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

 

[Marcopf]

Can you now confirm that the boiler runs continuously once it cuts in and you have a high flowrate??

Whenever I turn on a hot tap, regardless of any parameters, the boiler cuts in immediately and runs continuously until the tap is turned off again

So you’re saying I have to turn down the shower to half speed for it to stay hot?

 

[SJB060685]

Whenever I turn on a hot tap, regardless of any parameters, the boiler cuts in immediately and runs continuously until the tap is turned off again

So you’re saying I have to turn down the shower to half speed for it to stay hot?

What John and I are saying is your flow rates are far to excessive for the boiler output. If you regulate the flow rates and your situation improves then we have our answer.

 

[Marcopf]

What John and I are saying is your flow rates are far to excessive for the boiler output. If you regulate the flow rates and your situation improves then we have our answer.

OK, bye bye shower, it was good while it lasted (August/September) 😟

The boiler is maybe 10+ years old. Can anyone tell me just how old from the serial number?
And if I was to replace the boiler, which brand and capacity would you recommend as professionals?

Thanks

 

[John.g]

And maybe there never was a problem with the boiler?, the first few posts gave me the impression that the boiler was only running for a few minutes even with a high flow rate.

Some manufacturers coyly suggest that you can get say 18 LPM from a 25 kw boiler with a hot water temp of 40/45C or a delaT of 35C but neglect to say that this is only true while the heat bank lasts for a very short period, some of them especially oil fired combi makers will give a far more realistic figure of someting like (for this boiler) stating 90 litres of water available at a deltaT of 40C or whatever.

 

[SJB060685]

The serial number will say which month and year it was produced, im sure I have a chart somewhere.
You have a high flow rate so to get what you want a larger output is required. Brand wise everybody prefers different. You might want to consider a system boiler and unvented cylinder, rather than a combi.
For now adjust the flow rates down and see if things improve, we at least then know we're in the right place.

 

[Marcopf]

I’m confident that you are. Turning down the tap to a flow rate of around 16/17 the hot water keeps coming. In retrospect, I’m surprised that the Worcester tech support said nothing about flow rate. Thanks for all the time and knowledge you’ve all shared. I have learnt a lot. It needs to be a combi because of the house layout and very small loft space. It’s a pity because the shower on full is really nice, especially after years of a crappy underpowered electric jobbie, and does not feel in any way excessive. Here it is

 

[John.g]

I think around 40kw is around the limit, so in winter with a cold water temperature of say 5c and a required hot water temp of 45C will mean a max flow rate of 14.3 LPM. Your 25 kw boiler under the same conditions will give a flow rate of 9 LPM. I suppose its no harm to remember that any shower that gives a flow rate of 10 LPM or more is considered a power shower so you are in heaven compared to a electric shower with a flowrate of ~ 4 LPM!.
You also got that 23 LPM from a tap so if you are happy with a showering temp of 40C then you will still get > 10 LPM from water at 5C.

 

[exedon2]

Totally agree with what the guys have said re flow rate
Shower probably worked better to start with due to warmer incoming cold supply, as a stop gap
Solution I would try a flow restrictor on shower certainly Briston do them down to as low as 6lts a minute
Won't be a great shower but it might at least stay warm.

 

[Marcopf]

And maybe there never was a problem with the boiler?, the first few posts gave me the impression that the boiler was only running for a few minutes even with a high flow rate.

Yes, it was. After 3 mins the water started cooling and by 3:40 was luke warm. But my flow rate measurement with a 1litre jug was inaccurate so I redid it with 12L container and digital stop watch and calculated the flow rate at 23Lpm

 

[John.g]

Can't see your post again Marc while logged in, even though there is really nothing really wrong with the boiler do you mind still measuring that flow rate with your normal shot water temperature in case that the PHE is fouled and the flow rate is actually worse than the theoretical, the boiler would then start cycling on/off if this should be the case.
[automerge]1603927852[/automerge]
Also with HW setting at max please.

 

[Marcopf]

Hi John. Sorry if I’m being thick here, I’m very tired, but can you spell out exactly what you want in steps. I just looked at your post #29. Are you asking me to repeat this, whilst checking if the boiler is cycling on/off? I also don’t know what temperature 4 and 6 represent on the HW stat dial. It might be an idea to lower this, no?

 

[John.g]

No Marc, forget about the boiler cycling on/off, just turn the HW temp to 6, it should always be at this in my opinion, then keep reducing the flow rate until you get your required hot water temperature and then measure this, initially open the hot water tap fully to exhaust the heat bank before turning it down. You don't have to do it tonight....anytime will do at your convenience.

 

[Marcopf]

In the shower there’s a mixer control for temperature and a control for flow. So you want me to set the desired temp with the mixer then run the shower on full flow until it goes luke warm. Turn down the flow rate until the water comes back up to the temperature I want, and then gradually increase flow rate to a point where it keeps that temperature, indefinitely without dropping. In other words find the maximum flow to keep the temperature I want permanently. Then measure what this flow rate is. Is that correct?

[automerge]1603934747[/automerge]
I just thought, because the temperature in the shower is set with a mixer tap, rather than a thermostat, you’re not going to know how much cold water has to be mixed with the hot to get the desired temp so it’s not going to be possible to measure the output of the boiler, is it?
[automerge]1603935648[/automerge]
Is there any sure fire way to see if a PHE is functioning properly - can you just take it out and look at it? Can you put it through a descaling process? How much would it cost toreplace a DHE for this boiler? If it needs to be done I would like to get it done under this care plan

Thanks

PS I have the serial number if anyone can interpret it for me to get the boiler age

 

[John.g]

Same problem again Marc in not seeing your post (#42) while logged on on so have pasted it on here again......

"In the shower there’s a mixer control for temperature and a control for flow. So you want me to set the desired temp with the mixer then run the shower on full flow until it goes luke warm. Turn down the flow rate until the water comes back up to the temperature I want, and then gradually increase flow rate to a point where it keeps that temperature, indefinitely without dropping. In other words find the maximum flow to keep the temperature I want permanently. Then measure what this flow rate is. Is that correct? "

Yes, that is correct but I wanted that flow rate tested via the hot tap only (first) in case the shower mixer isn't working properly and not allowing the combi's full thermal output.
I see in post #35 that you were getting 16/17 LPM of hot water and if you are happy with this then forget repeating this part of the test, my cold water temperature is 12C at the moment so theoretically the hot water temperature should have been 34/35C, if the flow rate was reduced to 13 LPM then the hot water temp would be 40C but looking pretty good overall re boiler and PHE.

So, yes, carry out your shower test as above and measure your flow rate.

 

[Marcopf]

The reason I thought you meant the shower, even though you said tap, is that you talked about setting it to the desired temperature - I have separate hot and cold taps in the utility room so I can obviously only adjust the flow of the hot tap, not it’s temperature.

 

[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
[automerge]1603970580[/automerge]

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