I'm replacing all my radiators and boiler due to terminal system failure. At the same
adding 5 new rads. My house is a long thin terrace and the main CH pipework seems to
be in twin straight lines down the centre, with long branches off to each room, like a
My question is:- all the literature shows the main circuits running in ... well ... a
circuit around the house directly under each rad, with short tees upwards. Is this
only acceptable way to arrange a CH system? Or can I continue with my cunning plan to
replace all the rads but leave the pipework alone?
Hope you can help.
There's nothing wrong in principle with your system of plumbing. What you
need to ensure in practice is that the main arteries have sufficient
capacity to feed all the radiators - including the additional ones. If in
doubt, add up the heat output of all your radiators and come back here.
I would expect (without seeing the figures!) that you need 22mm pipe (or
even possibly 28mm) for the arteries. If they are currently 15mm they will
almost certainly need replacing with larger pipes.
You can plumb it anyway you like, provided that each section uses big enough
pipe for the flow at that point. For example, you can snake round from
radiator to radiator. You can have a major trunk with long tails to each rad
(like you have). You can have all the radiator tails having their own runs
all the way back to manifolds near the boiler (or even a manifold per floor
linked by a trunk pipe). Do whatever is convenient.
Whilst you are repiping, consider subzoning with an S-Plan-Plus system. Even
just having two zones allows different timings and temperatures for upstairs
bedrooms and downstairs receptions.
Provided that the disaster that befell your system didn't affect the pipes.
(And also assuming the previous pipes were adequetely sized).
You can stick with this arrangement, which is practical for the type
of house - I did one myself this way once.
If you are adding radiators, you do need to make certain that the
pipework is adequate at each point.
Begin by checking the output ratings of the existing radiators by
comparing with the closest types in manufacturer data sheets and
accounting for whether they are double or single and with or without
fins on one or both panels.
If you are adding new radiators, is it because the existing system was
inadequate or because some rooms didn't have them? It would be
sensible to do a proper heat loss calculation for the property.
Either you can use tables of U values from the British Standard, or
there are programs from the radiator manufacturers that allow you to
plug in the wall, window, floor and ceiling areas for each room and to
determine the heat loss through the surfaces. You then add in amounts
for air changes. This will give you the required amount of heat for
Don't guesstimate because you might be wildly wrong. Older terraced
properties generally have not unreasonable heat losses in the main
part of the house, but huge ones in the rear extension because of the
solid external walls.
Once you have done that, you can use
to check and size the copper pipes.
In most properties, having 22mm for the main trunks is adequate unless
it's a very large place in which case 28mm may be needed nearer the
boiler. The thing to watch is that not too much radiator capacity
is run from branched off 15mm sections. It is the capacity that
counts, not the number of radiators. The pipe sizings and lengths
affect the flow rate. The flow rate determines the amount of heat
that is transferred.
To email, substitute .nospam with .gl
Thanks for all the advice. I have indeed done heatloss calculations. The rear two
downstairs rads total 3756w and the front two total 2730w. There are two branches
upstairs serving rad outputs of 2945w and 2736w respectively. The only pipework in
is the main section between the boiler and the second upstairs branch.
Somewhere it says that 15mm pipework can support a max of 6000w including the pipe
at 46w per metre uninsulated. Is this about right?
Now I am wrestling with radiator outputs. Wickes latest leaflet publishes outputs in
calculated at (triangle)T50 rating (uh?). This differs from their previous publicity
about 8%. The rolltop rads look reasonable quality, though.
You often have to adjust manufacturers' figures to suit your installation.
The actual output of a radiator is a function of the difference between the
mean radiator temperature (usually half way between flow and return temps.)
and room temperature.
I've just looked at a Stelrad brochure, in which the outputs are quoted for
a 60 degC difference. A typical system will have a difference nearer to 55
degC (Flow 80, Return 70, mean 75, room 20). The brochure has a conversion
table which gives a factor by which the output has to be multipled for
different temperature differences. For 55 degC it is 0.898
In other words, the radiator will, in practice, have an output about 10%
lower than the standard published figure. You can, of course, adjust the
figures for other scenarios - but it is something which you do need to do
rather than just accepting the raw figures.
That's a starting rule of thumb. Be careful though, because if the
pipe runs are long, (as they can be in this kind of scenario relative
to a semi or detached house which is more square in shape) then it may
need to be reduced. I would check using the CDA white paper
calculation method. The thing most likely to catch you out is too
much radiator output on a lengthy 15mm branch.
First of all, dump BTUs and work entirely in watts, metres, degrees C
etc. It's a really bad idea to mix units together because the
conversion factors are obscure and sooner or later you'll make a
If you take a given physical radiator, the heat output depends on the
water temperature drop across it and the temperature difference to the
First of all you have the Mean Water Temperature which is basically
half the difference between flow and return temperatures.
With a conventional non-condensing boiler the flow design temperature
is 82 degrees and return is 70, so the MWT is 76 degrees.
Secondly, you have Mean Water To Air temperature (MWTA) which is the
difference between MWT and the room temperature. So if the room is
heated to 21 degrees then MWTA is 55 degrees. MWTA is the delta-T
number that you mentioned (delta is a Greek letter symbolised with a
kind of triangle and generally used to denote a difference between two
things). Sometimes it's written as dT.
The standard method of radiator output measurement (a Euronorme
method) uses higher temperatures than are the common 82/70 used in UK
heating systems. The specified output numbers are for this higher
figure. So manufacturer's data sheets provide a derating table and a
factor that you use to multiply to give the output at actual
operational temperature. Generally the figure is 0.89 for this
common case - i.e. a radiator specified at 1000W will have an output
If you go to one of the radiator manufacturer web sites - e.g. Stelrad
or Myson, they have data sheets which show the tables.
If you were going for a new condensing boiler, these can run at 82/70
but will run more efficiently at 70/50. The impact of this is
that larger radiators are needed because the derating factor then
works out to 0.6 typically. I don't remember if you said you were
going for a condensing model but if you are and are changing all the
radiators as well, then it's worth sizing on this basis. You would
get some energy saving from it, but mainly when it's really cold.
During the spring and autumn, the boiler would modulate down because
the heat isn't required anyway and radiators sized for 82/70 would
still give enough output.
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