Radiator sizing

[David Hearn]

I have a cold conservatory that I want to heat - how should I work out the best size of radiator to use? The kitchen opens directly onto the conservatory and neither have any direct heating in them (though the boiler adds a little heat to the kitchen).

A 400w flat panel electric heater does nothing really. A 2kW convection heater with fan makes it much better - however I don't use it enough to get a good idea of what its like long term (ie. too hot, not enough heat etc) due to the cost! Currently we just allow the heat from the living room to move into there and its not too cold. Closing off the living area makes it very cold.

My plan is to get a Screwfix radiator 600mm (H) x 800mm (W) which is either

1.1kW for a single or 2.1kW for a double and plumb it in.

Our current old gas boiler is going to be replaced very soon. The old boiler claims 13kW output (18.4kW input), and the new one claims 14.7kW output (17.8kW input). We don't think there's any problem with the sizing of the current one (heat loss calculator estimated 10-11kW I think including hot water) so an extra 1.7kW should over the 2.1kW of the new radiator.

All the rads have TRVs (except one next to room stat).

What's the problem if I put in a rad which was too large? With the TRV it shouldn't make the room too hot. What other problems are there? We don't really want to put one that is too low - though I guess the single, 1kW one being on all the time *might* be enough to keep it warmer.

Any advice?

Thanks

David

[Christian McArdle]

It uses too much wall space.

Consider using fan convectors too. When I build my conservatory, I plan to install a big fan convector above the door. It won't use valuable wall space and is a fraction of the size of an equivalent output radiator. OK, they're not totally silent, but they're much quieter than an electric fan heater.

For example, a Myson 20-14 has almost the same surface area as an 800x600 radiator (it is actually 1175 (W) by 360 (H), ideal for mounting high up above the door opening). However, at normal fan speed, it has an output of a staggering 5.3kW, boostable to 6kW by increasing fan speed. Of course, this means that you are likely to install a smaller model, such as a Myson 10-6. This has a normal output of 2.3kW, boostable to 3kW. It is sized at just 645 (W) by 360 (H), which is about the width of a single door.

The Lo-Line, designed to be mounted low down just like a radiator is 635 (W) by 470 (H) for this level of output. The air blasts outwards somewhat, providing more instant heat than a radiator and making shelving practical above without adversely affecting the heat output as much as a conventional radiator convector.

Alternatively, if the wallspace indicates a "vertical" solution, the Slimline is well overpowered at 3.4kW (boostable to 5kW) at 395 (W) by 800 (H). All these models have a slow speed option that would enable lower powers to be generated, but with less noise.

Oversizing convectors isn't a bad sin, as it has the advantages that the room will heat quicker and that a slower (thus quieter) fan speed can be selected.

Also, if possible, it is a very good idea to have a conservatory on a totally separate zone. The difficulty of achieving this depends on your current system layout. Having a seperate zone allows independent timing, so you only heat the conservatory when it is expected to be in use, which could save you a bomb in running costs in a part of the house that is subject to large heat losses.

Even if the layout of the system doesn't make installing zones easy, it is still possible to fix a programmable thermostat to the fan convector. The heat output will drop markedly when it turns the fan off, both preventing overheat in the room when high temperatures are desired and providing a reasonable drop in temperature when "off", even when the CH water is circulating. Further drops could be made by installing a zone valve (with microswitch disconnected), although obviously the system will only fire when there is a call for heat from the main house heating zone. The fan convector will only fire if there is a call for heat from the conservatory AND the house.

Having blinds, particularly on the roof lights, is also a good idea.

Christian.

[Capitol]

Beware, these things can blow cold air round the room in normal operation under some circumstances. They work well under constant heating conditions, but not when the boiler is cycling. A normal rad loses it's heat very slowly, a fanned rad is almost instant.

Regards Capitol

[Christian McArdle]

conditions,

The one I have automatically cuts the fan when the circulating water drops below around 45C. Indeed, they recommend that you just connect it up to permanent mains and control it just using zone valves with no electrical connection. When the flow stops, the water gets cold and the fan cuts out. I varied their recommendation by connecting it up to the zone valve input so it would cut the fan as soon as the thermostat is satisfied. However, had the zone valve not been adjacent (such as being installed on a whole house radiator circuit), I would have followed their recommendation for ease of wiring.

Christian.