Central heating advice needed!

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Mandrake
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Re: Central heating advice needed!

Post by Mandrake »

RichardW wrote:3. By the sound of it the boiler stat is on the way out too if you are getting 85C at the pump with it turned down - do you get kettling in the boiler when you turn it up? It's unlikely you are getting vapour in the pump - it wouldn't just whine it would cavitate and sound horrible before it got to that stage! More likely it is suffering a partial seizure (see 1 above)...!
Got home tonight and could hear the pump is noisy again - not a loud whine but the "cavitation" noise... ? Here is a quick video of it, not sure how easy it is to make out on the recording because it does tend to exagerate how loud it is - it's certainly quite audible when opening the cupboard door but not that loud. Flow temperature was about 60-65 at the time and the boiler hadn't fired for at least 10 minutes.





In person it sounds like cavitation in the pump to me, maybe with a touch of bearing whine when you get a bit closer ? All the noise is coming from the pump and the surrounding pipes (including the ABV loop that you can see running above the pump) and listening with a screwdriver confirms it. No noise from the boiler heat exchanger at all!

Knackered pump, air getting in somewhere in sufficient quantity to cause cavitation, or both ?! All the radiators were bled yesterday - no air in any downstairs radiators but a small quantity in the two upstairs ones.

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Mandrake
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Re: Central heating advice needed!

Post by Mandrake »

Looks like I do indeed need a new pump, and I think I've worked out the manner in which the pump is failing. If I'm right it's not the "obvious" culprit of a failed bearing/bushing. (Although those are probably worn and noisy too)

Tonight it was running for 3-4 hours with the flow temperature turned down without problems then suddenly it started whining again with a dry bearing type noise so I quickly measured the flow temperature - which was 75 degrees. On a hunch I decided to turn the flow temperature up to maximum to try to provoke it to see what would happen, within a couple of minutes as the temperature climbed to 80 degrees the pump suddenly started to get really quiet, with the whine fading right away, but interestingly rather than slowing down the motor pitch increased as if the motor was speeding up due to a loss of load...

When in this really quiet but seemingly spinning faster than normal state there was no audible water flow through the bypass radiator (normally clearly audible) and switching the speed switch on the bottom also didn't seem to change the motor speed as judged by the (very quiet) pitch of the motor. There was no kettling from the boiler and no vapour lock as far as I could tell.

I assume that the impeller is an interference push fit on the end of the shaft, if so my conclusion is that when the pump gets hot the impeller starts slipping on the shaft! This will make a horrible worn bush sound and possibly cause cavitation if it allows the impeller to rock on an angle too. Then when the pump gets even hotter the impeller loses grip on the shaft completely allowing the shaft to free-wheel at a high speed while the impeller is hardly turning at all, thus the quiet high pitched motor noise and lack of pumping action!

After turning off for half an hour or so it cools down enough for the impeller to grip the shaft again.

New pump time for sure...

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Mandrake
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Re: Central heating advice needed!

Post by Mandrake »

wurlycorner wrote: I think the angle yours was at is probably not ideal, so having rotated it level is probably a good thing. That's just a hunch though - you could probably find the data sheet on the internet to confirm that?
In light of my previous post I've ordered a Grundfos UPS2 15-50/60 which is the modern drop in replacement for the pump I have now. (Among some of the benefits of the newer model - less than half the power consumption for the same flow rates and pressures! :) )

Here is a technical manual for it:

http://www.cerpadla-neptun.cz/foto/man_ ... 403147.pdf

Page 4 installation and startup clearly states "The pump must always be installed with horizontal motor shaft." and even has a picture of a spirit level next to the pump. So I guess that settles it. Needless to say I will be installing the new one with the shaft horizontal...

RichardW
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Re: Central heating advice needed!

Post by RichardW »

Sounds plausible - if somewhat unusual! When I stripped out the old CH from our house, I found that the pump shaft was actually sheared above the impeller - no idea if it happened when I was taking it out, or if it was running like that! Never gave any trouble though.... You'll need new valves as well - chances are you won't be able to separate them from the pump.

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Mandrake
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Re: Central heating advice needed!

Post by Mandrake »

Yeah it does seem unusual I agree! :lol:

But I can't see any other explanation for the observed behaviour - if continuing to heat the pump up past the point where it has got noisy causes the pump to audibly increase in speed as if it was suddenly unloaded, (motor pitch increases quite a lot) and at the same time get really quiet, and the pumped flow virtually stops, I can't see what else it could be other than the shaft continuing to spin by itself with the impeller hardly moving! I have seen this increase in pitch and going really quiet behaviour a few times before when hot but I had not checked the flow through the radiators until now to notice that it was no longer flowing...

Looking at the diagram of the new pump in that PDF it has a ceramic shaft with a "Composite, PP or PES" impeller, whatever that is, but it's quite possible the old pump (being a 20+ year old design) has a metal shaft and/or impeller. I guess we'll find out during the post mortem after it has been replaced. :twisted:

Yes I'm going to replace the isolation valves - I can get the pump out easy enough as I've already had the two large nuts on the pump loose to turn it around and they are not frozen at all, but the shut off valves themselves (screw driver slot types) are totally seized meaning that it will be another full drain down to replace the pump... so if I'm going to that trouble it'd be silly not to replace the isolation valves at the same time - they're not expensive.

The new pump also has an identical head unit mounting flange to the original such that you can actually replace just the head without disturbing the pipe work at all, however again with faulty shut off valves requiring a full drain down I may as well just do the job properly and replace the whole thing, then if the pump ever fails in the future I can easily exchange the pump head without a drain down or disturbing the pipes.

I'll also have a spare ABV on hand to fit when draining down - pretty sure that when I drained the system last time some muck has got stuck in the ABV valve, as it has been working perfectly since February but then about 2 days after the drain down and refill it seems to be stuck partially open all the time even when the differential pressure is low and regardless of what setting I turn it to... [-X

I don't mind this kind of work really, it's not a lot different to working on cars! :lol: As long as you're equipped with the right tools and understand how the system works its no harder than a difficult job on a car really. I just don't want to be doing these kind of repairs in late October... :twisted:

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Re: Central heating advice needed!

Post by RichardW »

They never give trouble in the summer.... :lol:

At least you've still got walls though....

Image

It's OK, it was intentional, got some supports on the other side!

Image

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Mandrake
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Re: Central heating advice needed!

Post by Mandrake »

Best comeback ever. :rofl2:

Mind you, if you had seen our kitchen or bathroom a few months ago... (still not finished, but getting there)

That's some mighty thick walls you've got there :shock: thicker than the walls in our 1930's bungalow which has pretty solid walls...

I bet you notice the cold coming through what's left!

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Re: Central heating advice needed!

Post by Peter.N. »

I have an oil fired Rayburn running a standard indirect system which is more of a series of modifications than anything. My pump makes that cavitation noise when I only have one radiator on in the lounge, it basically only runs two large radiators and the hot water. Its something to do with the flow rate through the pump, its obviously sucking air in but from where I don't know. As soon as I turn on the second radiator it stops, this obviously increases the flow rate through the pump

I have had to fit a valve to control the flow rate through the hot water tank coil otherwise its starts circulating through the header tank from the expansion pipe so needs carefully setting up. I had thought of fitting a variable pump bypass valve as an easy way of regulating the flow - but like so many other things, haven't got round to it yet.

Peter

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Re: Central heating advice needed!

Post by isisalar »

I replaced 2 of those Grunfoss pumps over the time we had our old central heating system, about 25 years. One of the few parts I've ever bought that I've thought was cheap, and a doddle to fit if the stop valves are working.
We had a new Worcester Bosche combi boiler installed about 10 years ago but it's already clapped out and will be replaced very shortly. Apparently this is good for a combi, they normally only last about 8 years. The old boiler lasted 25yrs+ my parents one is still going 40 years on, so don't be in a rush to replace your, presumably old, boiler Simon.

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Re: Central heating advice needed!

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@ Mandrake - the wall is about 500mm thick - pretty std for a 'solid' stone wall (if a few bits of stone held together by 160 year old lime mortar can be called solid!). It's pretty cool in there, but this is more to do with the hole he cut in the other end to allow fitment of a new window. It's surprising how effective a sheet is at keeping the heat in the living room!

@ Peter - Sounds like your pump is oversized - giving 'pumping over' where it throws water out of the expansion pipe is one sign of this, as it being noisy when throttled (this is not cavitation per se - it's just that centrif pumps don't like being run too far up their curve, and it makes them vibrate)

@ isialar - we've got a modern condensing system boiler fitted just over 8 years ago, and it needs replacing.... leaking from the heat exchanger. Whilst I could replace the exchanger it's half the cost of the boiler, and the pump is noisy, and the expansion tank loses pressure over time.... Virtually the same boiler is still available, so it's a relatively easy change out, as the back plate, piping and flue should fit directly.

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Mandrake
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Re: Central heating advice needed!

Post by Mandrake »

Peter.N. wrote:My pump makes that cavitation noise when I only have one radiator on in the lounge, it basically only runs two large radiators and the hot water. Its something to do with the flow rate through the pump, its obviously sucking air in but from where I don't know.

[...]

I have had to fit a valve to control the flow rate through the hot water tank coil otherwise its starts circulating through the header tank from the expansion pipe so needs carefully setting up. I had thought of fitting a variable pump bypass valve as an easy way of regulating the flow - but like so many other things, haven't got round to it yet.
As Richard says it sounds like the pumped side of the system is over pressurising when radiators are closed down causing it to pump out through the overflow pipe into the header tank - if the pressure in the feed pipe exceeds the head of water in your header tank that will happen... This will also be the likely source of air getting into the system, because as soon as you start circulating water back to the header and then down through the header feed you are picking up air and circulating it into the system.

An adjustable automatic bypass valve is a good idea - it's mandatory on new installs I believe but it's worth adding even to an old system like ours which is over 20 years old. (I insisted on having it added to ours even though the engineer said we didn't really need it because we have a bypass radiator) This way you can limit the maximum pressure differential to less than the head of water that your header tank provides - if your header tank was 5 metres high you'd want to limit the pressure to no more than about 0.5 bars for example. (Most valves let you set it between 0.1 and 0.6 bars)

Also have you checked your pump to see if it has more than one speed ? Ours has three speeds and whilst it doesn't overpump into the header on the highest speed it does cause excessive flow noise around the house and is much more reasonable on the 2nd speed where the system is quite (except for the faulty pump at the moment!) and pumps more than fast enough to heat up radiators quickly.

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Mandrake
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Re: Central heating advice needed!

Post by Mandrake »

isisalar wrote: We had a new Worcester Bosche combi boiler installed about 10 years ago but it's already clapped out and will be replaced very shortly. Apparently this is good for a combi, they normally only last about 8 years. The old boiler lasted 25yrs+ my parents one is still going 40 years on, so don't be in a rush to replace your, presumably old, boiler Simon.
Hard to be sure but I think this boiler was installed in 1993 (that's the publication date of the owners manual) so that makes it 22 years old, although to be honest the design seems older than that to me! :lol:

Don't worry, I'm in no rush to replace the boiler, in fact I'm doing what I can to keep it going as long as possible. Apart from the boiler stat that seems to be getting a little bit inaccurate and inconsistent in its temperature cutoff point it has been working fine since we moved in. Surprisingly you can still buy a new boiler stat for it for £30, however I have a better idea: :twisted:

http://www.amazon.co.uk/gp/product/B00B ... ge_o02_s00

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Zelandeth
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Re: Central heating advice needed!

Post by Zelandeth »

Astonishing how efficient some of the (decent!) Older boilers are too. The green energy guy from the council just about dropped his iPad when discovering that our 35 year old Thorn Apollo only just missed out on an A rating by a few percent.

Given it has a total of five moving parts (gas valve solenoid, medium temp switch, high temp switch, and the ignitor) I'm in no hurry to replace it! Interface consists of 240V mains and a single wire that tells it there's a call for heat...can't complain at that. It's a compact little unit though, but easily keeps up with our not small five bed house for both heat and hot water.

Peter.N.
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Re: Central heating advice needed!

Post by Peter.N. »

Mandrake wrote:
Peter.N. wrote:My pump makes that cavitation noise when I only have one radiator on in the lounge, it basically only runs two large radiators and the hot water. Its something to do with the flow rate through the pump, its obviously sucking air in but from where I don't know.

[...]

I have had to fit a valve to control the flow rate through the hot water tank coil otherwise its starts circulating through the header tank from the expansion pipe so needs carefully setting up. I had thought of fitting a variable pump bypass valve as an easy way of regulating the flow - but like so many other things, haven't got round to it yet.
As Richard says it sounds like the pumped side of the system is over pressurising when radiators are closed down causing it to pump out through the overflow pipe into the header tank - if the pressure in the feed pipe exceeds the head of water in your header tank that will happen... This will also be the likely source of air getting into the system, because as soon as you start circulating water back to the header and then down through the header feed you are picking up air and circulating it into the system.

An adjustable automatic bypass valve is a good idea - it's mandatory on new installs I believe but it's worth adding even to an old system like ours which is over 20 years old. (I insisted on having it added to ours even though the engineer said we didn't really need it because we have a bypass radiator) This way you can limit the maximum pressure differential to less than the head of water that your header tank provides - if your header tank was 5 metres high you'd want to limit the pressure to no more than about 0.5 bars for example. (Most valves let you set it between 0.1 and 0.6 bars)

Also have you checked your pump to see if it has more than one speed ? Ours has three speeds and whilst it doesn't overpump into the header on the highest speed it does cause excessive flow noise around the house and is much more reasonable on the 2nd speed where the system is quite (except for the faulty pump at the moment!) and pumps more than fast enough to heat up radiators quickly.
Iv'e cured the circulating through the header tank problem by fitting a valve in tank feed from the boiler, if you adjust it jut right it limits the flow sufficiently to stop it, otherwise you get a header tank full of hot water! but I still get the cavitating sound when I turn one radiator off, the pump is on its lowest setting, definitely sounds like air in the system. I think you are probably right though, I did initially have more radiators but have found cheaper ways of heating the other rooms.

Peter

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Mandrake
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Re: Central heating advice needed!

Post by Mandrake »

Sounds like an automatic bypass valve would solve both your problems, I just got a Corgi one (for 22mm piping) for £14 to replace the one that seems to have died/jammed when I do the pump next week.

The advantage over having just a relief valve left slightly open is that it stays completely shut when the differential pressure is below its set threshold so you still get maximum flow when all radiators demand it with none wasted by a bypass loop, it only starts to open above a set pressure and regulates that pressure fairly accurately maintaining a decent flow through the boiler and pump. So you could set the pressure below that which over pumped to your header tank and have confidence that it will not happen.

It will also reduce the strain on your pump a lot with only one radiator open and make it sound a lot more happy. This is the one I got which arrived in the post today:

http://www.amazon.co.uk/gp/product/B00L ... ge_o00_s00

I've had a look at it and it seems to be of quite good quality especially given the price, and looks quite a bit more sturdy than the one currently fitted.