heating a conservatory

richard murray

hi im looking for soom ahelp about my new conservatory. it is a 4.5mtr x 4.5 mtr build that i fitted 2 rads into which are fed of the house system. the rads are 40x1200 mm and 500x 400mm.. could you please advise if you think that is sufficient as the room is totally unusable as it is and that is with the heating on full. when you leave the hall and go into the dining room it is nice and warm, but when you leave the dining room and enter the new build it is baltic. the builder is telling me thats normal, but i dont think so. thanks

GMcD

It's the glass

Cheap glass with high heat loss = huge radiant cooling panels that require huge heating system size and energy use to maintain comfort. Even if you get the "air" temperature at 75F in that room, you'll still feel cold due to the poor/low mean radiant temperature.

If higher performance glass was used, then you'd have a much smaller heating requirement, much smaller energy bills, and a higher inside surface temperature of the glass, so the glass isn't acting like a radiant cooling system in the winter.

The "fix" for this won't be cheap. The lowest capital cost solution would be to add more heating system "stuff" to the room, but watch your energy bills go up, for the rest of the buildings' life....

Brad White

Geoff is spot-on right as usual

Your mean radiant temperature is everything when it comes to comfort. Not much between you and outer space and absent a low-E or other reflective barrier you are toast or anything but.

Maybe that IS normal, for your builder- but it need not be that way. I will respectfully submit that conservatories do tend to be "sacrificial spaces" with 3-season comfort at best.

While you could stand to pump more heat into that space (and pay the cost of same as Geoff notes), the short, least expensive answer I can give is to admit defeat and retreat when the room temperature tells you to. You may also want to drain down those radiators to inside the building, unless you have glycol in the system.

Even giving your plight in metric measurements does not make the problem seem smaller for some reason

richard murray

heating

thanks for the reply . i have looked at the plans and the spec for the glazing is 1.8w/m2k aa rated,u value for the roof and windows,double glazed units. i have got an overall output of around 4400btu,s from the 2 installed rads at the moment. what do you suggest next?
thanks

Maine Doug_68

Kind of strange

that we call such an energy consuming room a "conservatory".

Brad White

Pithy, Doug.

Acerbic and pithy. I like that.

GMcD

Glass \"ratings\"

Another sales hype that has become gospel- the Published window "u" values. Reading the fine print, one finds that the published U value is the "centre of glass U value" which is the best lab condition measurement at the centre of the glass. That U value fails to take into account the framing edge losses, the thermal bridging at the double glazed seals, and the radiant surface temperature effect of the glass. The true measure of the glass U value is the "overall" U value that captures the edge losses, framing losses and thermal bridging, and it's generally 50% to 90% higher (worse) than the published "centre of glass U value.

As an example: I'm working on a project where the sealed glass unit has a centre of glass U value of 0.27 (imperial) in winter. When I run it on my handy Lawrence Berkeley Labs Free Software - Window 5.2, the true overall U value with the framing system being used is actually 0.45 (imperial). Nearly 67% worse than the sticker sez. If I selected my perimeter heating system based on the centre of glass U value, I'd be getting a call next December.

LBL Free software at: http://windows.lbl.gov/software/default.htm

What many people consider "high performance glazing" is really crap compared to what you can get for real "high performance glazing", but the baseline in North America is cheap=efficient.

If it was my conservatory I'd be getting my Visionwall or Heat Mirror Rep on the phone and using a window and framing system that would give me all the daylight, with an overall U value of around 0.12 (imperial) (0.68 W/sq.M-K). Yes it's expensive, but my heating system would be minimal, I could sit and tie flies year-round in total comfort, and my operating costs would be very tiny, for the life of the house....

Based on a fairly simple estimate of the skin area of the Conservatory, based on the dimensions you stated in your initial post, I come up with a skin heat loss of around 14,675 Btuh (4.3 kW), and that doesn't even include infiltration or floor losses. Even with the proper amount of installed heating elements in there, you'd still feel a chill from the cold glass. A combination of floor heating plus perimeter heaters would at least take the edge off, and heat up the inside surface of the glass a bit to reduce the radiant cooling chill.