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]

As I thought. The original heatslave with the bentone downfiring burner.
I suspect the DHW hex. Are you in a hard water area? Have you ever had sludge problems within the system?
If the boiler fires when the DHW stat calls for it and remains firing and the pump is circulating then this says the control side is probably OK.
Anything that can impede the transfer of energy will cause this problem.

I suggest you find an experienced engineer to attend and explain to him/her what we've discussed.

What vintage boiler is this SJB, any link to this particular model?

 

[SJB060685]

What vintage boiler is this SJB, any link to this particular model?

Hi mate. This is the early greenstar heatslave, not the mark 2. This particular model had downfiring burners, either Riello or Bentone. Google greenstar heatslave service manual. If you're still stuck i will upload the file from my ipad in a bit, im a bit busy at the moment.
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Here is the service manual. If you scroll through to electrical flow chart I think there is a misprint on one of the relays. When the DHW stat calls for heat it should de energise feed to diverter and return for HW.

 

[Marcopf]

Hi a bit of an update:

Firstly, I got a bigger container to fill and more accurate timing. 12 litres took 31 seconds so 23Lpm

I ran the hot tap until is was look warm then it took the boiler 5m 35s to heat to ‘4’ on the stat

Turned the stat to ‘6’ (max) and the boiler fired for a further 3mins

So with the stat at 6 opened up the hot tap fully. At around 3 mins the water was losing heat, 3:40 it was luke warm

Ran the same test again with stat on ‘4’ and got same results, although this time I kept my hand under the tap as much as possible the temp seemed to vary a bit. I could keep it there for 30 secs (and felt it vary a bit I’m sure) before pulling away and obviously running towards 3mins I could put my hand there again

Thanks

 

[SJB060685]

Ok. Ive relayed this on to some of my associates and it would appear the flow chart i had is wrong and quite frankly useless.
The 18/25 KW output is only rated for a maximum of 18 LPM, you say you had 23? By my calculations, to keep up with those flow requirements you would need an input of around 60 KW, you have at max 25 KW.
Try regulating the flow rates through the taps by either restricting the valve on the cold inlet to the boiler or at your main stopcock. Try this and see if it makes any difference.

 

[John.g]

I can't see your post #27 Marc when I log on so I have copied and pasted it here.

"i a bit of an update:

Firstly, I got a bigger container to fill and more accurate timing. 12 litres took 31 seconds so 23Lpm

I ran the hot tap until is was look warm then it took the boiler 5m 35s to heat to ‘4’ on the stat

Turned the stat to ‘6’ (max) and the boiler fired for a further 3mins

So with the stat at 6 opened up the hot tap fully. At around 3 mins the water was losing heat, 3:40 it was luke warm

Ran the same test again with stat on ‘4’ and got same results, although this time I kept my hand under the tap as much as possible the temp seemed to vary a bit. I could keep it there for 30 secs (and felt it vary a bit I’m sure) before pulling away and obviously running towards 3mins I could put my hand there again"

The attachment above that SJB kindly supplied states that the total primary water contents are 69 litres so if one assumes boiler contents at 25 litres then the real volume water in the heat bank is ~ 44 litres, also the max heat bank temperature is 72C so I would think that your tests bear this out.
Can you now confirm that the boiler runs continuously once it cuts in and you have a high flowrate??
Also remember that once the heat bank is exhausted then a 25 kw boiler will only raise the temperature by 15.5C with a flow rate of 23 LPM resulting in a lukewarm temperature of 25.5C (assuming cold water at 10C.) To get a reasonable hot water temp of say 40C (from 10C) will require the flow rate to be reduced to 12 LPM and probably a fair bit more since the water is not being taken directly from the heat store but is passing through a PHE.
You might do this, just keep reducing the hot tap flow rate slowly until you have a comfortable temperature (boiler still firing) and then measure this flow rate.

 

[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.
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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?

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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?
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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.