installing a Mechanical Ventilation Heat Recovery (MVHR) system to allow me to ventilate the house while recovering heat. From an energy performance point of view passive vents are the worst as you are heating the house for the heat to then escape out of the walls.

 

[dishman]

From my knowledge MVHR systems are only worthwhile and useful in modern, highly insulated houses, designed with the system in mind. If used in an old property, the house would have to be substantially upgraded/modernized to a very high specification in a very planned/considered way.

They [MVHR] are not heating systems per say but rather a system for a house that won't need much heating (due to the high spec design/insulation), enabling it to keep the heat inside while providing good fresh ventilation.

It is unlikely to be suitiable/worthwhile for an old property with standard insulation with just vents blocked up.

It is one of these technologies that has to be fitted to a very specified property. You can't just retrofit it to an old property.

You may find the system ineffective and far more costly to run then you think.

Futhermore, passive vents do not let in and out as much cold as as one would think (I wont go into that now). But provide good background ventilation.

 

[evilmrkipling]

They are normally found in more modern airtight homes. However I am looking for a solution to ventilate the home without having to rely on simply having holes in the walls. That's why I'm looking into MVHR systems, also open to other ideas.

 

[dishman]

Ok,but if you really want a MVHR you have to do it properly, as I said it is not just a case of blocking up obvious ventilation holes and sticking in a MVHR system just to give you ventilation without the cold air. Your old property will still have to be brought up to modern standards (air tightness and insulation) for it to perform anywhere near like it is meant to. You have to look at the structure of the property (if it is even viable to bring up to modern standards), its current level of insulation....you should also do a pressure test etc. (just because you have blocked obvious ventilation does not mean its air tight as there will be many "holes" to fill in an old property).

If you just stick a system in it will basically suck cool air into the property, futhermore the property wont be insulated enough to retain enough heat, so the heat exchange element may not even work. So you have basically bought a giant suction machine.

In the mean time,blocking up the ventilation you have now is a bad idea as you have exacerbated your condensation issue.

Regardless, going back to your initial post, mechanical ventilation (with humidistat) in the bathroom and kitchen are a must, you can get one with backdraught shutters to prevent cold air being blown back in).

At some point with old housing stock, there is a insulation/ventilation balance. You can only do so much with an older property in terms of bringing it up to modern standards, and then it becomes a cost/performance balance. especially if it is a non-standard construction type.

Ventilation has become a bit of a dirty word compared to insulation, but it is just as important for a house and the occupants to have good fresh air coming in. It is about finding the best balance you can with your type of property. Passive air bricks do not let in a huge amount of cold air (far less than one thinks), and if you bring the rest of your insulation up to standard (a pretty cheap thing to do), they are often a small compromise whilst providing the ventilation you need.

Regarding alternatives.......I found this brief simple guide on the homebuilding website which has a good list and summary.

http://www.homebuilding.co.uk/advice/key-choices/heating/ventilation-options

 

[SimonH2]

Ok, if you really want a MVHR you have to do it properly, as I said it is not just a case of blocking up obvious ventilation holes and sticking in a MVHR system just to give you ventilation without the cold air. Your old property will still have to be up to modern standards for it to perform anywhere near like it is meant to. You have to look at the structure of the property (if it is even viable to bring up to modern standards), its current level of insulation....you should also do a pressure test etc. (just because you have blocked obvious ventilation does not mean its air tight as there will be many "holes" to fill in an old property).

If you just stick a system in it will basically suck cool air into the property, futhermore the property wont be insulated enough to retain enough heat, so the heat exchange element may not even work. So you have basically bought a giant suction machine.

I'd disagree with some of what you write.
Yes, it would be ideal to "upgrade" the structure, but I don't see why it's necessary for an MVHR system to work.

In essence, you are sucking out the damp air at source, just the same as having an extractor fan in the bathroom and kitchen - which alone are normally enough to deal with the bulk of the problem. For both rooms, you cannot get away from them being the "leaky bucket" in your analogy (especially if you have gas cooking) - both produce lots of moisture which is not avoidable, and so it's quite reasonable to treat the problem (as is normally done) by removing the moisture rather than futile attempts to not create it (fixing the leak in the bucket).

So, having established that a couple of extractor fans will deal with the bulk of the problem, we can either have fans that suck the warm air out and waste the heat we've put into it, or we can try and recover that heat and return it to the building by way of pre-heated forced make up air. Ie, put in an MVHR unit.
It is true that the building will lose a lot of heat through poor insulation - but that won't affect what the MVHR unit does, it simply means the house costs more to heat regardless of how you ventilate it.

I've first hand experience of using a dehumidifier. My GF's house is a 60's ex-council house and one of the first things I noticed was that it had a very damp feel to it, and signs of mould in the usual cold spots. I was able to borrow a decent dehumidifier, and it really did make a huge difference (just sat on the landing) - beyond what you can achieve by just opening the windows and letting all the heat out with the damp.
I'm in the process of collecting offcuts of Correx from a local signmaker, and when I've got enough I'll be making a DIY MVHR unit - since the commercial ones invoke the Barnsley battle Cry (HOW MUCH ) once you get to a size of unit worth having.
I know it's not going to be perfect, but if it can deal with the damp without costing a fortune to run then it'll be worth while.

 

[dishman]

I disagree with what you disagree with....

As I said, the leaky bucket analogy was crude, it is less about "not creating the moisture" but more about dealing with it at the source. I should have said "dealing with it" at source rather then "not creating".

Having the correct ventilation system at the source of the moisture is the best/most efficient way to deal with it, rather than let it permeate the property cause condensation then having to remove it.

Your correct an MVHR system will/can work like an extractor. However it is designed as a whole house solution, and while I am sure it will have an effect of some degree, if it is installed in a house that is not up to a standard it was designed for, you lose a huge amount of the systems benefit/efficiency for a much higher system cost. Why have a glorified extractor system which is working at a small percentage of it's intended efficiency, when you could install an equally good standard extractor system, with humidistat etc for a tiny cost.

The point of an MVHR system in a highly insulated house is that the moist heat is trapped in the house, it can only go through the MVHR to escape and therefore a lot of the heat is recovered. If you put it into a poorly insulated house with poor air tightness, only a tiny percentage of the hot moist air may actually make it into and through the system. Hot moist air in a concentrated space such as a bathroom/kicthen may have more chance of being processed by the MVHR system before it escapes, but even then with the intermittent use a of a bathroom/kitchen, I don't see it being a worthwhile investment just to reduce damp/condensation and get a tiny amount of heat recovery.

Don't get me wrong, I think it is a great system. But only if you get the full benefit of it by upgrading your house (insulation/air tightness) to a standard the system was designed for.

Otherwise, it is a weird halfway house where you install a MVHR system but only get a tiny portion of the benefit. The cost will outweigh any energy savings of the system, and you basically have a glorified extractor system.

However is the OP is planning to fully upgrade his property insulation/airtightness then I am sure it will be a great solution.

 

[SimonH2]

However it is designed as a whole house solution ...

But if your extract side terminals are in the rooms that make the moisture …
And your supply side terminals are located such that they create a general (if not very strong) movement of air towards those damp areas, then that will tend to restrict how far the damp air moves about the house.

As to cost, that's why I'm making one - it won't cost much more than a couple of extractor fans up front. And as I find them, I intend dealing with the leaks - it's already been upgraded in the past with modern UPVC windows and doors.

 

[dishman]

I still feel that using a whole house MVHR system specifically for a bathroom and maybe the kitchen, to reduce condensation and recover a small amount of heat is a inefficient use of a system.

Without a pressure test on the house it is unlikely you will find the leaks easily. And even though you may intend to do this, my response was aimed at the OP and their intentions, as it sounds like their house has very little current insulation nor the required air tightness (even if they have blocked up vents)...although they did not confirm this..

There is another option however, which I feel would be more suitiable for your [the OP] situation....a much smaller scale "through the wall" MVHR. Basically similar to a standard extractor system but with a small scale MVHR system built in. It is aimed at single room use such as a bathroom/kitchen and avoids having to install a system in the loft...ducting etc.

http://www.vent-axia.com/range/hr25.html


I also found this review of sorts which describes a situation similar to that of the OP:

http://andyduffell.com/techblog/?p=1270

Hope that is useful and maybe gives you some options to consider....