Oversized radiators and underfloor heating

ajbaker

Hi all.

Im in the middle of installing my own plumbing for the central heating (yes I have a Corgi guy to do the boiler) and have done the required calculations to pick the appropriate radiator sizes.

Problem is, being rather obsessed by aesthetics, I have found a radiator that will look fabulous in my lounge. Its heat output is around 7000 btu at 50 degrees, while the room requires a maximum of 4800. The question is, can I purchase this radiator and use the TRVs to limit its heat output? Would this cause a problem or inefficiency elsewhere in the system? (I guess it may be better to use the lockshield to limit it as well.)

I was originally planning to put the room thermostat in the room in which this rad will go (with no TRV) but I could quite happily put the thermostat elsewhere and use a TRV in this room.

Also, regarding underfloor heating. Is there a big different between the commercially available underfloor systems and installing a long run of microbore copper pipe under the floor. The design I am considering will sit below a suspended floor on which I just have floor boards. The gap under the floor boards is around 25cm high so will not take long to heat up. Also it is ventilated to help the damp problem (very old property). Instead of purchasing an underfloor heating system I was considering a very long run of microbore to run in parallel lines around 30cm apart under the floor joists. Ideas on how effective this would be? (Would the much reduced temperatue returning to the combi prove to be inefficient?)

ajbaker

I didnt realise this would be such a trick question. Am I just being stupid and missing something obvious, or is there not a problem having a much larger radiator than required?

Plasticman

Haven't got a clue but as nobody has answered yet I thought I would bump the thread up a bit!

micko2602

if im right I think underfloor heating works better when layed into a surface, like concrete or some sort of screed. This acts as a conductor to evenly heat up the area, without this I would think you would have to heat the floor cavity first before the room. I would avoid using microbore for this job too, best use a plastic pipe which will bend easier and last longer.

ajbaker

Thanks for the response thus far.

I'm still hoping someone will be able to answer re oversized radiators - time is ticking on eBay!!

As for the underfloor heating, thanks for your comment micko2602. The reason I thought of using microbore is that it will convect more heat than plastic, and even larger 15mm copper. Also, I find microbore easier to bend because it will actually remain in that shape! I believe you are right about normal underfloor heating being embedded in the floor - is anyone able to explain why, since the heat transfer in concrete is low (?) Yes it would result in heating the underfloor area - I wonder how quickly it could heat an area 3.6m squared x 35cm high?

Plasticman

Found this link:

http://www.irish-energy.ie/uploads/documents/upload/publications/Radiators_-_Size__Position.pdf

See the reference to over sized radiators - while it doesn't sound like best practice, it seems that it will work.

robv_3

Over size rads allows a lower water temperature to be used within the heating system making it more efficient. If run at higher temperatures bigger rads can cause the room temperature to over shoot and rise above the temperature set on the thermostatic valves or room stat. Unless the rad is way oversized you should be ok.

Its important that UFH is designed correctly to work at its best and this always includes high levels of insulation to prevent heat loss. For this reason its very expensive to install in a older property. Using microbore in a vented space under the floor will at best be *very* inefficient. IMHO it will not be enough to heat a room.

ajbaker

Thank you robv. Given the various free radiator size calculators I have found on the web (I ignored those which didnt take into account solid walls and single glazing) the room calls for a rad between 4500 - 5000 btu. The radiator I am looking at is rated at 7000. That's 40% oversized at best. Presumably this would fall outside your threshold of "Unless the rad is way oversized you should be ok."

As for the underfloor heating, thanks for your comments. I did not imagine it would be sufficient to heat the room alone, but was not considering how efficient it would be.

economiser

I don't see any problem with an oversized radiator if you are using TRV's. After all except in the most extreme weather conditions all radiators are oversized. There is an advantage in that it will heat up the room more quickly on start up.

robv_3

ajbaker wrote:

...the room calls for a rad between 4500 - 5000 btu. The radiator I am looking at is rated at 7000. That's 40% oversized at best. Presumably this would fall outside your threshold of "Unless the rad is way oversized you should be ok."

I would use a rad thats 40% oversized if needed.

It used to be normal to calculate heating requirments using a outside temperture of -1 degress but now its more normal to use -3 degrees which will require larger rads. Redo the calculations using -3 and it will only be 20% oversized