Water not getting hot - boiler cycling

[SBDIY]

I've had this problem for over a year and am struggling to solve it. I've had 2 plumbers look at it and also had the boiler serviced and yet the problem persists.

When there is a demand for CH and HW my boiler operates as it should, fires up and gets up to the set operating temperature and continues like that as long as there is a demand. When there is only a demand for HW the boiler fires up but quickly reaches the set point and trips out without any regulation of the burner. It continues like this as long as there is a demand for hot water but the water takes a long time to get hot on account of the boiler coming on for only about 20 seconds at a time and then going off for 3 or 4 minutes. I've changed just about every part on the system (tank thermostat, pump, diverter valve and header unit). I've run sludge remover through the system and drained it out yet the problem persists. I'm getting to the limit of my ability and the next thing I'm thinking of doing is checking the thermistors in the boiler. Then again it was serviced by the manufacturer (Gloworm) only 2 weeks ago and given a clean bill of health. It seems to me that the hot water is not escaping from the boiler fast enough and so the boiler shuts down to protect itself. It was working perfectly fine for the past 20 years until a year ago and then suddenly just started doing this after I drained it to fit a new rad. I have drained it many timed before and never had a problem. I don't think its air as I've bled the system and it runs very quietly apart from this problem.

I really hope someone can help me solve this.

 

[PlumbBobBob]

hi sbdiy welcome to the forums. so this only happens when you have hot water on? not when you have both hot water and heating. i think there must be a blockage somewhere on the system that is not common with the heating. the only place i can think of is the coil in the cylinder. with the heating and hot water both on can you check the temperature of the feed and return pipes for the coil and see what the temps are like.

 

[steadyon]

It could well be that in draining it sludge / debris from the system, or from the feed and expansion (header) tank, has got into the system and lodged somewhere in the DHW circuit. Try removing the diverter valve and poking some net curtain wire (or similar) through the pipes. If you meet a lot of resistance then renew the appropriate pipes. Sludge remover won't remove heavy encrustations or solid debris, and, if you have a blockage, running sludge remover through the system probably won't have got to the affected parts as they have little or no circulation.

 

[SBDIY]

Hello Steve, thanks for the reply.

It does only happen when there is a demand for HW only but when I run it with a demand for HW and CH at the same time I get hot water ok. The feed and return pipes to the cylinder get hot as they should do, feed hotter than the return. The tank was replaced in 2005 when I got a new boiler. The problem would suggest a blockage of some sort but I do get HW when I run the CH at the same time.

 

[PlumbBobBob]

hm i'll put my thinking cap on then lol

 

[lame plumber]

big old air lock in the coil, no water movement so your boiler is cycling. drain down and fill via a drain off cock on the ground floor to push air thro with motorised valves locked open manually for a first effort

 

[PlumbBobBob]

lame he's getting hot water when heating is on as well just not when by it self. could be a problem with the diverter, is it fitted round the right way? A towards heating and B towards cylinder.

 

[SafeGasInstall]

there is not a problem just boiler riches temperature too quick as cylinder can not absorb the heat quick enough ...
what is the set temp at boiler ? stop spending your money on stuff you dont need to change

 

[SafeGasInstall]

also visit from a heating engineer will sort things out for you , and it will be cheaper

 

[mache]

Hi, First of all let me say I am not a plumber and I am a new member (today). I am however an engineer who has been involved in fault tracing. You mention that you get hot water when CH and DHW requested so there appears to be no blockage in the DWH heating circuit. Restriction possibly.
When DHW requested only then ther appears to be no flow or extremly restricted.
A point was made regarding the diverter valve causing an issue. Possible incorrect positioning ??

What is the situation when CH only selected. Is the flow only to the radiators or is ther bleed though the cylinder coil as well. i.e can you feel hot water being pumped through the return pipe from the cylinder. If you get that on CH demand only it would reinforce that the diverter valve is faulty/misspositioned. As to fix or replace if this is the case....you need to ask a plumber about that :smilewinkgrin:
Regards
Martin

 

[SBDIY]

there is not a problem just boiler riches temperature too quick as cylinder can not absorb the heat quick enough ...
what is the set temp at boiler ? stop spending your money on stuff you don't need to change

The system has worked perfectly well for many years and has always worked perfectly well just heating the HW. The boiler is set at the same temperature as it always has been but I've tried changing it but it makes no difference.

 

[SBDIY]

also visit from a heating engineer will sort things out for you , and it will be cheaper

As I said in the opening post I've had two plumbers look at it and also had the boiler serviced and discussed the problem with the guy who did it but he wasn't able to shed any light on it.

 

[SBDIY]

lame he's getting hot water when heating is on as well just not when by it self. could be a problem with the diverter, is it fitted round the right way? A towards heating and B towards cylinder.

Yes mate, the diverter was the first thing I thought of and replaced that a year ago and it is fitted correctly, exactly as the old one came off.

 

[SafeGasInstall]

what is make and model of boiler ?

 

[SBDIY]

Hi, First of all let me say I am not a plumber and I am a new member (today). I am however an engineer who has been involved in fault tracing. You mention that you get hot water when CH and DHW requested so there appears to be no blockage in the DWH heating circuit. Restriction possibly.
When DHW requested only then ther appears to be no flow or extremly restricted.
A point was made regarding the diverter valve causing an issue. Possible incorrect positioning ??

What is the situation when CH only selected. Is the flow only to the radiators or is ther bleed though the cylinder coil as well. i.e can you feel hot water being pumped through the return pipe from the cylinder. If you get that on CH demand only it would reinforce that the diverter valve is faulty/misspositioned. As to fix or replace if this is the case....you need to ask a plumber about that :smilewinkgrin:
Regards
Martin

Thanks Martin for taking the time to offer some advice. I'm certain its not the diverter because the problem started initially before I replaced the diverter and the header unit. When I have a demand for CH only the feed pipe to the coil doesn't get hot but does when I then demand HW. The symptons seem to point towards a restriction of some sort in the DHW pipe/coil. Although the water gets hot when there is a demand for HW and CH I think it takes longer for the HW to come up to temperature than it used to. Problem is how to find the restriction if there is one?

 

[SBDIY]

what is make and model of boiler ?

Gloworm 15HXI and is 8 years old.

 

[Nostrum]

Run the system from cold on h/w only. When the boiler goes off for the first time is the return pipe hot or cold?

 

[mache]

As I say I am new to this and feel a bit cheeky stepping in here. So it does look like a restriction in the DHW circuit. Are you able to view the complete piperun from cylinder back to boiler. I wonder if a some point in the run a gate valve has been fitted to balance the system when CH and DHW are requested. could it be that some debris has caught at that valve and added an extra restriction, or possibly when isolating parts of the system for a previous maintenace job the valve wasn't opened back up enough afterwards ??? just a thought.
Martin.

 

[SBDIY]

As I say I am new to this and feel a bit cheeky stepping in here. So it does look like a restriction in the DHW circuit. Are you able to view the complete piperun from cylinder back to boiler. I wonder if a some point in the run a gate valve has been fitted to balance the system when CH and DHW are requested. could it be that some debris has caught at that valve and added an extra restriction, or possibly when isolating parts of the system for a previous maintenace job the valve wasn't opened back up enough afterwards ??? just a thought.
Martin.

Hello Martin. There is a gate valve and I replaced that last Friday as a last resort and just in case it had become blocked as you suggest. Unfortunately it hasn't made any difference.

 

[SBDIY]

Run the system from cold on h/w only. When the boiler goes off for the first time is the return pipe hot or cold?

s
I'll try that in the morning but I expect it will just shut down after about 20 or 30 seconds and the pipe will still be cold. I'll let you know tomorrow exactly what happens.

 

[Ermintrude]

Has the system got an auto bypass fitted ? When heating and hot water on the pressure between flow and return low so bypass closed. When hw only and if is a restriction pressure diff high and bypass opens and effectively shorts flow and return... Thus boiler thinks demand is satisfied. Pump stops pressure goes bypass closes and all starts again?

 

[SBDIY]

Has the system got an auto bypass fitted ? When heating and hot water on the pressure between flow and return low so bypass closed. When hw only and if is a restriction pressure diff high and bypass opens and effectively shorts flow and return... Thus boiler thinks demand is satisfied. Pump stops pressure goes bypass closes and all starts again?

There isn't an auto bypass fitted, just a gate valve which I have also replaced at the same time as replacing the other one and which I have turned open just a crack to allow water to flow which is what I'm told it should be.

 

[SBDIY]

Run the system from cold on h/w only. When the boiler goes off for the first time is the return pipe hot or cold?

OK did that this morning. From cold the boiler took about two and a half minutes to get up to 74deg and then shut off. Feed pipe to coil was hot and the return pipe just a little warmer than it was before. Boiler took 3 minutes to switch on again and then the cycle began again.

When the system runs with CH and HW demand the boiler normally sits at 73 deg displayed temp and remains at that as long as there is a demand. When there is just a demand for HW only the boiler gets up to the cut off very quickly once it has been running for a while, normally about 20-30 seconds. Now and then the boiler does try to regulate at 73 deg but only for a short time before it goes up to 74 deg and cuts out.

 

[GasmanxxxR1]

is the hw pipework in 15mm or 22mm

 

[galaxy_plumbing]

Feed pipe to coil was hot and the return pipe just a little warmer than it was before.

looks like a blockage in the coil or somewhere on return from cylinder.

 

[SBDIY]

is the hw pipework in 15mm or 22mm

HW is 15mm

 

[SBDIY]

looks like a blockage in the coil or somewhere on return from cylinder.

Accept that but after only two and a half minutes and starting from cold with a tank full of cold water, with the pump running on slow speed, would you not expect most of the heat in the feed pipe to be given up through exchange via the coil? The return pipe does get hot after a while but I'm agreeing with your suggestion of a restriction in the pipework in the DHW system. probably somewhere from the tank / coil onwards.

 

[galaxy_plumbing]

is there any valve fitted on the hw return from cylinder up to the 3-port valve? if it was working ok for years then something must off happened and more likely it sounds like blockage.

 

[SBDIY]

is there any valve fitted on the HW return from cylinder up to the 3-port valve? if it was working ok for years then something must off happened and more likely it sounds like blockage.

No, the only two valves are the ones from the 3-port diverter (DHW) to thew coil feed and the by-pass from the feed side of the diverter back to the boiler. Things do seem to be pointing to a blockage or restriction but if so then the problem then becomes where and how to shift it?

 

[mache]

There isn't an auto bypass fitted, just a gate valve which I have also replaced at the same time as replacing the other one and which I have turned open just a crack to allow water to flow which is what I'm told it should be.

I think I would be tempted to open that valve further. If when CH and DHW are on demand you are not getting adequate feed to the radiators then choke back the cylinder return to provide back pressure and force more flowthrough CH side.

 

[SBDIY]

I think I would be tempted to open that valve further. If when CH and DHW are on demand you are not getting adequate feed to the radiators then choke back the cylinder return to provide back pressure and force more flowthrough CH side.

I don't have a problem getting heat into the radiators, the CH side is working perfectl;y well. The problem is only in getting HW when there is only a demand for HW.

 

[GasmanxxxR1]

With the pipework to the hw being in 15mm this increases the chances of it being a blockage in the pipework or coil.

New Cylinder and 22mm pipework needed probably

 

[GasmanxxxR1]

15mm circuit is too restrictive for the boiler, from memory the hxis are sensitive about water flow rate

 

[SBDIY]

15mm circuit is too restrictive for the boiler, from memory the hxis are sensitive about water flow rate

Interesting suggestion however the boiler was installed in 2005 and worked perfectly up until about a year ago when the problem started after me draining the system, which I have done on several occassions. The suddeness of the start of the problem seems consistent with a restriction or partial blockage of some sort. If the tank needs replacing then fair enough but i'm obviously reluctant to spend up to £300 on getting a new one installed if it doesn't solve the problem.

If the boiler is sensitive to flow rate as you suggest then again the affects of a blockage of some sort might be the explanation?

 

[PlumbBobBob]

i think your cylinder would cost more than £300 to change, but of course it all depends on your location.

 

[galaxy_plumbing]

how long is 15mm hw run? it could be replaced with 22mm, that would be cheaper than replacing whole cylinder

 

[SafeGasInstall]

gasmanxxxR1 well done ! Excellent thinking

Op repipe F&R to 22 or if you have good budget 28mm , and turn boiler stat to 65

 

[Nostrum]

You need to check the return at the boiler on first fire for heat. It could be that it is finding another path back to boiler with less resistance. In fact I would put money on that as the boiler wouldn't run for 2.5 minutes if there was no circulation at all. Wind the by pass right down to nothing. Have you bled the top of the coil since re filling? Air can be really stubborn depending on how it is piped, crack the top nut and fill a small bowl up with water to check for air. A picture of the cylinder may help.

 

[SBDIY]

i think your cylinder would cost more than £300 to change, but of course it all depends on your location.

Yes probably, just going off what it cost me to get it changed 8 years ago.

Although the consensus of opinion seems to be suggesting a partial blockage I'm still not convinced on account of when the boiler burns (for 20+ seconds at a time) the pipes get hotter quite quickly (both feed and return). I attribute the time taken to heat the domestic hot water to be more due to the cyclic nature of the burn time which only burns for 20 seconds in every 3-4 minutes. As a result the water being pumped through the coil isn't (on average) hot enough to heat the water quickly.

I'm still wondering if one of the flow / return thermistors could be faulty but don't know if that would account for the problem?

Somewhat baffled.

 

[SBDIY]

You need to check the return at the boiler on first fire for heat. It could be that it is finding another path back to boiler with less resistance. In fact I would put money on that as the boiler wouldn't run for 2.5 minutes if there was no circulation at all. Wind the by pass right down to nothing. Have you bled the top of the coil since re filling? Air can be really stubborn depending on how it is piped, crack the top nut and fill a small bowl up with water to check for air. A picture of the cylinder may help.

With the by-pass closed and only a demand for HW the boiler fires up and shuts down in less than 20 seconds. There is a bleed valve at the top of the 15mm feed pipe directly above the tank about a metre away and I bleed this every day and usually take a bit of air out each time. The system is pretty good at giving up air to be honest. Not sure how I'd bleed it as you suggest but I've attached a picture of the feed pipe / connection.

Thanks for your help with this by the way - really appreciate you taking the time to try to help.

 

[GasmanxxxR1]

As i said in previous posts, the 15mm pipe is too restrictive and causes the boiler to cycle, 15mm pipe can only carry so much energy and just cant get the heat away from the boiler quick enough

 

[SBDIY]

As i said in previous posts, the 15mm pipe is too restrictive and causes the boiler to cycle, 15mm pipe can only carry so much energy and just cant get the heat away from the boiler quick enough

Yes I understand and see the logic in what you are saying. It doesn't though tally with the fact that it has worked perfectly well, without any problems for 30 years, including the past 8 years since the 15kW Gloworm Boiler was fitted. It would though fit with the pipes being partially blocked with sludge but I have ran some Sentinel through it to clear it out. Maybe I need to try something stronger?

 

[Mickplumb]

Been following your thread was system power flushed when new boiler installed
if not would recommend you get it done with all rads shut hopefully the larger pump will shift any blockage ,get them to fit a magna clean on primary return to boiler while your at it

 

[Ermintrude]

Disconnect flow and return from cylinder , connect cold mains to it and a drain on flow. So water is going up the coil and flush out any pap. I would fit a jet auto air vent aswell.

 

[SBDIY]

As a different line of thought, I mentioned in an earlier post that I wondered if the problem could be something to do with the flow/return Thermistors. Earlier today the boiler shut down with fault F26 which when I checked the manual means Return Thermistor Restriction. Anyone know what that means and could it be relevant to the problem I've got?

 

[GasmanxxxR1]

its indicating you have a problem, its not causing the problem imho, too much difference in temp between flow and return

 

[SBDIY]

its indicating you have a problem, its not causing the problem imho, too much difference in temp between flow and return

Good point - makes sense. I think I will check the two Thermistors though just to make sure they are both responding consistently to changes in temperature. Probably ok but at least eliminates them from the investigation before looking at things more difficult and time consuming.

 

[Nostrum]

The 15mm isn't the issue, there are plenty around with 15mm circ's. They used to do it to balance the coil so it didn't take preference to the heating circuit. It's only 15kw boiler.

i have had a similar problem once before on a heating circuit. The radiators stopped almost overnight. I checked everything and in the end chucked some sentinel x800 jetflo in and within 2 hrs they were working again. Still never found out what caused it and the water was gin clear.

before doing this though, I would isolate a pump valve and carefully crack the nut in the picture with a bin bag underneath to catch the water, this will push water through the bottom of the coil and out the top just to check for blockage or air, the coil could be bend and rising up trapping air although unlikely. Make sure you open the pump valve again.

 

[Nostrum]

Good point - makes sense. I think I will check the two Thermistors though just to make sure they are both responding consistently to changes in temperature. Probably ok but at least eliminates them from the investigation before looking at things more difficult and time consuming.

if there was a thermistor fault you'd get he same issue with the heating...

 

[PlumbBobBob]

Good point - makes sense. I think I will check the two Thermistors though just to make sure they are both responding consistently to changes in temperature. Probably ok but at least eliminates them from the investigation before looking at things more difficult and time consuming.

you shouldn't be messing about inside the boiler, you could make the problems worse.