Hoping someone with some heating / plumbing experience might be able to help me on this one.

I've just had a new radiator fitted in my kitchen (big beast of a rad designed to pump out 8500 BTU) to replace a smaller rad as it wasn't big enough for the space.

After spending to have this done we're gutted to find that the radiator isn't getting as hot at the others. I've closed the lockshield on all the others and it now gets hotter than it did but not as hot as I would expect or as hot as the other radiators are. I've now opened the lockshields and "balanced" the system as best I can and the heat seems more even in the house now.

Two questions I've got iare….

1. If I've balanced the rads and the house is now "more even" in temperature (although this rad is still not as hot as I would expect) will my boiler run more or the same as before – I basically don';t understand that if I am allowing less water to get into each radiator, especially those closer to the boiler / pump will I use more gas to keep the whole house warm even if the temp of the CH water is set to 60degrees?

2. The offending rad is the furthest away from the boiler, having its pipes go from the back bedroom to the front of the house then downstairs under the house all the way to the back extension (a run of 25-30m overall). I have a radiator in the dining room which is fed from the boiler directly above it (a run of about 4m) which looks like it could have pipes taken from this one under the house and to the kitchen giving an overall run of about 7-8m making it closer to the boiler – if I did this would this make this radiator hotter as it is closer to the boiler and gets "served" quicker?

This is doing our heads in as we wanted it to heat our kitchen up to make it useable in cold weather and isn't working.

Barney have you checked for air in the radiator?

Turn all the rest off, does it get hot?

Pete – yeah – bled all rads.

Paul – I turned all the others off and it got hot but not as not as the others get when they are hot…

Is it as hot as the rad it replaced?

no, the old one used to get so hot I couldn't comfortably touch it but this one I can hug and it doesn't burn me. Although at the top and bottom it does get really hot – just in the middle it isn't hot.

Its one of these in 1800 x 600 double

Ultraheat Planal

Actually, I've turned all the rads off again and will see how hot it gets (will get the temp probe on it)

Given I know nothing about central heating, maybe it just means the new rad is more efficent at tranfering heat to the air than the old one. The water temp is the same, but it's losing more heat (which is good) so is colder.
The above could also be bo***ks.

heat flux equation for a heat exchanger:

Q = UA delta T

where:
Q = heat flux
U = overall heat transfer coefficient (we'll assume this is constant for both radiators)
A = surface area (much bigger on your new radiator)

delta T = ((temp water in – temp air)-(temp water out – temp air))
ln((temp water in – temp air in)/(temp water out – temp air))

increasing the surface area will be balanced by an increase in Q and a decrease in the temperature of the water leaving the radiator. given that heat dissapates more quickly in metals than liquids the radiator temperature will be more uniform than the temperature of the working fluid inside. This will lead to an overall lower temperature of the radiator metal.

Sorry I guess all of that is of no real practical use to you if your kitchen is still cold though!

Radiator is slowly getting very hot – but bottom 1/3 is quite cool compared to the top 1/3

If I do (2) above will this increase the flow to increase the time to heat the rad as it will hopefully get more of its fair share of heat as its not at the end of a long line of rads nicking the hot water.

Try this, follow step by step.

HOW TO BALANCE A CH SYSTEM
If some of your rads are not getting hot you will have to balance the system.

Failure to correctly balance a system can lead to complaints of inefficient operation and criticism of the operational abilities of thermostatic radiator valves, when in fact the valves are not the source of the problem.

Common complaints include:

Rooms are slow to reach temperature even though radiators are sized correctly

Some rooms with correctly sized radiators never reach temperature during cold periods

Room temperatures fluctuate particularly if the TRV is on a low setting

Balancing procedure:

You will need either two clip on pipe thermometers (£12 each) or an infrared thermometer (about £30 from Maplin). I have used both and prefer the infrared as it has a faster response, is more accurate and can be used for other things, e.g checking the temperature distribution over the surface of a rad.

1.Turn boiler off and allow to cool
2.Make sure HW is turned off
3.Open all LS valves fully; remove all TRV heads and open all manual rad valves fully.
4.Turn the boiler temp to max and bring system up to temperature.
5.Check, by feel, the order in which the rads warm up and make a list.
6.Adjust the bypass (if fitted) as per mfr instructions
7.Measure the temperature differences between the flow and return pipes at the boiler. What it should be will depend on the boiler -refer to the installation manual. If the temperature is not right, you need to adjust you pump speed (higher speed gives smaller difference and vice versa). Set the speed to give as a difference as close to the mfrs recommendation as possible.
8.Turn the boiler off.
9.Close all lockshield valves. Leave TRV heads off.
10.Restart boiler
11.Go to first rad in list made in step 5
12.Attach the pipe thermometers (if used) to the flow and return pipes near the rad valves and open the LS valve a quarter turn.
13.Wait until temperature has stabilised on the two thermometers (or use the IR thermometer to measure flow and return temp in the pipes adjacent to the valves). If the difference is smaller than that at the boiler, close the LS valve a fraction, or vice versa. Wait until the temp has stabilised and check again.
14.When you are happy with the first rad, move on to the next on the list.
15.Repeat steps 12, 13 and 14 for each rad on the list.
16.When you have reached the last rad, check the boiler temp difference and adjust pump speed if necessary.
17.Go back to the first rad and test it is still giving the correct difference
18.And so on, going round and checking the temperature difference until they are all as near the required drop as you can make them. Don't expect perfection
19.Replace TRV heads and set to required temperature
A very small change in the amount a LS valve is open can have a considerable effect on the temperature difference. Because of the way LS valve are constructed, it is virtually fully open when it is one and a half turns open. So most adjustment will be within one turn from closed.