Water not getting hot - boiler cycling

[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.