CH problem - radiators not heating up

[JohnM]

We live in a large old house that has been converted into 5 flats. There is a large boiler and about 60 radiators. There are two radiators on the highest level (second floor) that are a long way from the boiler and don't heat up. We've tried turning off a number of other rads in the house to increase the pressure and this does get them hot after about an hour, but after a couple of days they cool down again. We've bled them and as far as we can tell there's no air in them - however they have bleed valves that you use a screwdriver with rather than a key, and on one of them the screw doesn't tighten up. Is it possible that this valve is letting air into the system? Or could the TRVs on them be both faulty ? Any other ideas gratefully received !

Thanks
John

 

[FamilyWiggs]

Sounds like the system may need balancing.

 

[Craig]

yep - very much sounds like the system needs balancing. http://www.diydoctor.org.uk/projects/balancingcentralheatingsystems.htm

There is loads on the web about this - you basically want the lockshield valve on the rads "nearest" the boiler (i.e. those getting water first) to be open the smallest teensy amount possible. You want a very significant difference between the water going in and the water coming out - you can use pipe thermometers if you like but it’s very easy to tell with your hands.

For the faulty bleed screw, just replace if you can - it’s possible that it’s leaking

I also believe you are supposed to bleed & fill when cold?

You can always install a more powerful pump.

For the TRVs, sometimes screwing them all the way in and then back out can help "clear" them. Our plumber would also hit them with a big hammer. Doubt that fixed them but he liked it...

 

[Andrew Bodenham]

Previous comments about balancing the whole system are entirely correct, and the bleed screws should be renewed if you have suspicions about their sealing capabilities. If they aren't sealing though they ought to be leaking water, unless the pump is managing to suck air in through them, which may explain your problem.

Is it an open vented system (e.g. a header tank somewhere in the attic) or a more modern closed and pressurised system? If its an old system then the pipes / valves feeding the rads may be sludged. The best way to check this woud be to drain and remove the rads, then open the valves and check what sort of flow you get. There are various chemical system cleaners which can be added to remove sludge - if it's a problem then the whole system would benefit from flushing out.

 

[skier-hughes]

If its an old system then the pipes / valves feeding the rads may be sludged. The best way to check this woud be to drain and remove the rads, then open the valves and check what sort of flow you get. There are various chemical system cleaners which can be added to remove sludge - if it's a problem then the whole system would benefit from flushing out.

I was also thinking this.
Graham

 

[Penners]

I thought that TRV's made it un-necessary to balance the system. Is that wrong?

Surely if all the rads have TRV's the system becomes self-balancing - all you have to do is make sure that each TRV is properly set to allow that room to reach just the right temperature.

Experts - please shoot me down if that's wrong.

 

[skier-hughes]

I suppose it depends on size of boiler, but if the first 58 rads were set to 30' would there be enough heat to heat up the other two?

I suppose we should also ask the question, what size is the large boiler? Maybe it was designed to heat 58 rads :wink:

 

[Craig]

As noted, the TRVs would achieve the same effect if you turn them all right down. However, the aim is to have a system that works...

 

[Andrew Bodenham]

The TRV's will certainly have some throttling effect, but from a design point of view you have to assume that they are all open and 'calling for heat', so the system still needs to be balanced, unless the overall system resistance is very small, or the pump is heavily over-spec'd.

 

[Penners]

unless the overall system resistance is very small, or the pump is heavily over-spec'd.

Which, on a 60-rad system, isn't very likely!

 

[JohnM]

Thanks for all the good advice - I suspect that balancing may be the cause. Someone suggested a more powerful pump - there are actually 2 pumps on the system, because there are two separate circuits - the orginal cast iron one (which was originally gravity-fed) and a newer copper one (which I know isn't a great idea !). The pump on the copper circuit is quite small and is set to it's maximum setting. The pump on the cast iron system is very large and powerful, and in the past has always been set on it's lowest setting - 1 on a scale of 1 to 3. I've changed this to 2 without improving matters but perhaps that's because it will only affect the copper circuit indirectly. Maybe I should try 3 ?

It is a vented system with a header tank that is only slightly higher than the two problem radiators. The system has been drained and refilled recently and was flushed two years ago when the boiler was replaced. It probably does contain some sludge but I'm not sure why that would only affect the two highest rads - would it not tend to accumulate lower down ?

I don't have the spec of the boiler to hand but as I mentioned it was replaced two years ago and I'm sure it was sized correctly.

Cheers

John

 

[Pford75]

John - I would also have a look at this page >http://www.letsfixit.co.uk/html/plumbing_faq.html

Specifically, the section headed "Q: I have a cold radiator could it be a sticking TRV, what can I do?"

 

[Aidan]

> Is it possible that this valve is letting air into the system?
Quite possible; you could replace the air vent in about 5 minutes. You can get low pressures in some systems (i.e. < atmospheric) which will allow it to draw air in through valve glands, or anywhere that you might otherwise get a leak from.

>Or could the TRVs on them be both faulty ?
Could be, but it's not likely to affect only 2/60 rads & those 2 in the same place. Check the pins spring up; don't pull them hard, they can come out.

>yep - very much sounds like the system needs balancing.
Maybe. I think it's a pump pressure problem mostly; balancing helps.

>You can always install a more powerful pump.
You then get a more powerful problem. It was a gravity system (big pipes, low resistance) so the pump is quite likely to be oversized. It formerly worked with NO pump. It's also had a modern boiler added (higher resistance compared to sectional cast-iron jobs). It could be a whole lot of things. A inverter drive pump would allow infinite variations.

>If its an old system then the pipes / valves feeding the rads may be sludged. The best way to check this woud be to drain and remove the rads, then open the >valves and check what sort of flow you get.
No, just isolate & take any problem rads outside and hose them out. If there is a sludge problem, it indicates a corrosion problem, often air being drawn in. A magnet that is attracted to copper pipes is a certain indicator.

>Surely if all the rads have TRV's the system becomes self-balancing - all you have to do is make sure that each TRV is properly set to allow that room to reach just the right temperature.
>Experts - please shoot me down if that's wrong.Penners

Pass the 12-bore, Carruthers.
It's an urban myth put about by work-dodging plumbers. The TRvs are fully open until the room temperature approaches the set point. Until then, the most favoured circuits (rads nearest the pump), will act as short-circuits and will get all the flow. The furthest rooms will only get heat as the lower rooms heat up; this could take a long time.

>I suppose we should also ask the question, what size is the large boiler? Maybe it was designed to heat 58 rads skier-hughes
Unless grossly under-sized, it should cope with part-load conditions, like the current weather. It's a pump & static presssure problem.

> unless the overall system resistance is very small, or the pump is heavily over-spec'd.
>Which, on a 60-rad system, isn't very likely!

Many plumbers are clueless about sizing pumps. Those that do know how often don't bother, but take a flying guess.

> Someone suggested a more powerful pump - there are actually 2 pumps on the system, because there are two separate circuits - the orginal cast iron one
>(which was originally gravity-fed) and a newer copper one (which I know isn't a great idea !).

Oh dear. That's probably not a good sign, unless the installer knew his stuff about low-loss headers and primary-secondary pumped circuits. You're more likely to find that it was installed by a gang of moonlighting fairies.

>It is a vented system with a header tank that is only slightly higher than the two problem radiators.
Could you convert it to a sealed system? It would fix any air-being-drawn-in problems. The boiler has to be suitable (manual re-set high limit thermostat) but it probably is if it's 2 years old.

Check the F&E tank is correctly set up. It should have a 2 or 3 inches of water covering the cold feed outlet with the system cold. Most of the space in the tank is to accomodate expansion when the system is hot. Ensure the water doesn't get near the overflow with the system hot. It's a big system so the tank should be bigger than usual.

> The system has been drained and refilled recently and was flushed two years ago when the boiler was replaced.
And corrosion inhibitor replaced? Any rads needed replacing due to corrosion/leaks?

> It probably does contain some sludge but I'm not sure why that would only affect the two highest rads - would it not tend to accumulate lower down ?
It would, but it restricts the pipes and the furthest rads, being least favoured, get affected first.

Boiler make and type? Condensing boiler?
Do you do the plumbing yourself?
How does it work with one boiler supplying 5 flats? Are the fuel bills split? Is it on all the time? Weather compensation?
How does the domestic hot water work? Any problems with that? Is there a secondary DHWS circulating pump?

 

[JohnM]

Hi Aidan,

I hope you can make sense of my comments on your points:

>You can always install a more powerful pump.
You then get a more powerful problem. It was a gravity system (big pipes, low resistance) so the pump is quite likely to be oversized. It formerly worked with NO pump. It's also had a modern boiler added (higher resistance compared to sectional cast-iron jobs). It could be a whole lot of things. A inverter drive pump would allow infinite variations.

The replacement boiler is a sectional cast-iron job.

>If its an old system then the pipes / valves feeding the rads may be sludged. The best way to check this woud be to drain and remove the rads, then open the >valves and check what sort of flow you get.
No, just isolate & take any problem rads outside and hose them out. If there is a sludge problem, it indicates a corrosion problem, often air being drawn in. A magnet that is attracted to copper pipes is a certain indicator.

I must try that !

>Surely if all the rads have TRV's the system becomes self-balancing - all you have to do is make sure that each TRV is properly set to allow that room to reach just the right temperature.
>Experts - please shoot me down if that's wrong.Penners

Pass the 12-bore, Carruthers.
It's an urban myth put about by work-dodging plumbers. The TRvs are fully open until the room temperature approaches the set point. Until then, the most favoured circuits (rads nearest the pump), will act as short-circuits and will get all the flow. The furthest rooms will only get heat as the lower rooms heat up; this could take a long time.

>I suppose we should also ask the question, what size is the large boiler? Maybe it was designed to heat 58 rads skier-hughes
Unless grossly under-sized, it should cope with part-load conditions, like the current weather. It's a pump & static presssure problem.

> unless the overall system resistance is very small, or the pump is heavily over-spec'd.
>Which, on a 60-rad system, isn't very likely!

Many plumbers are clueless about sizing pumps. Those that do know how often don't bother, but take a flying guess.

> Someone suggested a more powerful pump - there are actually 2 pumps on the system, because there are two separate circuits - the orginal cast iron one
>(which was originally gravity-fed) and a newer copper one (which I know isn't a great idea !).

Oh dear. That's probably not a good sign, unless the installer knew his stuff about low-loss headers and primary-secondary pumped circuits. You're more likely to find that it was installed by a gang of moonlighting fairies.

You're right, the additonal copper piping and rads was installed by a bunch of moonlighting fairies back in 1973 when the building was converted into flats on the cheap by a developer. They buried some of them in concrete which has caused us a huge problem recently when they started to leak.

>It is a vented system with a header tank that is only slightly higher than the two problem radiators.
Could you convert it to a sealed system? It would fix any air-being-drawn-in problems. The boiler has to be suitable (manual re-set high limit thermostat) but it probably is if it's 2 years old.

Check the F&E tank is correctly set up. It should have a 2 or 3 inches of water covering the cold feed outlet with the system cold. Most of the space in the tank is to accomodate expansion when the system is hot. Ensure the water doesn't get near the overflow with the system hot. It's a big system so the tank should be bigger than usual.

I dont think there is any air being drawn in at the header tank - there is at least 3 inches in it when cold, and it doesn't get near the overflow when hot.

> The system has been drained and refilled recently and was flushed two years ago when the boiler was replaced.
And corrosion inhibitor replaced? Any rads needed replacing due to corrosion/leaks?

Yes, but that was all flushed out by the leak I mentioned above. Now we've repaired the leak we are adding the inhibitor this week (and getting the faulty air vent fixed).

> It probably does contain some sludge but I'm not sure why that would only affect the two highest rads - would it not tend to accumulate lower down ?
It would, but it restricts the pipes and the furthest rads, being least favoured, get affected first.

Boiler make and type?
Potterton NXR3, oil fired - fitted by a very competent local firm.

Condensing boiler? No

Do you do the plumbing yourself? No !

How does it work with one boiler supplying 5 flats? Are the fuel bills split? The fuel bills are split pro-rata based on the size of the flats.

Is it on all the time? We usually run it all the time in winter because we think it is more economical to do that than to let the system cool down and then have to heat up again (which takes over an hour). It would be useful to be able to turn the temperature down a little at night and back up in the morning but this is controlled by a knob on the boiler, which is located in a below-ground boilerhouse outside the house. It would be good to automate it !

Weather compensation? When the weather is cold, we run it for longer.

How does the domestic hot water work? It's totally seperate - each flat has its own hot water cylinder (electric).

Any problems with that? Is there a secondary DHWS circulating pump?[/quote]

Regards

John