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Thread: Underpowered wet radiators?

  1. #1

    Default Underpowered wet radiators?

    I'm just sizing a radiator for my son's bedroom. The room is about 1000 cu feet, one external wall, which faces south-east (i.e. very good for sun). At the moment the loft insulation is poor, and it is single glazed. Both of these will be changed in the next year or so.

    The central heating system is a wet system, with a new boiler.

    2 questions:

    1. I've looked at various online calculators for working out the radiator size in Btus. The results vary widely between about 1500 and 5000. What gives?

    2. We're concerned about GCC, energy consumption and cost. So we're quite prepared for the radiator to be slightly 'underpowered'. If this means that on the very coldest of days the room is a bit chilly that's no problem. I intend to insulate the loft as heavily as I can, and when the time comes to fit a new window (which is about 2 sq metres, by the way) it'll be double (maybe even triple) glazed, with reflective glass, argon filled if I can get it etc.

    I've read somewhere that a slightly underpowered radiator which is on all the time is better than an overpowered one which is constantly turning on and off (with the TRV).

    Your opinions please.

    Oh, I'm in the UK, north of England, not up a mountain or anything.

    Many thanks, Mike.

  2. #2

    Default more info

    The boiler is a gas combi condensing boiler. Vaillant something or other. Just that I read somewhere that condensing boilers may or may not work best at 'full stretch' but it wasn't clear what the poster meant.

    Cheers, Mike

  3. #3
    Homeowner geniescience's Avatar
    Join Date
    Nov 2005
    humid summers hot, humid winters cold


    Quote Originally Posted by mikemacsween View Post
    just sizing a radiator ...the loft insulation is poor, and it is single glazed. .... a new boiler. ... vary widely between about 1500 and 5000. .... concerned about GCC, energy consumption and cost. .... prepared for the radiator to be slightly 'underpowered'. ... intend to insulate the loft .... constantly turning on and off (with the TRV)....
    too many variables mixed together, so nothing is clear yet. Define GCC and TRV for the average reader please.

    Your "new" boiler has already established for the forseeable future (the next few decades) what your cost and expected performance is going to be.

    Which radiator in a 10' by 16' room (that has an 8' ceiling, a window and an exterior wall) won't make any difference at all, to your cost as a house. How your house was built, with its thermal bridges in its structure, is a far far more important variable. Then how it was insulated when someone first set out to insulate it, is the next big factor.

    Assuming the room in questoin is not in a basement, its floor will be warm, except near the outer wall. Will the room's sides (walls) be warmed by other heated rooms, or is there an unheated space like an attic next to the room? Above it, is the roof space insulated? Spray foam is good both as an insulator and as an air barrier, vapor barrier.

    Iit is true that a large area heater giving low heat and staying ON all the time gives heat more as radiated and less as convected. That is a comfort concern, and it is significant in terms of how you feel in general when you spend time in the room. It does not affect the amount of heat loss to the outside; insulation is everything. If the room is lofty (with a high ceiling), then you do want less convection and more radiation.

    I have a 10' x 17' room similar to yours. It has an exterior wall, with a door (with a window in it) to a balcony, and a small window. For the last six winters I have had no heater in it. The room is bearable except near the outer wall. The floor-wall junction is the most apparently cold spot, since feet get cold. When I place a small electric device on the floor, its small heat changes the heat distribution pattern. What "device"? Anything: computer, kettle, chargers, etc.

    A heater takes up a little space itself, but it changes drastically how you use the space, since you cannot place furniture or "live" the same way when heat is pouring out of a heater designed to shoot it out in one way or another.

    Hope this all makes sense, and helps reassure you that any heater that suits the occupants needs for design and layout purposes is the best choice.

    Last edited by geniescience; 10-21-2007 at 06:18 AM.

  4. #4
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default radiator

    Unless this is a stand alone radiator, it will only be on when the rest of the system is on. Between times, the room will be cold if it loses it heat faster than the rest of the building, and when the heat is on the room will be chilly if it cannot heat the space during the time the heat is on to the other rooms.

  5. #5
    Plumber Cass's Avatar
    Join Date
    Nov 2005


    Like HJ was stating if the rest of the house is insulated well and the furnace does not cycle often enough the room may be cold.

    A small electric ceramic heater may make a world of difference if you find the room colder than you like.

    About $25.00-35.00 at big box stores.

    Be sure the circuit will handel the load as some will draw quite a few watts/amps.


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