central heating issue

Postby openhouse » Fri Jun 27, 2008 10:30 am

All the radiators gets warm, but no one gets really hot.
New Boiler fitted, and I've had 3 different plumbers having a look.
The problem seems to be in the return pipe, as the water reaches any radiator very hot, but comes back warm.
This theory is not entirely convincing, though.
If the water is hot enough in the way in, why the radiator itself does not get hot?
And I'm talking all of them...
Any clues in your own experience?
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Simply Build It

Postby ericmark » Sat Jun 28, 2008 2:25 pm

Back to principle of operation.
1) Boiler heats water
2) Pump circulates water
3) Radiators cool water
The faster the water flows the less differential between incoming temperature at radiator and out going temperature if too fast then the return water is too hot and the boiler will start to close down if to slow the boiler will get too hot and again close down so we have three ways to control the flow.
1) Pump speed
2) Lock shield valve
3) TRV Thermostatic radiator valve
So if pump speed too slow you will get problem as you relate, or it lock shield valve too restricted or if the temperature is too high for setting on TRV.
Since it is June the latter is possible, they also stick so that also could be problem normally they have replacement knobs so they can be manually controlled that would be first test. Or just remove heads.
Next is Pump normally you can hear it and feel it but a magnetic drive so opening bleed screw if not sealed system and seeing change in how quick water comes out with pump on and pump off will show if running.
Unlikely all lock shield valves would have same problem which brings me to last and that’s muck in the pipes my sisters I needed to use tyre compressor to free pipes loads of thin copper slivers must have been in from new. Others in street had the same problems maybe poor quality pipes. No way to check this other than remove pipes.
One thing I have not covered is motorised valve. If this has not activated this also could cause the problem.
I think you have posted in wrong section and it does not make sense that three plumbers have such an open answer were they really looking for problem or had they come for a drink and you collared them!
You should hear some of the things I tell people in the pub. Engine with straight crank shaft, overhead sump and leather pistons and best is they seem to believe me?
You would not believe how many central heating systems there are. If you want help you need to tell people a lot more about it. Open or vented, does it heat domestic water, combi or conventional, if latter Y, S, C, or W plan, has it got motorised valves and if so how many port, are TRV fitted. And try posting in Central Heating & Air conditioning & Ventilation section.
What I can’t understand if new boiler fitted why was it not all sorted when it was commissioned?
All best Eric

Postby openhouse » Sun Jun 29, 2008 4:33 pm

You're right saying I posted in the wrong section.
Sorry, and thanks for your reply.
Trying to give you some more informations, the boiler newly installed is a Combi, providing hot water and heating. When it was installed, hot water was running good (as it still does) and as the radiators were all getting warm,
being a very hot day we assumed everything was working fine.
We only realized during the winter (as usual) they don't go better than that.
There are no thermostatic valves, and anyway as you also said,
they cannot be all wrong in the same way.
About the 3 plumbers, at 45 pounds a pop, I'm in no mood for jokes.
The first I called back was the boiler installer, being CORGI and all.
He seemed very puzzled, and said he could try to clean the system
(running some chemicals, I guess) but with no guarantee of success.
What he did guaranteed, though, was the price he was going to charge.
No less than £200, depending on complications.
The second was advised to me from the shop were I bought the boiler.
He seemed again not very sure on what to do. Suggested something about changing the radiators, but still as a try.
Now, all I would like to do is to be ready for the winter, but according to these people I should do so 'trying' this and that, with no guarantees apart from the price they charge.
The third one, at least, did not mess about.
In his point of view, the only way is to re-run the pipes as it's not possible
to guess where the problem is.
A bit radical, maybe, but at least it would solve the problem.
I'm still not sure what is the best option here,
but is still not clear to me how the water is boiling hot up until the radiators pipe, with the radiators always warm.

Even your theory of having the pipes blocked at some point, I'm not sure can apply here, as all the pipes 'bringing' hot water are hot until all the radiators. If there was a blockage I don't think they would be, at least not all of them.

Thanx anyway for trying to help.
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Postby ericmark » Mon Jun 30, 2008 1:42 am

Please remember I am an electrician rather than a heating engineer but we still get involved from time to time.
Thought one if replacement boiler the old system may have had a bypass valve so when radiators are turned off, there is a route for the water to circulate, if this was stuck open it could cause the problem. There will also be one inside the boiler but I am sure plumbers would have found that if that was fault.
As you say to affect all radiators the restriction would need to be on main feed or main return so an inspection to find any redundant valves from old system.
I tend to agree with you general muck would be likely to affect just a few radiators. I have seen filters to catch muck if there is one this could well be blocked. Some systems have a manifold and others just a large pipe with smaller ones coming off it if there is a manifold they would be my start point.
History may help why was boiler changed what was there before?
The new boilers are not off or on like old types but have variable output so if there is water bypassing the radiators it will reduce boiler output and you should be able to see difference in flame between domestic hot water and central heating. One the old system it was on or off and it is possible any bypass water would not have affected the old type. If there is a bypass the return water at the boiler will be hot.
Any single radiator which is full on can also cause the problem for example my house has a myson radiator with has a thermostatic fan and no restrictor valve on old system this is no problem but on new system it would cause premature shutting down of the boiler.
Very little heat is radiated from the pipes so the pipes being hot and radiators only warm is what would be expected with low flow rate.
My son has a problem with his central heating in that it is not balanced properly with the lock shield valves and on start up all the hot water tends to go through one radiator and then the others are slow to warm up. But he has thermostatic valves on all radiators so as soon as the radiator passing the water warms up the valve shuts off that radiator and forces water through the others and once house is warm the thermostatic valves control the flow and everything works A1 it is only with a cold house he has a problem. But you have no thermostatic valves so any radiator full open with your system would not auto switch off when warm and would turn down boiler.
It is all to do with being condensating old boilers did not mind hot water returning as long as water leaving did not exceed a set limit but new boilers must have return water cool or condensation part will not work.

Postby htg engineer » Mon Jun 30, 2008 9:15 am

Simple checks first.
Boiler thermostat set to maximum ?
Pump running, strong enough to circulate the water ?
Was an external bypass fitted, when the combi was installed ? if this is fully open, the water can sometimes return at the same temperature as the flow - result: boiler at maximum temperature - radiators not.
At boiler, check flow and return pipes. Flow should be hot, return warm.

If the flow is hot as you say, but the radiators only get warm - not hot, there's really only two things that could be wrong.

Radiator valves, restricting the flow.
There's a partial blockage somewhere in the return pipework.

Radiator valves fully open ? both sides of each radiator, they may seem to be, but you will have to remove the valves and check the spindle and washer, check for debri in the valves. If they're old valves and you're going to the hassle of draining and removing the valves, would it be worth fitting new radiator valves ?

I can't see anywhere where you have said the radiators or valves have been removed or renewed ??? checked for sludge etc.

If this fails, you need to start checking the return pipework. What size pipe feeds the radiators ? any manifolds (manifolds is normally where you get the blockages).

Also worth a try, turn off all radiators but one. Gets hot ?

I'm guessing, there's either a blockage - or the pump is struggling. Yes it's a new boiler, but they still go wrong.

htg engineer
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Postby openhouse » Mon Jun 30, 2008 11:15 pm

First of all, thanx to the two of you and to this forum that I've found more helpful than the three (not so wise) man I've had around.
To give you the best picture I can, I try to answer:
-boiler thermostat set to max temp.? YES
-pump strong enough to let circulate the water? I wouldn't know how to check this out myself, but, having had three plumbers around...
Still, if you could be more specific on how to check it myself, I'll give it a try.
-external by-pass fitted? Not that I'm aware of. But it may help knowing that we're talking about a very small 1 bed first floor flat with only 5 radiators to feed, the boiler being a Valliant 831.
-at the boiler: flow very hot? YES; return warm? YES, and is the same to all the radiators, unfortunately radiators included (in the warm).

About your last two options:
-Radiator valves restricting flows: apart from having checked all of them
more than once,
I would have had them changed at a first place, but even the plumber said
that would be very unusual that they're all having the same problem.
-Blockage in the return pipes: size pipes feeding radiators? I would say 15mm.

If the blockage is the problem, I was thinking of running new pipes this time through the loft, as it would be virtually impossible lift the floor to check where the problem is.
Would that be a solution, or am I going from bad to worse?
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Postby htg engineer » Tue Jul 01, 2008 12:14 pm

The Vaillant 831 has an internal bypass fitted, so an external one is not required and would not have been fitted.

15mm pipework, would be very unusual for this to become blocked unless you have a serious sludge problem.

Turn off all radiator, apart from the one nearest the boiler. If this still doesn't heat - i'd be tempted to get manufacturer out to check the boiler over first. It may be something as simple as the gas pressures are incorrect.

Did all radiators get hot with the old boiler connected ? has this problem only occurred with the new boiler ?

Re-piping the radiators could solve the problem (depending on the answer to the question above). If they worked before, then probably a foult on the boiler. If they've never worked properly, it will be a pipework or radiator/valve problem.

htg engineer
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Postby openhouse » Wed Jul 02, 2008 8:49 am

The boiler is due to be serviced in the next few days.
Apart from the gas pressure you already mentioned, anything else I can ask
to check?
When the old boiler was in place, as far as the previous owner is to be believed, everything was working fine.
But when I took over, we also decided to move it from where it was to where it is now, and that is why we also bought a new one, as we were told that moving a boiler can give problems.(by the way, is that true?)
So, when the new boiler was in place, some pipe work it's been re-done to adjust to the new situation and all I can think of is that it's not been done properly.
I haven't try yet to leave only the nearest radiator on turning the heating,
but I'll do it soon and let you know.
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Postby htg engineer » Thu Jul 03, 2008 8:18 am

Moving a boiler can sometimes cause problems. But moving and replacing with a new boiler shouldn't. Basically because all the components are new and the pump will be at it's best performance. As long as pipework is relatively level, and not up and down all over the place.

If pump is running as it should then the water will circulate as long as there are no restrictions. Such as faulty or closed radiator valves, sludge in system, damaged/kinked/crushed pipework. As you seem to have checked all these. I would say it's a blockage or problem in the return pipework, a problem with the boiler or the pump isn't strong enough.

have you checked the flow and return isolation valves on the boiler are fully open ?

When the boiler is on, does it reach temperature quickly and the flames cut off ? when the flow gets hot, do the flames go out ?

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Postby Perry525 » Thu Jul 03, 2008 12:01 pm

As I understand it, a radiators job, is to radiate heat.
You say that the water in the pipe, is very hot when it arrives at the radiator, the radiator never gets hot, only warm. And presumably the water pipe leaving the radiator is warm to cold?
This heat doesn't just disappear it warms the radiator and moves into the room.
If you have a continental system?
Where the radiators run at 50 degrees C or less?
And the heat input from the boiler is by our standards low, then the heating engineers design is off the mark.
So, what is the output of the boiler?
What is the designed output of the radiator?
Look at the output water temperature on the boiler,
is it 70-90C after an hour running?
Keeping in mind that central heating is usually designed to raise the temperature of a room by 3 degrees C every hour, what is the starting temperature of the room when the heating is turned on? How long do you run the heating before you expect the room to be comfortable?
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Postby htg engineer » Wed Jul 09, 2008 2:43 pm

As I understand it, a radiators job, is to radiate heat.
You say that the water in the pipe, is very hot when it arrives at the radiator, the radiator never gets hot, only warm. And presumably the water pipe leaving the radiator is warm to cold?
This heat doesn't just disappear it warms the radiator and moves into the room.

....... but to heat the radiator the water has to circulate - if the return is blocked the water/heat cannot circulate.

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Postby Perry525 » Wed Jul 09, 2008 5:27 pm

You miss the point!
The pipe running up to the radiator can only heat up, if the cold water that was in the pipe has moved forward along the pipe or into the radiator and back to the boiler.......
Thereby making space for the new hot water!
If the cold water moves, the hot water moves into its place, therefore, the pipes are not blocked, however, if there is a connection between the outgoing hot water pipe and the return, making in effect one continuous easy loop most of the hot water will not bother to travel through the radiators.
If the radiator does not heat up, the radiator load is so big and the heat input so low, that the heat is radiated at almost the same rate as input., therefore lukewarm, no great heat output.
One needs to track the hot water down the pipe to find the bridge.
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Postby htg engineer » Thu Jul 10, 2008 9:47 am

Yes, if there is a bridge. I cant see anything in the post to suggest this is anything other than a 2 pipe heating system, with a flow and return to each radiator.

Therefore if the pipework is partially blocked the water will not circulate effectively, it obviously is circulating as the posts says the flow is hot, the radiator and return is warm.

If the Flow is Hot but the radiator is not - it's either a problem with the radiator valves or the return pipework RESTRICTING (not stopping) the flow of water.

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Postby htg engineer » Thu Jul 10, 2008 9:49 am


Any progress ? what did the engineer that serviced the appliance find ???

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