Combi hot water cycling hot cold hot cold

[Lukepricephotography]

Morning all, ok so since having a new Worcester greenstar 8000 combi installed I’ve been struggling to get constant hot water… it runs hot for a minute or two then cold then hot etc.
the system has been power flushed, magnacleansed, radiators agitated (very little sludge came out) it has mesh filters on the cold in and central heating return as well as the magnaclean unit. the incoming cold is also fully softened.
the central heating flows well, but the boiler and pump switch on and off even when there is still a temp delta better desired temperature and the thermostat.

in terms of investigations… the pump has been replaced. The heat exchanger has been replaced… yet the problem persists.

importantly the issue is DIRECTLY RELATED TO HOW COLD IT IS OUTSIDE…. If it’s particularly cold say 0degC the cycling is much quicker… it’s it’s a really warm day say 20degC then the hot water is almost constant.

any thoughts as this is driving me insane I’ve now had 6 plumber visits over 18months without success.

When I had a conventional boiler system I never had an issue, but I was trying to be more eco!

 

[Deleted member 120897]

Who decided it needed a new heat exchanger if it’s that new? Have you had Worcester out under the warranty? Try running the hot water, and then feel the cold pipe, you may have reversed connections.

 

[Lukepricephotography]

Who decided it needed a new heat exchanger if it’s that new? Have you had Worcester out under the warranty? Try running the hot water, and then feel the cold pipe, you may have reversed connections.

British gas, they installed the boiler and have been coming out trying to sort it.

it would make sense it cycling if it was blocked, but it’s not made a difference. And the power flush magnacleanse was super thorough, a week to soak in chemical, then each radiator individually. hardly anything picked up on the magnets.

I do have two rising mains (house used to be two properties)… but the one is isolated with a stopcock and the other is the one feeding the whole system. I guess there could be some leakage past the stopcock into the system but that would only be on the cold taps side.

 

[Chuck]

It's not clear to me whether the cycling affects (a) the domestic hot water (from the taps), (b) the radiator circuit, or (c) both.

For (a) when you run a hot tap, what's the flow rate in litre per minute (or how long to fill a jug of known volume? For (b) how many radiators of what sort of size and type (single/double) do you have?

Can you clarify what you mean by "a temp delta better desired temperature and the thermostat", please?

 

[Lukepricephotography]

It's not clear to me whether the cycling affects (a) the domestic hot water (from the taps), (b) the radiator circuit, or (c) both.

For (a) when you run a hot tap, what's the flow rate in litre per minute (or how long to fill a jug of known volume? For (b) how many radiators of what sort of size and type (single/double) do you have?

Can you clarify what you mean by "a temp delta better desired temperature and the thermostat", please?

morning chuck.
so the cycling appears to affect both the central heating and the hot water… the hot water bothers me more though when I’m trying to take a shower!

size of system. Ok so my mains water in is regulated down to a maximum 5bar with a PRV to protect the water softener. With this in place I get between 17 and 21 liters per minute into the house measured at the tap furthest from the rising main.
there are 6 radiators and two towel rails. The radiators are three single and three double.
the system Is plumbed with 22mm for majority, only dropping to 15mm for the legs to radiators and taps themselves.

the boiler is a Worcester greenstar 8000 life 40c

As for delta, simply that if I’ve got the heating set to 25deg desired, even if the thermostat reads 16degs the heating doesn’t run continuously. It runs then shuts down, then runs, then shuts down.

any more info required just shout! I welcome any thoughts!

luke

 

[Ben-gee]

Sounds like boiler is oversized

 

[Chuck]

any more info required just shout! I welcome any thoughts!

When you are running the shower, how many litre per minute are being delivered as measured, e.g,, by the time needed to fill a jug?

Do the radiators have thermostatic valves (TRV's) attached, and what setting are they at?

 

[Lukepricephotography]

Sounds like boiler is oversized

Eeeek I hope not…. But surely that would be British gases liability to have provided the correct sized boiler?

 

[Lukepricephotography]

When you are running the shower, how many litre per minute are being delivered as measured, e.g,, by the time needed to fill a jug?

Do the radiators have thermostatic valves (TRV's) attached, and what setting are they at?

Ok so I get 17lpm hot 17.5lpm cold at my en suite shower furthest from the boiler. I get 18lpm from the shower in the bathroom closest to the boiler.. and I get 18.5lpm combined if I open various taps and then combine the outputs.

 

[Lukepricephotography]

Eeeek I hope not…. But surely that would be British gases liability to have provided the correct sized boiler?

if I remove the incoming cold PRV then I get 25lpm at 7bar. But my water softener monarch branded states a 5bar limit… do you think I can up the pressure on this safely for improved flow and get the boiler into size again? I’d also have a better shower too I guess

if not, what about running two water softeners in parallel so the flow is split across them and pressure would be reduced?

 

[Chuck]

if I remove the incoming cold PRV then I get 25lpm at 7bar. But my water softener monarch branded states a 5bar limit… do you think I can up the pressure on this safely

No. If the manufacturer states a limit of 5bar, do not exceed it.

Finally, when you open a hot tap how much flow do you get?

It the flow rate for hot alone is decent I suspect that you have a boiler fault, which will need Worcester Bosch to attend and sort it out for you under warranty.

 

[Chuck]

I do have two rising mains (house used to be two properties)… but the one is isolated with a stopcock and the other is the one feeding the whole system. I guess there could be some leakage past the stopcock into the system but that would only be on the cold taps side.

Not related to your boiler issue but this description worries me because, if I've understood correctly, it means that you must have a dead leg(s) in your cold water plumbing. This puts you at risk of legionella, etc. Needs reviewing and either fixing or a risk assessment and a schedule of regular flushing and testing implemented.