Boiler Cycling and Pumping Over

[parky]

Hi

I have just moved into a new house and it's always nice inheriting someone else's heating system problems just before the coldest part of the year.

The system is as old as the house (18 years) including the boiler. A Potterton Profile 50e, open vented, fully pumped system with tri-port valve. There are 17 rads (one in the bathroom on a bypass circuit), and a Grundfos 15/60 pump. The vent pipe is connected to an air separator, and the order of the plumbing looks normal (boiler feed, vent, cold water feed, pump, tri port valve).

And here is the problem.

I noticed that the boiler is cycling once the circulating water reaches a "hot" temperature. So it stays on fine from cold, but once warm the boiler will only stay on for 2 mins, then goes off for 1 min, then back on for 2 mins - and cycles this way until the tank stat and room stat are satisfied. When the boiler is in its "cycle off" state, turning the boiler control up a notch fires it back up again, so it appears that the boiler stat is working OK.

I have measured the temp of the water and the drop across the boiler is not great (40-50 return and no more than 60 on the output). It never gets above 60 despite putting the boiler on maximum. I don't leave on max as the one time I did the boiler overheat tripped. But it seems happy on Number 4 (although the boiler itself gets very hot to the touch)

I recently noticed that one of the rads was cold in the middle. Although I have a invoice showing the system was powerflushed a year ago, I took that rad off and got a lot of muck out of it. It now gets hot throughout.

I have now also noticed that when both HW and CH are on, there is a constant trickle of water from the vent pipe into the f&e tank (I have tested this with the boiler off, just the pumps and valve operating and it pumps over regardless of whether the boiler is on or not). Note, if only HW or CH is selected then there is no pump over.

I am guessing 60 is too cool for the boiler output when the boiler is on No4, so there is not enough heat exchange going on in the boiler (sludge/scale in the exchanger?) and this could cause the cycling, but reading these forums indicates that pumping over is normally caused by a blockage between the vent pipe and the pump? (or could I gave both problems) Can an Air Separator go faulty or get blocked? It is a simple copper can type. I have tried putting a magnet on the pipes around that area (as advised) but can't get it interested.

I have just half drained the system and put in two bottles of Fernox F3 and topped it back up again, so will leave it for a week, but if anyone had any thoughts on the problem I would appreciate it (as I am not hopeful with the F3). BTW the f&e tank had a nice coating of brown sludge on the bottom which I left undisturbed as much as I could.

Should I expect to have to get it powerflushed after one year (or does it indicate that the powerflush didn't clear it out properly?

its worth adding that the rads do all get hot and I do get hot water. The house could be a bit quicker at warming up but its not too bad - I am more concerned about the long term damage that could be going on (eg stress on the boiler when cycling)

Sorry for long story but have noticed a lot of questions get answered with questions so I thought I would try and get everything covered first off

Thanks

Dave

 

[rojaca]

If system is pumping over then your system will be full if 5hit. The sludge is most likey the cause of the cycling and the poor flow temperature.
I would think that you are possibly looking at a blockage or poor system design as the primary fault though.
How far above your the htg cistern water level does the vent pipe extend?
How high above the pump is the htg cistern?
What speed is the pump on?

 

[DP]

Pump speed is not a factor that should result in pump over. 450mm height from water level in the header tank should suffice.

Pump over is mainly caused by poor system design. In a correctly plumbed system pump over can still occur but that is normally due to blockage between vent and feed pipes.

Drawing of installation showing vent, feed, flow, return and pump etc would help.

 

[parky]

Thanks for the replies

The boiler is wall mounted on the ground floor, the hot water tank, pump and Tri port valve are on the 1st floor (pump is about 1 foot off the 1st floor level). F&E tank is in the loft mounted on a table, so tank is about 2.5-3 meters above the pump. The vent pipe loops over the f&e tank about another 1 foot higher.

I have turned the pump down from 3 to 2 and it seems to stop the pumping over (but not he cycling) so I am only hiding the problem by doing that (although nice not to have a f&e tank full of hot water.

The vent and feed pipes are only 1inch apart (if that) so could there really be a blockage in that short length?

If the circulation is slow due to sludge, would you still normally expect a boiler output temp of 80 rather than 60? Does 60 indicate a sludged up/scaled up heat exchanger (and could F3 help this if I leave it long enough)

I have noticed that as the boiler gets hotter, just before it cycles off, I can hear a lot of gurgling (like air) in the pipes through the boiler and then shortly after I can hear the air separator fizzing (doing its job I guess) but I have bled every rad and the pump, in fact the only part I have not bled is the top of the hot water loop as the nut is well and truly seized. I don't know if any of this is relevant.

24 hours after the F3 treatment and no change in the cycle times or boiler output temp so far.

I am charging up my camera to get some photos of the system - will post them shortly

Thanks for your help

Dave

 

[rojaca]

Now I may get a lot of stick for this. The profile do seem to have a pretty poor anti cyle on them- if at all. I have noticed it on other ones but to my disgrace can't say I have ever investigated further (Baxpoti Help?)
Your feed and expansion pipes should be within 150 of one another so this is sound.
The noises you can hear sound like the heat exchanger is kettling- most likely the sludge- it may need a heavier dosing of chemical to clean it thoroughly and will certainly need far more than 24 hours.
As far as the height of the cistern and expansion pipe they sound fine.

 

[parky]

Thanks - sounds like the best thing to do is wait the week (I put in 2 bottles of F3 due to size of system -cheek of a certain well known DIY store charging twice what a local plumbers merchant charges for it!)

I have never linked to imaged before but hopefully the link below will take you to some annotated photos if they help shed any light?

//www.diynot.com/network/parky/albums/

let me know if you cannot access them

cheers

Dave

 

[D_Hailsham]

I have measured the temp of the water and the drop across the boiler is not great (40-50 return and no more than 60 on the output).

The Profile 50e should have a temperature drop of 11°C.

17 rads is a lot for a 14kW boiler. Have you worked out the total kW of your rads? Use the info in Stelrad Elite Catalogue to work it out.

If your rad total kW is more than the boiler kW, the water will never get up to temperature.

 

[heatingman]

the boiler stat is shagged, this is a job for an RGi. The system is full of srud which is building up in cold feed/air seperater, doubt F3 will shift it. Easiest to cut out and replace.

 

[parky]

D.Hailsham, thanks for the link. Have just run around the house measuring all the rads and the total comes to just short of 20KW (excluding the HW tank - what is that normally?) so yes looks like it is always going to struggle (I should have realised as I knew the boiler was new with the house, but the house has been extended, so more rads)

I have struggled to measure the temp drop due to B&Q only having 1 rad thermometer in the whole of the south east of england, but by switching it between the two pipes it looks like I am only actually getting a 5 degree drop (55 in and 60 out)

Heatingman, Are air separators prone to blockage if there is a lot of sludge around, and it would certainly explain the pumping over (and why slowing the pump down stopped it.) Presumably it does not pump over if only on HW or CH as those individual circuits provide more resistance than the sludged up separator - but when CH and HW are running the separator is providing a high point of resistance causing the pump over?

Do you think my boiler stat could be s#####d even though it switches the boiler on or off if I turn it up and down? As you say that is a RGi job, so with that and the undersized boiler I am probably entering new boiler territory. (but not till I've got the system clean)

So I think I will give the F3 a week drain and refill (twice) the system, then replace the separator. If that doesn't solve it then it sounds like I need to start saving.

 

[D_Hailsham]

Have just run around the house measuring all the rads and the total comes to just short of 20KW (excluding the HW tank - what is that normally?) so yes looks like it is always going to struggle (I should have realised as I knew the boiler was new with the house, but the house has been extended, so more rads)

Including the cylinder that's a load of 22kW and you only have a 14kW boiler! Keep your fingers crossed that either it does not get really cold or your rads are oversized for the heat loss.

If you are interested, you can work out how big a boiler you need by using the EST online boiler calculator

Is your house well insulated? If not you can get grants to help, even if you are not on benefits. Check out Energy Savings Trust Grant Search

it looks like I am only actually getting a 5 degree drop (55 in and 60 out)

Not surprising with an undersized boiler

So I think I will give the F3 a week drain and refill (twice) the system, then replace the separator.

The longer you can give the F3 the better.

 

[parky]

Fortunately the house is only 19 years old, so is cavity wall insulated, double glazed and pretty good loft insulation so even in the fairly cold snap recently the house still warms up pretty quickly. Doesn't ever get really cold in Lewes.

Thanks for the other link. It was suggesting that 14KW was right for the original house, but looks like a 16KW or 18KW would be better for the extended house. Shouldn't need 22KW, it just has too many rads I think (we keep 2 of the bedroom TRVs turned right down as they are normally empty - but have got them open for now during the F3 cleanse

Is there a maximum time that the F3 can be left in for? I think as it is all working I won't take on replacing the air separator till after christmas. Need to hone my soldering skills first.

 

[D_Hailsham]

Fortunately the house is only 19 years old, so is cavity wall insulated, double glazed and pretty good loft insulation so even in the fairly cold snap recently the house still warms up pretty quickly. Doesn't ever get really cold in Lewes.

I know what you mean. You might be able to guess where I live

It was suggesting that 14KW was right for the original house, but looks like a 16KW or 18KW would be better for the extended house.

I hope you did not have the extension done, it might have been better to change the boiler at that time

Shouldn't need 22KW, it just has too many rads I think

But when you eventually change the boiler the 22kW rads will earn their keep. Rad outputs are quoted for an 11°C flow/return difference, which is the same as your existing boiler; condensing boilers work with a flow/return difference of 20°C. Because of this the rads give out about 15% less heat. Assuming you actually need 18kW you would have to install 18 X 1.15 = 20.7kW; and you have 22kW, which is not far off. Your new boiler will run very efficiently and at a lower temperature.

Is there a maximum time that the F3 can be left in for?

Fernox suggest a week, but longer would not do any harm

 

[parky]

I guess the clue is in the name Yes not too cold and we have a wood burner if it gets too bad.

We have only just moved in. The extension was done 12 years ago. I could have put two and two together when I saw the boiler was older than the extension though.

Let's hope the boiler lasts until the proposed scrappage scheme comes in. That will help.

I will report back on progress.

Cheers

dave

 

[D_Hailsham]

I guess the clue is in the name

Got it.

Let's hope the boiler lasts until the proposed scrappage scheme comes in. That will help.

Not in your case, unfortunately. The Profile 50E is a Band E boiler and the scheme will only apply to Band G or worse.

 

[parky]

Doh!