Aircon in loft struggling to bring temp down

[b737]

In my loft I have created a clean, and slightly insulated storage area by boarding and installing some foil bubble wrap insulation where I store and occasionally read books (its not a conversion but just tidy storage). Its size is approx 3m x 3m and is pitched roof. The foil comes down from the pitch about half way and then drapes to the floor. There is still a dirty area behind these foil walls.

During these hot UK months the insulation hasn't been thick enough to stop this area getting up to 33-35C which is about 10C too hot to read up there. I therefore purchased a 5000 BTU aircon unit and have the vent going down the loft hatch and out of the bathroom window (2.5 to 3 mtrs). Its a little further than the standard hose comes with so I had to add a 1.5 meter extension which I know may make it a little less effective.

The problem is the unit isn't doing as good a job as I expected it to. After 3 hours of running late afternoon (when the sun is getting less strong) its still up near 29C up there so it has done something but not as much as I had hoped for.

I am wondering why this could be, could it be any of the following reasons:-

1. 5000 BTU isn't enough for a 3 x 3m area?
2. Is having the loft hatch open causing all the cool "air-con" air to fall out of the loft hatch, and more hot air from downstairs to rise up into the loft? Should i block off the hatch when up there?
3. another reasons?

Thanks
M

 

[Dan Robinson]

Having the hatch open will be a major factor. Our flats in Japan are purely AC and one of the first things I was taught was to keep the interconnecting doors closed at all times.

As for the 5000btu capacity being enough? Don't know, possibly not in this weather. But certainly start with ducting through the loft hatch rather than having it just open.

 

[stem]

There are lots of things to be considered when sizing AC units. Building construction, direct sunlight, electrical appliances, number of occupants, other heat sources etc.,

AC units designed for habitable rooms will be OK in for example in a standard bedroom, where the temperature is generally less than outdoors during the peak daytime temperature. (For example 23 degrees indoors when it's 27 outside.) In such cases a 5,000 btu air conditioner left on for a couple of hours can drop the temperature by about 6 to 7 degrees to a more comfortable level. In such instances as a rule of thumb, a 5,000 btu AC unit would be OK for a room of about 3m x 3m.

However, lofts that are not designed as habitable rooms and are not properly insulated will get much hotter, such as the 33 to 35 degrees you mention. The sun shining on the roof will heat up the roof surface and that heat will be transferred by conduction to the internal surfaces. In addition, most roofs are designed to allow a flow of air through them so more heat will be coming in there. As a result 5,000 btu's will probably be woefully inadequate to make much of a noticeable difference. If I had to guess, I would think to get rid of so much heat you would need at least 2 or 3 times more cooling capacity.

 

[Dan Robinson]

Just checked one the units for a living room.... It's rated "skill" is 5.6kw. That's got to handle a relatively poorly insulated room of about 30sqm.

In our bedroom, we have one half the capacity and it's just under half the size of room.

Bearing in mind we have near 40 heat outside and 98% relative humidity.... A factor I think you have not taken into account when judging your comfort temperature.

We set the AC to about 24 at night for sleeping as the air is much dryer and therefore, higher temperatures are ok.

 

[Dan Robinson]

Just checked though rooms, and the bedroom Is down to temperature much quicker that the living room.

 

[b737]

This is the one I have got and in question:-

https://www.amazon.co.uk/dp/B073RB912B/ref=pe_3187911_189395841_TE_3p_dp_1

I understand that a loft is a lot harder to cool but are there any suggestions to improve on what I already have without spending another £450 on a new unit that is more powerful.

Do you think the hatch could be part of the problem or am I clutching a straws.

 

[Dan Robinson]

Do you think the hatch could be part of the problem

Did you read my post?

 

[b737]

Did you read my post?

For some reason I missed that whole post from you and started reading from post #3.

I won't be able to close the hatch fully when up there so was thinking of making some sort of insulation pad/bag and just lay it over when up there... Any suggestions.

 

[Dan Robinson]

Anything would do for a test. You just need to hinder the flow of air. I think you'll be surprised. But, it can take a good while for a small ac unit to do any good.

 

[John D v2.0]

The other main problem will also be that foil bubble wrap isn't really insulation. The u value will be very little, mostly the air tightness will be helped.

 

[rsgaz]

Think about all that hot air that those portable machines create/blow out the back hose for a minute.

That air must be replaced by 'other' air, just like a bathroom/kitchen extractor, otherwise you'd end up in a vacuum with your ears popping! They blow out hot air at an alarming rate. With the loft hatch open, that air is being replaced by 'other air' from inside the house, which is hot anyway I assume. Most of which will be sucked in via the very same window that the hose is poking out of.

Even if you could seal up the loft hatch hose exit, that 'replacement air' has to be gotten from somewhere, so it will be sucked in from outside through the eaves vents from outside, which is also hot air!

Those units are fighting a losing battle, they are always sucking in additional hot air from elsewhere adding to the problem they are supposed to be solving. You need to massively up the BTU rating to combat that. Their suggested suitable room sizes are a joke!

The proper wall mounted AC units with the separate unit outside don't have this problem at all, because they are re-circulating the same air inside the room, making it colder each time, over and over again, rather than having 'new heat' to have to deal with.

 

[b737]

ragaz,

That is very useful and makes sense. Thank you.

With this in mind and the fact the other air has to come from somewhere else, then if the air on the landing is cooler than in the loft then surely I would be best having the lid open and have it closed if the air in the eves etc is cooler than the landing?

Does that logic work in this scenario?

 

[rsgaz]

Does that logic work in this scenario?

The logic works, sure, but the unit just isn't powerful enough to overcome the issues I mentioned. Sadly, the more powerful the unit, the more air it expels, and the more air it needs to draw in! To answer...

are there any suggestions to improve on what I already have

If you could leave the unit on the landing, vent the hot out of the window and add a hose/duct to the front and just have the cold duct going into the loft, if would work much better. This way, the loft will be under slightly 'positive pressure', forcing the existing air out of the eaves and filling it with cold. The unit will still, of course, be sucking more hot air toward it, but that 'negative pressure' area then remains on the landing, rather than drawing hot air into the room you are trying to cool. I bought...

https://www.appliancesdirect.co.uk/p/airflex15/electriq-airflex15

... a month ago and loved it(I say loved, past tense, because it broke down already and has been sent back. now they can't replace it yet because there is no stock ). Rather than just bunging the hot air hose out of a window(therefore sucking more hot air back in through the very same window), I put the hot air duct through a hole in the wall and then to allow the 'replacement air' in, open a window elsewhere in the house, accepting the fact that that room will be getting even hotter, drawing in from outside. And rather than have the unit in the room I was trying to cool, I put it in the doorway, just blowing the cold in, therefore keeping the 'negative pressure' zone outside the room, rather than drawing the 'extra air' into the room.

 

[bernardgreen]

5000 BTU isn't enough for a 3 x 3m area?

As mentioned you need to reduce the heat entering the climate controlled area to a level lower than the capacity of the air con unit.

The area could be 5 x5 m and be cool if the heat getting into the area was less than 5000 BTU.

Include the heat in the air entering the area to replace the air that is being expelled from the area by the air con unit.

increase the effectiveness of the insulation around the area

Also consider the suggestion from rsgaz about the unit being on the landing, it is a good one.

 

[b737]

Thanks all

Any suggestions on how to duct the cold air from the front of the unit. Would obviously want to ensure a good seal etc..

This is the unit I have:-

https://www.amazon.co.uk/dp/B073RB912B/ref=pe_3187911_189395841_TE_3p_dp_1?tag=diynotcom-21