Water not getting hot - boiler cycling

[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.