Hi Chaps, thanks for the feedback.
wurlycorner wrote:
I had exactly the same pump in the old back boiler system that was in my house (I've since fitted a condensing boiler). It was mounted vertically rather than horizontally.
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?
The installation instructions for the Grundfos pumps I have found (many seem to be those silly international picture only manuals with ticks and crosses...) clearly show that if the pipe work is horizontal the pump must be on it's side with the shaft horizontal, and definitely not with the shaft facing up vertical.
I don't like the idea of the thermostat shutting off the pump and leaving the boiler running, at all! That sounds totally wrong.
I agree, neither do I - whether it was "normal" back in the day (which Richard suggests below) I still don't like the idea, and I don't see why I can't adapt some more "modern" ideas to the control of the boiler, even if the boiler itself is old.
My system had a 2 way valve that switched the flow between the hot water tank coil, or radiators. Does yours not have this? Presumably you are able to tell it to heat water rather than the rooms? Which would suggest it does???
If it does have said valve, is it sticking? Is there a thermostat on the hot water tank? Is that working correctly, or demanding heat all the time?
I'm afraid this system is a lot simpler/cruder than your system was - there is no 2 way valve at all, the indirect loop of the hot water cylinder is not pumped, it's only thermo-syphon. The feed for the central heating goes from the front top right port on the heat exchanger to the pump, with the return coming to the front right bottom. Meanwhile the loop to the cylinder is pipes to the rear left hand side bottom and top.
I suspect that when the central heating pump is running that the vertical flow of water in the heat exchanger helps "encourage" the flow in the un-pumped circuit that shares the same heat exchanger, but with the pump off it will be entirely thermo-syphon, eg very slow and ineffective.
There is no stat on the hot water cylinder - at least for the indirect heating part. The immerser element has it's own thermostat but that is independent of heating the hot water from the boiler where you just get hot water at whatever temperature the boiler is set to!
Deanxm wrote:The pumps can be mounted at any angle but head up like yours was, the shaft loads the inner bush and air gathers at the bush in the head wearing it out, the bleed screw you mention controls end float of the impella shaft but also allows you to bleed air off the end bearing, do not opoen it while the pump is running as in some situations it can draw air in, although no big issue as it will collect at the rads.
I think that's exactly what has happened - for various reasons (air, vapour etc) an air pocket was forming at the bleed screw end bearing causing it to run dry. Also because the heat exchanger was vapour locking when the stat turned the pump off (but left the boiler running, trying to maintain the flow temperature in the heat exchanger without any flow) the pump was suffering from 2-3 minutes worth of vapour lock and dry bearings when the stat next came on, it was taking that long for it to clear itself and get pumping again. During that time the bearing would have been overheating and wearing out. This could have been going on for years for all I know as it was already doing that when we moved in.
Since I turned the pump around to exactly level it has been behaving
much better - just before I turned it even with at least one radiator open all the time (bypass radiator) and the boiler stat set to about 65 degrees it would run OK for a couple of hours then suddenly burst into a loud squeal - that has not happened since I turned the pump around. Also now that the boiler is wired to go off with the pump the vapour lock after the stat cycles off and on is greatly mitigated. Previously it would sit there vapour locked for several minutes sometimes, now it whines for maybe 5-10 seconds then clears the lock and gets on with things. Still not ideal but a big improvement. Adding pump overrun should eliminate what's left of that problem completely I think.
Deanxm wrote:Fitting trvs is all good but the pump is a bit big for your size system, I personally would go with a 15-50 but reduced floiw could be causing cavitation as the trvs close down, if the room stat is still calling for heat though the system will be trying to run to just one rad which can cause cavitation.
The pump is actually a 3 speed pump. I don't know what speed it was set to when we first moved in as I didn't even realise the pump had a speed switch at the time - the guys who came in to do the TRV's either left it at full speed or set it at full speed, which was causing quite a bit of noise around the system, and I think was causing a bit of cavitation.
I think like you say, the full speed (6 metre head ?) is a bit much for the system, it's a bungalow with a converted loft, but it is quite a high roof with the cistern right at the very apex of the roof, so there is probably a 5 metre head of water in the system. The lowest speed is too low to remove heat from the boiler fast enough while the highest speed does cavitate a bit, it seems fairly happy at the middle speed, which is where I have left it the last few months.
Deanxm wrote:Alternative is the pump has picked up some crud from the power flushing which is common or the diverter valve sticking.
By diverter valve are you referring to a valve to divert the pump between the heating loop and the hot water cylinder ? If so it doesn't have one, the pump only feeds the central heating with the cylinder being only thermo-syphon. Or do you mean the automatic bypass valve ?
In which case I think it's possible it may have got a bit of crud in it when I drained the system down because I have noticed since then that adjusting it doesn't really do anything any more. Previously if you deliberately closed all the radiators and adjusted the bypass pressure (0.1 - 0.6 on this valve) you could clearly hear a difference in the noise of the pump as the differential pressure changed, and you could also judge by turning the screw where it started to get tight, how high the pressure was. Now it doesn't seem to make any difference which makes me think it might be stuck partially open.
That might also contribute to the problems as it would mean even with the bypass radiator open some of the return flow would be short circuited back to the boiler without any heat being removed...If I change the pump I think I'll change the ABV as well or at least remove it and try to blow it clear.
RichardW wrote:1. The pump is almost certainly knackered, but if it runs quietly now you've rotated it then I'd ignore it till it gets worse. New ones are not dear though. Unlikely it is sucking air in - there's will be best part of 0.5 bar in the pump even when it is running.
As above, the pump is much better now that it's running horizontal, but I think the damage has already been done to the bearings - it runs near silent when cool but once it gets to about 60 degrees it starts to get audibly noisier and noisier with a dry bearing sound, even if it's not the loud scream from when it was at 45 degrees. Not only that, I'm only running the boiler stat at the minimum at the moment while I get to the bottom of the various issues - it won't cope with running at the higher temperatures a cold winter will need without making nasty noises, so I'm thinking seriously of replacing the pump at the end of the month - the bearing has been put under strain for probably years with a combination of poor mounting angle and wall stat induced vapour locking...
RichardW wrote:2. Wiring the pump through the wall stat and the boiler through the timer was std practice for a gravity primary / pumped secondary system. You don't need to worry about heat soak - the gravity primary will take care of that.
But it doesn't - if by gravity primary you mean the thermo-syphon hot water cylinder, it does nothing to prevent vapour lock - until I rewired the boiler to go off with the pump a couple of days ago every time the stat went off if it came back on in 10-60 minutes or so afterwards you'd get a massive vapour lock of the pump with it screaming out with no water in the bearings and no ability to pump. It could take many minutes to clear this vapour lock. There just isn't enough flow in the thermo-syphon system to remove the excess heat from the heat exchanger after the pump goes off to prevent the water boiling in the heat exchanger, causing a vapour pocket. Maybe the pipes to the cylinder are blocked up, but it seems unlikely when they are 28mm and the 8mm microbore around the house is
not suffering from any blockages that I'm aware of.
If you want to improve the efficiency then you can fit a tank stat - wire the boiler off the common and the HW to the 'calling for heat' side, and the CH to the 'satisfied side'. That way the boiler will be powered when the tank tank is calling for heat, and when the CH is running, regardless of the state of the tank.
I could do that but when the pump is not running the boiler is far to slow to heat the hot water. The immerser can heat the tank from cold in about 45 minutes, the boiler with the CH pump running can do it in about an hour but when the pump is not running it was taking a couple of hours at least (with the boiler flame off most of that time due to the cut out temperature being reached) to heat the water. Presumably due to insufficient flow. If the loop to the hot water cylinder was pumped I'm sure it would have been much more effective.
RichardW wrote:Been away over the weekend.... my 2ps worth.
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)...!
With the stat set right down the average flow temperature measured at the pump (IR gun pointed at black tape) is about 65 degrees, which seems a bit high to me for the minimum setting. Even on this minimum setting it
sometimes peaks as high as 75 degrees. So yes, I think the stat is starting to misbehave and getting out of calibration. It has a sensing capsule pushed down a long hole (about 3-4") into the top of the heat exchanger, thinking it might not be making good thermal contact I pulled it out and put a little bit of copper grease on it, that seems to have made it a bit worse for some reason. Grr....
If I turn the boiler up I can get to about 80 degrees before I start to hear any noise from the heat exchanger, it starts to sound a bit like the hiss of steam, I wouldn't call it kettling though which I presume would sound like the early stages of bubbling a kettle. Above about 85 degrees (measured externally on the pipe, so probably a bit hotter inside the heat exchanger) you can hear it start to bubble and the pump will then suddenly vapour lock and start screaming. So the boiler stat is definitely letting it go too hot if you turn it up.
As for cavitation, I have observed that too at considerably lower temperatures, possibly as a result of air getting in (from where ?) where it will sound like cavitation from the pump, but no noise at all from the heat exchanger. Not sure whether that is air or vapour that is causing that, it only seems to happen after the system has been running for several hours.