Greenstar Heatslave not producing vet limited amount of hot water

[Marcopf]

Hi, would be extremely grateful for any pointers on this problem

The full name of the boiler is the Worcester Greenstar Heatslave 18/25 Oil combi. The boiler has a 69L heatbank. We leave the water permanently set to on.

When I run the hot tap nearest the boiler i get hot water as expected for the first couple of minutes. At 2 mins the temperature starts to fall and by three mins the water is luke warm.

This is not enough to complete a shower without the wife having to rinse conditioner out of her hair in cold water

The boiler is firing up as soon as I start drawing water From the tap. As I understand it, you are initially drawing hot water from the heatbank and the boiler keeps replenishing that with hot water until you turn the tap off.

It seems the boiler is not able to keep up with the rate the water is required at and I’m inclined to think that the water starts getting colder after the hot water that was stored in the heatbank has been used up. If turn off the tap, the boiler keeps going for a while (topping up the heatbank, I presume) and wait for a while you can draw hot water again, but not for long.

Thanks

 

[John.g]

Yes you CAN adjust its temperature by adjusting its flow rate as explained above, the less the flow the hotter the water gets and vica versa.....the same way as your electric shower, you turn the temperature control up or down to change the temperature but all you are doing is changing the flow rate to achieve this.
You have already "proved" this yourself, a flowrate of 23 LPM was only giving a tepid temperature but 16/17 LPM was giving hotter water.

 

[Marcopf]

Yes you CAN adjust its temperature by adjusting its flow rate as explained above, the less the flow the hotter the water gets and vica versa...the same way as your electric shower, you turn the temperature control up or down to change the temperature but all you are doing is changing the flow rate to achieve this.
You have already "proved" this yourself, a flowrate of 23 LPM was only giving a tepid temperature but 16/17 LPM was giving hotter water.

No that’s not the case. The temperatures are the same, both too hot to shower, but when on full throttle the temp drops to luke warm after 3.5 minutes whereas at 2/3 flow rate it stays hot indefinitely. I’m not claiming any knowledge here, just reporting what happens

Anyway, someone who services boilers just came and was checking things out and noticed that the feed feed from the boiler for CH was hot when the HW was flowing and the CH was turned off. He’s sending someone else round more experienced or qualified to take it further

Thanks

 

[SimonG]

See this most times with a combi conversion, but normally on the bath tap. I fit a valve under the bath now to prevent full bore flow.

 

[Marcopf]

See this most times with a combi conversion, but normally on the bath tap. I fit a valve under the bath now to prevent full bore flow.

I’ve got rid of the bath now, as you could never get a hot one within a reasonable time frame. the service guy suggested we might restrict the flow into the boiler, but I said I cann’t see that that’s any different to controlling the flow in the shower, only I have more control that way. I might be wrong
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No that’s not the case. The temperatures are the same, both too hot to shower, but when on full throttle the temp drops to luke warm after 3.5 minutes whereas at 2/3 flow rate it stays hot indefinitely. I’m not claiming any knowledge here, just reporting what happens

Anyway, someone who services boilers just came and was checking things out and noticed that the feed feed from the boiler for CH was hot when the HW was flowing and the CH was turned off. He’s sending someone else round more experienced or qualified to take it further

Thanks

John G. - the penny’s just dropped. It’s quite hard to set set the desired temp from the tap as I’m not convince that holding your hand under it is the same as what temp you’d like falling on your whole body. Then there’s the running through all the pipe’s and falling as a sprinkle from a fair height. Also, it was difficult to keep the temp steady - it seemed to vary

in the end I measured 12l in 1:34 which is about 8Lpm

And also this time the CH pipe didn’t get warm. I’m thoroughly confused now

 

[John.g]

Yes you CAN adjust its temperature by adjusting its flow rate as explained above, the less the flow the hotter the water gets and vica versa...the same way as your electric shower, you turn the temperature control up or down to change the temperature but all you are doing is changing the flow rate to achieve this.
You have already "proved" this yourself, a flowrate of 23 LPM was only giving a tepid temperature but 16/17 LPM was giving hotter water.

No that’s not the case. The temperatures are the same, both too hot to shower, but when on full throttle the temp drops to luke warm after 3.5 minutes whereas at 2/3 flow rate it stays hot indefinitely. I’m not claiming any knowledge here, just reporting what happens

Re your new post, what does 2/3 flow rate mean in LPM. If we assume that too hot means say 55c then it is thermodynamically impossible to have a sustained flow rate of more than 8.3 LPM (from cold water at 12C) once the heat bank is used up, if you are talking about a flow rate of say 16 LPM (2/3 flow) then starting from this base and with a fully heated heat bank then you will get very hot water for a short period of perhaps 3 or 4 minutes and it will then fall to 34C and remain at this temperature indefinitely, if you are happy with this then fine, I prefer a showering temp of 40C in which case I would have to reduce this flow rate to 12.8 LPM to maintain this temperature indefinitely.

 

[Marcopf]

No that’s not the case. The temperatures are the same, both too hot to shower, but when on full throttle the temp drops to luke warm after 3.5 minutes whereas at 2/3 flow rate it stays hot indefinitely. I’m not claiming any knowledge here, just reporting what happens

Re your new post, what does 2/3 flow rate mean in LPM. If we assume that too hot means say 55c then it is thermodynamically impossible to have a sustained flow rate of more than 8.3 LPM (from cold water at 12C) once the heat bank is used up, if you are talking about a flow rate of say 16 LPM (2/3 flow) then starting from this base and with a fully heated heat bank then you will get very hot water for a short period of perhaps 3 or 4 minutes and it will then fall to 34C and remain at this temperature indefinitely, if you are happy with this then fine, I prefer a showering temp of 40C in which case I would have to reduce this flow rate to 12.8 LPM to maintain this temperature indefinitely.

Yes, sounds fair. At 16Lpm from the tap it stays hot, but too hot for a shower (at that point)
By the time it’s traveled to the other end of the house and through the shower head it’s not far off with a little cold mixed in

By the way, now that the boiler has been going for a bit to reheat the heat store that CH pipe has gone hot again 🤷‍♂️

 

[John.g]

Don't know why your CH pipe gets hot at times, maybe diverter valve partially sticking at times, may be worth changing but I think that boiler is performing to spec but a boiler service might be no harm if the guy who is coming can do it and give you a print out of the flue gas analysis.
Re the shower, I presume you have a thermostatic mixer, then if working correctly set it to your required showering temp and when it starts running too cold just cut back on the flow rate, you should then get a good feel for the required position of the flow control valve and just set it to this each time you require a shower.

 

[SJB060685]

A combi boiler should always give priority to HW. The manual i uploaded has a vey misleading electrical flow chart, whenever there's a demand for HW at the taps or by DHW control stat the boiler SHOULD, fingers crossed, divert the flow of water through the diverter valve and to the heatslave.
One would have to do some checks on the PCB and relays to confirm this.
As I mentioned in my original post, many things can impede the transfer of energy. If the boiler is dirty ie hasn't been serviced in a while this won't help, likewise if the nozzle and pump pressure arent suitably sized and set this was also affect the boiler output, then of course you have the boiler efficiency, not to be confused with combustion efficiency, which also has underlying affects of energy transfer through heat lost through pipes, casing, heat exchanger etc. You might find a good service and correct settings will improve your situation slightly, im not saying for definite but could do.

 

[Marcopf]

Right, the boiler’s had it’s annual service, a couple of bits were changed, but nothing pertaining to the issues we’ve been discussing. As mentioned another engineer/plumber coming shortly to do a few more tests. I’m really grateful for the help received here, it’s surprised me actually how generous people have been with their time and expertise. I’ll report back with the final outcome, in a few days hopefully.

Thanks