Airtight electrical back boxes

[tew1]

Thanks Bernard, from my experience, a lot of mass-produced airtight homes can be designed very sterile, and often, the designers omit all the 'normal' things people expect from a house, which is something I'm interested in. Fundamentally, homes need to become more airtight, because air movement usually accounts for around 40% of heat loss. There is no reason you can't have a normal comfortable home with a wood burning fire (you can get amazing balanced flue wood burning stoves), opening windows, architectural features e.t.c. in an airtight house-it can actually feel very reassuring to walk in and close the big chunky door and feel completely isolated from the cold and draughts outside. Often, the MVHR system hasn't been installed correctly, or the ducts are made of plastic which become lined with dust and lint due to static, or not big enough because installers are still unfamiliar with the technology in the UK. Or people don't get the filters changed in as much the same way they don't clean their gutters each year. Also, the heating system is often designed for a traditional inefficient house, and is completely overkill, creating uncomfortable waves of heat. Culturally, in Canada and Scandinavia, people are more accustomed to not having radiators (ducted heat) and tumble drying clothes as opposed to sprawling them around radiators in the house (which is incredibly inefficient, in winter by the way and causes excessive moisture). It comes down to good design and an understanding from contractors I guess.

 

[JohnW2]

... and tumble drying clothes as opposed to sprawling them around radiators in the house (which is incredibly inefficient, in winter by the way and causes excessive moisture).

Do I take it that they somehow dry the dryer's exhaust air and retain it in the house?

Kind Regards, John

 

[Aragorn84]

You could probably just duct the exhaust into the MVHR "stale" side?

They usually have the MVHR intakes in the bathroom and kitchen, where both moisture and heat are likely to exist in large quantities, so presumably its designed to handle the dampness etc?

That way the MVHR system deals with the heat from the dryer as it would any other waste heat?

 

[tew1]

no, you can't connect a dryer to the MVHR, because it will fill the entire system with lint. What I was saying, is dryers are more commonly used in colder climates like Scandinavia and Canada. In an airtight house, the dryer would need to be located in an unheated room e.g. basement or mechanical room, with a vent to the outside, so that you aren't pumping air out of the heated space through the dryer exhaust. Air drying clothes with heating is exceptionally inefficient, and causes excessive moisture in the air which often leads to mould

 

[JohnW2]

... In an airtight house, the dryer would need to be located in an unheated room e.g. basement or mechanical room, with a vent to the outside, so that you aren't pumping air out of the heated space through the dryer exhaust.

That's what I suspected - a very interesting definition of 'an airtight house'!!

...What I was saying, is dryers are more commonly used in colder climates like Scandinavia and Canada. Air drying clothes with heating is exceptionally inefficient ...

I would have said that tumble dryers are very commonly used in the UK. Whatever, as for 'inefficient', I don't know in what sense you mean it, but I would not personally have thought that pumping the heat generated by one of the most heat-producing (and most energy-consuming) appliances in a house into the outside world was even a half-good example of energy efficiency!

Kind Regards, John

 

[tew1]

I think you're missing the point John. In an airtight house, you can't dry clothes inside, which alot of people like to do, because it is very inefficient on heating, and causes serious moisture problems. Many people don't think about where to install an appliance like a clothes dryer in an airtight house. Like a naturally burning fire or boiler, obviously, it needs a supply of fresh air and an exhaust, so it must be placed outside of the conditioned envelope so as to avoid messing with the ventilation rate in the house e.g. a basement/mechanical room, as is common in Canada and the US.

 

[JohnW2]

I think you're missing the point John. In an airtight house, you can't dry clothes inside, which alot of people like to do, because it is very inefficient on heating, and causes serious moisture problems. Many people don't think about where to install an appliance like a clothes dryer in an airtight house. Like a naturally burning fire or boiler, obviously, it needs a supply of fresh air and an exhaust, so it must be placed outside of the conditioned envelope so as to avoid messing with the ventilation rate in the house e.g. a basement/mechanical room, as is common in Canada and the US.

I'm missing nothing, and understand exactly what you're saying. However, I was commenting on your apparently compartmentalised view of 'efficiency'. It's all very well having your 'airtight house' (which you're now sort-of qualifying as being a subspace within the house which is a 'conditioned envelope'), but if you also have a dryer (installed outside of the 'conditioned envelope') which is consuming 3kW of power, converting it into heat and then pumping most of it into the outside world, then the energy efficiency of the building as a whole (including the bits outside of the 'conditioned envelope') will clearly reflect that!

Kind Regards, John

 

[SimonH2]

On dryers, there are condensing driers which condense most of the water out - there's a heat exchanger so it cools the exhaust air while pre-heating the intake air, condensing much of the moisture as it goes. We have one at home, and it runs without any external vent, and without really making much moisture (at least not enough to notice or cause any problems). I assume (not having looked) that it uses a lot less electricity than a non-condensing model.

 

[SimonH2]

But back to the OPs question. How about stretching (or fitting loose) the VCL so it can go round behind a standard box screwed to a batten ? Add an extra layer in front of it if you are worried about the corners of the box tearing it.
Either use a conduit (taped round the hole it will need in the VCL if it's not just below the plasterboard), or run the cable in front of the VCL (which means it's against the back of the plasterboard).

 

[tew1]

On dryers, there are condensing driers which condense most of the water out - there's a heat exchanger so it cools the exhaust air while pre-heating the intake air, condensing much of the moisture as it goes. We have one at home, and it runs without any external vent, and without really making much moisture (at least not enough to notice or cause any problems). I assume (not having looked) that it uses a lot less electricity than a non-condensing model.

condensing dryers don't work, they spin the **** out of the clothes, they stay wet, and just get really hot and use more energy because the cycle usually goes on for hours instead of 40 mins for a normal dryer

 

[tew1]

John, you can only improve the efficiency of things within your control. People still need to cook, do laundry, dry hair, make toast, take a bath e.t.c. which are inefficient things that people must still be allowed to do uninhibited. What you can control is the efficiency of the building envelope and the way heat/moisture is regulated within it, and make it work around how people live instead of the other way around. It becomes inefficient if you try to retrofit energy intensive and old fashioned must-haves into it, without considering the implications of the term 'airtight', or if you take away things from people, leaving them hating the building and feeling like it can't be a home

 

[JohnW2]

On dryers, there are condensing driers which condense most of the water out - there's a heat exchanger so it cools the exhaust air while pre-heating the intake air, condensing much of the moisture as it goes. We have one at home, and it runs without any external vent, and without really making much moisture (at least not enough to notice or cause any problems).

Quite. That's obvioulsy the sort of thing I had in my mind when I wrote:

Do I take it that they somehow dry the dryer's exhaust air and retain it in the house?

I assume (not having looked) that it uses a lot less electricity than a non-condensing model.

Hmmm - never assume! I can't really think why it should use any less electricity (except by heating up the room air and therefore increasing its air intake temperature, thereby perhaps resulting in more rapid drying of contents - hence dryer 'on' for less time) - what thoughts were behind your assumption? However, at times of year when one needed house heating, it would obviously provide 'energy efficiency' (as compared with a vented dryer), since less 'other' heating of the house would be required.

Kind Regards, John

 

[JohnW2]

condensing dryers don't work, they spin the **** out of the clothes, they stay wet, and just get really hot and use more energy because the cycle usually goes on for hours instead of 40 mins for a normal dryer

It sounds as if you are talking about 'washer dryers', not condensing tumble dryers. The latter (at least, the ones I've seen) don't do any 'spinning' (just low speed 'tumbling') and don't take any longer than any other tumble dryer to dry the contents. there are just a lot more 'efficient' at times of year when one needs house heating.

Kind Regards, John

 

[SimonH2]

condensing dryers don't work, they spin the **** out of the clothes, they stay wet, and just get really hot and use more energy because the cycle usually goes on for hours instead of 40 mins for a normal dryer

You have obviously never seen ours then. It tumbles the clothes and dries them just like any other drier would. The difference is that instead of dumping all that expensive heat outside, it recovers a lot of it to heat the incoming air - ad in the process condenses a lot of the water out. Other may be different, but our doesn't dry the air and pass it round the loop again, it uses "new" air all the time just like a non-condensing one.

Hmmm - never assume! I can't really think why it should use any less electricity ...

Apart form the big heat exchange that transfers a lot of the exhaust heat to the intake air - so it doesn't need as much heating to reach the same temperature.

 

[JohnW2]

John, you can only improve the efficiency of things within your control. People still need to cook, do laundry, dry hair, make toast, take a bath e.t.c. which are inefficient things that people must still be allowed to do uninhibited. What you can control is the efficiency of the building envelope and the way heat/moisture is regulated within it....

Any gestures in the right direction are obviously beneficial, provided only that they result in a net benefit. However, what is your ultimate aim - cost saving? 'planet saving'? If either of those, you simply can't ignore a tumble dryer which is pumping hot air (heated with lots of your well-earned money) into your garden, just because it is happening outside of your 'building envelope' - far better to find a more energy efficient way to dry your laundry!

Kind Regards, John