Radiators colder at bottom hot at top

6 Sep 2005
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United Kingdom

I know this has been asked many times before but my question might be slightly different.

Boiler was installed a year ago and central heating cleaner added then. I bought the house and this winter the radiators were cold at the bottom. So, drained the system, flushed the radiators out with a hose put it all back.

I put more cleaner in when I filled the system back up.

I then balanced all the radiators in the system as they didn't seem to be balanced and now they all have a 10 degree Celsius difference between in and out flow on the radiators. However, they are still cooler at the bottom.

I am due to drain the system again and then put inhibitor in, however, I am unsure why the radiators are still cooler at the bottom. The cleaners been in for over a month and I manually flushed the system.

Does anyone know why they could still be cooler. There is about a 10 degree difference in temperature between the top of the middle of the radiators and the bottom in the middle.

Please help, I'm now confused. :(

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might still be sludgy. Radiators cool in the middle is typical of insufficient flow due to sludge.

Is the bottom of the rad the same temp as the Return pipe, or colder in the middle? Is the house warm enough even with this radiator pattern?
What sort of cleaner did you use, when did you add it, what colour is the water when you bleed it, are any of the rads fully hot, do you have TRVs, is the cylinder very hot (or is it a combi), if you set HW off at the programmer, does the HW still get hot, how old are the radiators, how many do you have and what size, is there a bypass and is it adjustable with a valve, how old is the pump and what speed is it set to, is it an open vented system or sealed, what is the boiler output and is it continuously firing with these cool radiators or does it turn itself off, if you feel the Return pipe at the boiler, is it hotter than the return from the radiators, what temperature is the flow at the boiler, what make and type of boiler is it.
Hi John,

Thanks for the reply.

The bottom of the radiator is the same temp as the return pipe.
The house isn't warm enough with this pattern.
I used No Nonsense clearn, but I am planning on using Sentinal for a 2nd flush. The cleaner was added exactly 1 month ago today.
The water is clear when I bleed it.
None of the rads are fully hot. The only way I can do that is by turning some of them off.
TRVs are fitted to all but the bathroom radiator although they don't seem to work. If they are on full or set to frost the radiator is the same temperature.
The radiators look old, I don't know how old though. None of them have fins fitted to them.
I think the pump was fitted last year before I bought the house. I can't see what speed it is set to because it is pushed upto the edge of the cupboard. :rolleyes:
It is an open vented system and the boiler output is 24Kw.
The boiler does turn itself off. The return pipe is about the same temperature as the return from the radiators.
Boiler flow temp is about 60 Celsius, return is about 50 ish. It is a condensing boiler.

The boiler is a Worcester 24Ri GB.

If anyone else can help with this problem it would make me a happier and warmer person. Could the issue also be because the pump isn't running fast enough?

The house is 3 bedrooms with sizeable radiators in all rooms. I can't see the pump speed because it was installed up against the side of the airing cupboard. :(

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I am not familiar with No Nonsense cleaner so it might work a different way. I say this because you report the water is clear. When you put in Sentinel X400, the water quickly goes jet black with dislodged sediment (unless the system is perfectly clean, which is rare with an open-vented system that is not new). Best to drain and rinse the old chemical out in case they are not compatible.

You do need to drain a couple of buckets of water out of the drain cock so that the chemical you add to the Feed and Expansion tank is drawn down into the system. Bale any mud out of the tank and sponge it clean first.

Note that Cleaner and Inhibitor are not the same chemical.

You say you can't see the setting of the pump. It is certainly worth trying to get at the knob and, if it is on slow speed, turning it up. If the pump is positioned so you can't get at it, you may have to close the pump valves, undo their connectors and turn or remove the pump so you can do this. You say the pump appears to be quite new, so it ought to be working properly and the connectors should not be seized. If you take it off, open the pump valves one at a time into a bucket to verify there is good flow.

You can also turn up the boiler thermostat. I usually run mine at 70C, but in the recent cold spell I turned it up to 80, which made the radiators hotter and the house warmed up faster.

You say the rads all have TRVs except for the bathroom one. Take the thermostatic heads off and see if the pins move up and down (sometimes they seize).
is the cylinder very hot, if you set HW off at the programmer, does the HW still get hot?
Hi John,

Thanks for the image of the pump. I had another look today before seeing this post and realised that the speed setting is accessible. It was on 2 out of 3.

Is it wrong/bad to put it onto 3? I assume it's okay to just try it and see?

I have checked all the TRVs and the valves move up and down freely on all of them.

When the hot water is off at the programmer the tank stays cold.

Thanks for the tip about turning up the boiler temperature when it gets colder. I'll bear that in mind, probably this week. :)

Sounds like the system has been piped ok if the water is clear after a full drain down. (feed and vent, pump position etc)

How long does the system take to warm up to a reasonable temp?

Have you got access to your gas meter? Could you identify what meter you have, U6 E6 G4 etc should be written on it somewhere, or describe what it looks like.

How far is the boiler from the meter, and what size gas pipe feed's it?

OP, can you turn the HW zone off and run heating on it's own?

I would run the boiler at a higher temperature (75 degrees C) and have flow to return differential adjusted to 20 degrees as opposed to 10.

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