How to calculate radiator size?

V

VictorDelta

Joined
18 Aug 2008
Messages
80
Reaction score
0
Location
Dorset
Country
United Kingdom
Our kitchen, where the family spend most of their time, has two radiators but they are not generating enough heat to make the room warm enough in cold weather.

Can anyone please recommend a website, or programme, which will help me calculate what size radiators we actually need? (Our boiler has spare capacity). Many thanks.
 
This might be suitable: http://www.bestheating.com/btu-calculator not sure as to the accuracy of the results or to any official guidance but it does seem to ask a fair few more relevant questions compared to the b&q calculator. You mention you have spare capacity, do you know this for sure? Also do you know the supposed BTU of the current radiators? Underfloor heating and/or plinth heaters might also help.
 
Heat loss calculations

You have say 4 walls, ceiling and floor
Walls will have windows or doors
Room will have between 1 and 2 air changes
Room will have design temperature
External assumed to be at zero degree or thereabouts
Each structure will have 'U' value

Heat loss equals room design temperature less 30 degree F times U value times area in feet giving you BTU lost through that fabric

E.g. A window 4 foot square, single glazed will have a loss of 4x4x1x(75-30)- room temperature is 75
Heat lost through the window.....16x45.......720 btus

Hope above helps and is a start, but am sure there are clever people here who will show you a better way of calculating room heat loss
 
Before you start calculating anything you should measure the flow and return temperature of the existing rads and compare their sizes with some new ones to see what heat output they should give.

Rads should be at about 70 C at the top. Too hot to hold even for one second!

Since rad outputs have been measured in kW since 1980, I would expect any calculator not to be using BTU any more. They are an ancient unit from the days of steam!

You can compare the sizes of your rads with new ones to find out their heat output.

When you have checked the rad temperature, compared the rad outputs and done your calculations then there is a little trick which only takes minutes to do.

That is to estimate the power required from the floor area. A very well insulated property needs 50 W, or less, Wsm. Most about 80 Wsm and less well insulated and solid walls about 100 Wsm.

Since new rads will be installed with TRVs then oversizing is no problem. Indeed it enables the circulating water to be at a lower temperature which makes a condensing boiler more efficient.

Tony
 
Many thanks again!

Agile/Tony - particularly like your floor area rule of thumb as a double check for the calculations. :)
 
Last edited:
Heat output for radiators still quoted in buts and kW, year 2017:p
Calculation shown not copied from anywhere!!! That is the way I remember, see no problem getting heat loss in btu and then look up the chart.
 
Hey DP , I'm not sure Agile is correct when allowing 2kw for HW??..:)
 
  • Thanks
Reactions: DP
You may find that 2kW is the figure recommended by the EST and also included in some of the calculators!
 
Steel, when one starts googling, as opposed to hands on graft and knowledge, everything is ok.
Thinking outside the box and based on practice experience is not allowed:whistle:

Bigger fools these HW cylinder manufactures who build cylinders with plus 20kW indirect coils.
 
You must log in or register to reply here.
Back
Top