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.

 

[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.

 

[SBDIY]

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.

That's another good suggestion mate and I can do that quite easily, as a non professional, as long as the missus can hold a bin bag .

I have had some X400 in the system a couple of weeks ago and by coincidence today bought some X800 to give it another go.

 

[SBDIY]

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

Yes, fair enough. I wouldn't mess with a boiler but as an electrical engineer I'm confident to test a Thermistor but you're advice is good and I'll take it.

Also the other point that if it were a Thermistor fault it would happen with the CH also makes sense too.

Thanks.

 

[Nostrum]

I wouldn't trust my misses to hold a door open

 

[Dotty]

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.

And to do that you need to be a GSR.

Read the forum rules appertaining to gas appliances.

Thread closed.