CH Pump vapour lock and noise issues

8 Oct 2015
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United Kingdom
Hi All,

This is going to be a long post especially for a first time poster so please bear with me. :)

First a bit of background. I consider myself fairly competent both mechanically and electrically - electronics/computers is my background but I'm also a keen DIY'er including doing all my own car repairs/maintenance etc, so a bit of pipe work/plumbing doesn't scare me and I have plumbed sinks, baths, replaced a couple of radiator valves that were leaking before etc.

However as an NZ expat of 6 years, until very recently I have not had a lot of hands on or live in experience with residential gas fired central heating systems as they are non-existent in NZ due to both the warmer climate and the lack of a piped gas in some regards I am still getting up to speed and learning the quirks and foibles of the typical UK central heating system from a maintenance perspective, especially the older gravity fed systems which I am now faced with...

We bought and moved into a 1930's Bungalow with converted loft in February, and along with a lot of other renovation work the heating was working but in need of a bit of attention. The system when we moved in was:

A gravity fed Glowworm hideaway 80B mounted in a dedicated cupboard in a utility room extension off the kitchen. We think this was installed somewhere in the early to mid 90's by the previous owners of the house. This is a very simple conventional boiler compared to the sealed combi-boiler we had at a previous rented address - no electronics at all, no modulated flame, no pump overrun just a gas vial style thermostat like you'd find in a refrigerator to turn on the power to the gas control box, and as far as I can see no separate over temperature cut out either.

The pump is a GrundFos Super Selectric UPS 15-60 mounted horizontally nearby, at the time we moved in with no automatic bypass valve fitted. There is a small vented hot water cylinder that has both an immerser element and a gravity fed indirect loop to the boiler.

In the hallway there is a bypass radiator and a rather old Honeywell wall stat which is currently wired to switch only the circulation pump, and the pump and boiler together are powered from a timer in the kitchen. As far as I can see there are no motorised zone valves anywhere.

The radiator piping is 8mm microbore being fed in a star configuration from a 22mm (?) manifold that terminates near the middle of the underfloor of the house. All internal walls are brick and plaster but there is a good metre or so crawl space under most of the house where the pipe work is, mostly dangling in mid air supported by gaps in the brick walls underneath the joists.

There are 9 radiators if you count the 3 side by side piped together radiators in the living room as one, otherwise call it 11. Half the radiators are old fashioned finless types and the other half are more modern finned types. Some of the radiators have significant external corrosion. (Oddly - only the more modern ones, the old ones are fine)

Most of the older ones were originally fed only from one side using those funny old single insertion microbore valves, including the living room radiators! (Needless to say the far ones did not heat up very well) There were no TRV's and several of the old valves were leaking through the top seal if you tried to close them.

With so much other renovating needing to be done I decided I didn't want to tackle this myself (and couldn't have in hindsight) so we called in a nearby heating engineer. Knowing that a new boiler/radiator/piping was completely off the table cost wise we agreed on:

TRV's on all but the bypass radiator and new lockshield valves all round, I asked for an automatic bypass valve to be fitted at the pump, all single entry radiators were converted to normal both side entry in the process of changing the valves, the 3 side by side living room radiators (which are piped between each other at the top and bottom) were changed to a conventional 15mm feed (plastic not copper as it turned out) right back to the main manifold which drastically improved their performance, all other radiators were left as 8mm microbore. They then did a full "power flush" and went on their way.

In this state the system has worked quite well for us over about 8 months. Despite its age the boiler has been nothing but reliable with all the radiators heating quickly and seeming to have plenty of heat output despite the microbore piping, (I was sceptical of the small pipe at first, but it does work) however the pump has caused me some concern.

For quite a while we would intermittently get a sudden whine from the pump that would take a few minutes to go away, typically when the system was fully warmed up and the thermostat had come back on. It took me a while to realise that it was the pump vapour locking and the whine was it trying to pump vapour instead of water...

This was only happening if the flow temperature was set over about "half way". Later on I discovered that the way the boiler and pump are wired means that the hall thermostat only turns off the pump *not* the boiler! I was a bit shocked when I first discovered that (in a face palm way) and I have no idea whether this was standard practice 20 years ago...

I realised it is probably done this way to allow the boiler to continue heating the indirect cylinder via thermo-syphon, however it seems to be a really bad idea to me because when the wall stat clicks off the boiler keeps running until it reaches its cut out temperature and then sits and heat soaks the stationary water in the heat exchanger until it boils - if you then turn the stat on again in 10-20 minutes you get instant vapour lock that takes a couple of minutes to clear.

The wiring for the boiler and pump are right next to each other, so I could easily connect the boiler to run from the wall stat switched feed so that it too is turned off by the wall stat - there is still no pump overrun but I'm sure it would be less likely to vapour lock than only turning the pump off....?

This tendency to vapour lock on a high flow temperature setting is probably exacerbated by the general inaccuracy and slow response of a gas vial mechanical thermostat and a lack of flame modulation. (The flame is either on full blast or not at all unlike a modern combi)

About a week ago I needed to do some work that required the system to be drained down including repairing a leaking radiator valve whose microbore pipe was not seated properly in the valve, and I decided to tackle this myself. The work was duly done and I believe I was careful in refilling and bleeding.

I started with the bleed valves closed, turned on the cistern tap to a moderate to low flow, bled the pump first, then started bleeding the downstairs radiators one at a time then moved to the upstairs ones. (Two of them) When the system was about half full I added a litre of Sentinel X100 in the cistern then finished filling and bleeding.

I then let the pump run for 10 minutes with the boiler disabled to give it plenty of circulation and mixing, stopped the pump and bled all the radiators again - with only a tiny amount of air from the bypass radiator and one upstairs radiator. I then let the pump run by itself for a while and was happy that there was no gurgling or other signs of air still in the system and the pump was nice and quiet then fired up the boiler and everything seemed fine.

However it seems that not all is well since then. Two problems, possibly related or not related. One is that the tendency to vapour lock has got much worse, more or less immediately. (I noticed it the following day) If I have the flow temperature set to a higher level (about 3/4) then the system runs fine until all the radiators are hot and TRV's are going off then I can start to hear the pump getting noisy and suddenly it will burst into full blown vapour lock even when it is still running - previously vapour lock would only occur if the flow temperature was turned right past maximum, or after the wall stat had been off for a while.

I have a layer of black tape wrapped around the output of the pump so I can measure the temperature with an IR temperature gun - when this occurs the temperature is reaching 85+ degrees, so it seems to be getting too hot despite the moderate flow temperature control setting that should be around 65 and the fact that a bypass radiator is connected and working fine. (The radiator is too hot to touch)

Turning the pump speed up from 2 to 3 or increasing the ABV pressure to the maximum 0.6 bars doesn't seem to prevent vapour lock occurring either. I'm finding that even with the control set near minimum (which should be about 55) it's still peaking out at high enough temperatures to cause noise.

I took a video of the noise the pump is making in this condition - at the time of the noise the flow reading just past the pump was 80 degrees:

This seems like the boiler stat may be getting a bit unreliable, and never having been happy with the accuracy of this stat I am thinking of adding an external digital controller that measures the temperature using a clamp sensor on the copper outlet pipe of the heat exchanger and controls the power to the boiler. That way I just turn up the existing stat to maximum (left in circuit as an over temperature cutout) and do the actual flow temperature control with a fast responding much more accurate sensor and controller.

The second issue that has me scratching my head though, is a lot of noise coming from the pump sometimes even when the temperature is much lower. In this situation the measured flow temperature has dropped to as low as 60 degrees and the heat exchanger in the boiler itself is silent, with the flame turned off, yet the noise coming from the pump remains constant, and sounds a lot like kettling:

When it gets into this unusual noisy state I have literally left it running for 15 minutes with the boiler off, pump running, bypass radiator is operating as it should be with decent flow, the temperature has fallen to 50-60 degrees yet this noise in the second video remains constant. However if I turn the pump off for 2-3 minutes then switch it back on it quickly goes nearly silent (and pumping normally) as if nothing had been wrong.

I have some theories on the cause of both problems but I'd really like some feedback on where I should be going with the diagnosis....

Am I looking at a failing pump bearing (or an impeller coming loose on the shaft) that is "OK" when cold but gets noisy when hot ? Am I looking at a boiler stat that is on its way out and intermittently causing the boiler to overheat ?

To me it feels like I have two separate problems, but I can't figure out how draining and refilling the system could have made things so much worse...especially the symptom at lower temperatures in video 2, which wasn't present before draining the system as far as I can remember.

Any suggestions appreciated. (No, new boiler is not an option :) )
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Nobody has any comments or suggestions ? :confused:

A bit of further testing last night may have turned up some more clues.

With the flow temperature set to minimum (about 60 degrees) after about half an hour the pump started getting noisy again sounding like it was cavitating even though the boiler heat exchanger was silent and a radiator bleeding check I'd just done had turned up no air.

The noise was persistent and fairly constant. After scratching my head trying to figure out where air could be coming from it suddenly occurred to me that an old, hard o-ring on the bleed screw of the pump might be leaking after being disturbed during the drain and refill, and allow air to be sucked in (as it vents the shaft which is the intake side of the pump ?) so I changed it for another o-ring - I didn't have quite the right size (slightly too small) but it was close enough so I fitted it and turned the system back on and bled it slightly while running. As I expected the old o-ring was very hard and pinched in the shape of the housing.

Silence! :D The pump then ran completely silent for over 2 hours at 60 degrees with none of the cavitation like noise it had just before that. So I think it's fair to say that there was some air ingress around the bleed screw due to a worn o-ring. Possibly as the housing heats up and expands the marginal o-ring starts admitting air.

Just when I thought I had the problem licked, after just over 2 hours of silent running the pump suddenly burst into emitting it's loud whine. The flow temperature was still on 60, the boiler heat exchanger was still silent, and there was no cavitation it can't have been vapour lock, and indeed checking a radiator for flow noise I could tell there was still normal flow despite the whine. (With a vapour lock the flow through the radiators drops to almost nothing) The only noise was a worn/dry bearing like whine.

The pump was very hot by now so I switched it off for a while. Today while doing more research on the problem I realised that the pump has been incorrectly installed. (n) The pipe work to the pump is horizontal and the information I've been reading says it's very important for the pump shaft to be oriented exactly horizontally so that both end bearings are submerged in water to be cooled and lubricated properly - this pump is mounted with the bleed screw end of the pump tilted up approximately 30 degrees in the air, has been that way since we moved in, and was not even commented on by the engineer that came to do the other work to the system...(n)

What I've read suggests this will be causing the top end bearing to run dry and overheat! I wonder if this is the second part of my issue. Any air admitted by a leaky o-ring would only serve to further reduce the water flow over the top bearing due to the air pocket that would form.

The cavitation issue that the o-ring fixed only occurred since I drained the system down and refilled it (probably through disturbing the o-ring) but the intermittent whine after running for a few hours was there before that.

So I'm going to try to slacken the mountings for the pump slightly so I can turn it around to horizontal, hopefully without needing to drain the whole system down again. The mountings do have slot style shutoff valves on either side but of course they are completely seized...
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I think you need a new pump. It sounds awful. Whether it's the cause of the problem or caused by the problem is uncertain but it sounds shot
It looks as if the pump shaft is not correctly fitted to be horizontal.

Like most, I expect, I have not read all of that but we are experienced at quickly coming to a conclusion.

Easy DIY task to rotate pump and then bleed air out of big screw.

Then see how it works.

It may have been seriously worn out already but possibly not.

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Can't be fussed to read it all, crikey it is like war and peace.

Pump shaft looks just above horizontal, which is exactly as it should be. The pump wiring housing is underneath which is not recommended because when you vent the impeller, as Agile wisely suggests, there is a danger that the water will short out the electrics.

The pump should have been installed with the wiring box to the left, right or on top but never underneath.

If the pump rotor is regularly filling up with air there's likely to be a more serious problem causing it, an experienced heating installer should look at it, because a new pump, if airlocked, won't last long either. I assume this is an open vented system.....

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