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

 

[SJB060685]

Is this a new problem? Did it use to work properly?

The heat store is NOT stored water for the taps, it is water stored at temperature to instantly supply heat running to the taps by means of a heat exchanger.

 

[Marcopf]

Hi, thanks for the information on the heat store. On the second point, we could never get enough water to use the bath and relied on a clapped out electric shower, so in August we had a shower put in that runs off the boiler. This worked well for a couple of months, but now cannot run for a normal shower length, the water goes to look warm within 3 minutes

 

[John.g]

Is Eco mode off and DHW temperature control turned to max? even the 69 litre primary heatbank on its own if set to max temperature of say 75C and using a heat exchanger if set to max temperature of say 75C should give the equivalent of 140 litres at 45C. Might be problem with Hx if in hard water area.?.

 

[SJB060685]

If this is a new problem then it could be a number of things John.
. Partially blocked hex.
. Sticky diverter valve.
. Boiler needs a service.
. Even the wrong nozzle installed on a recent service etc etc.

There is a response from the OP that hasn't been verified yet, hopefully that will give us a bit more information.

 

[Marcopf]

It’s not a new problem in that we’ve never been able to have a bath here with this boiler. When the shower replaced the bath it was good for a while now we can‘t even get a shower. The hot water tap wide open gives 15l/min for 2.5 mins before the temperature drops and by 3 mins it would get uncomfortable to stay in the shower.

I‘ve also been thinking that it’s a lot colder now than it was when we first had the shower installed in August and we’ve had the CH on on some nights as well, which would expose any issue further. One other thing is the burner fires up immediately when a tap is turned on, but tech help at Worcester say I should get 3 to 5 mins hot water before the burner kicks in

I was told it might be that the heat store isn’t being heated up and to check the sensor, stat, diverter and heat exchanger in that order. I’m going to get someone in. It’s just good to have some idea of what’s going on

Thanks

 

[SimonG]

Sounds like lack of primary water through heat exchanger.
If heatings fine then it's not the pump.
Probably the diverter.

 

[SJB060685]

It’s not a new problem in that we’ve never been able to have a bath here with this boiler. When the shower replaced the bath it was good for a while now we can‘t even get a shower. The hot water tap wide open gives 15l/min for 2.5 mins before the temperature drops and by 3 mins it would get uncomfortable to stay in the shower.

I‘ve also been thinking that it’s a lot colder now than it was when we first had the shower installed in August and we’ve had the CH on on some nights as well, which would expose any issue further. One other thing is the burner fires up immediately when a tap is turned on, but tech help at Worcester say I should get 3 to 5 mins hot water before the burner kicks in

I was told it might be that the heat store isn’t being heated up and to check the sensor, stat, diverter and heat exchanger in that order. I’m going to get someone in. It’s just good to have some idea of what’s going on

Thanks

Its most likely one of the above mentioned. My first suspicion is the diverter valve.
See if you can get by with the heating off for an hour and when the radiators are cold open a hot tap and give it 5 minutes and see if the rads start getting hot. If they do this would suggest the diverter is sticking.
There is another way to check but that would require taking the boiler cover off, which I suggest you leave to whoever attends.

 

[John.g]

Also with that flow rate of 15 LPM, the boiler should then fire continuously and not just run for a few minutes.

 

[Marcopf]

Thanks John G, that‘s what I thought

I tried turning up the HW stat to nearly max and running the hot water at 15Lpm again and it made no discernible difference in amount of HW before heat is lost

The boiler always fires up immediately whenever I turn a hot water tap on, whereas the Worcester tech support told me I should get a few minutes hot water before the burner kicks in

I also had the CH off with the boiler at max and the hot tap running full blast. The radiator next to the boiler was fully open, but I didn’t detect any heat there so does that rule out the diverter?

So all things considered what is the consensus on the problem?

Many thanks

 

[SJB060685]

If the radiator didn't start to get hot that should rule out a diverter valve.
The burner should ignite when the DHW thermostat senses a drop in temperature, this should only take a short time (providing im looking at the correct flow chart, there's a few for the heatslave models).
I still wouldn't rule out a problem with the HW heat exchanger.
When was the boiler last serviced? If a boiler is not regularly maintained then you could have problems with the nozzle output which would directly affect the thermal output, if the boiler is dirty inside this can have the same affect.

 

[John.g]

The PHE could be a problem but it was supplying "hot" water for 2.5 mins @ 15 LPM which is 38 litres of hot water so OK in some fashion but surprising that the 69 litre heat bank is then running out of hot water or only supplying cooler water for its remaining 30 litres or so.
Also re the boiler thermal output, 15 LPM should demand burner continuous running even at its full rated output of 25kw so even if its output has degraded it should still run continuously, so something is telling it to only run intermittently? The PHE temperature is also used as well as the heat bank temperature and the flow rate to determine the burner demand so possibly a problem with any one of these 3 items?. Even with no flow detected (< 2 LPM) the burner will still fire and start the circulating pump depending on the heat Bank and the PHE temperatures and some offset temperature.

 

[SimonG]

If the diverter is ruled out then it sounds like it could be the pump not kicking in when the flow switch activates.

 

[SJB060685]

OP could you take the front casing off the boiler and upload an image of the inside, so I can cross check my manuals for the right flow chart?

 

[John.g]

Page 51 might help in trouble shooting.

 

[SJB060685]

That is the manual for a greenstar heatslave 11 mate, the modern version. If the op can upload an image I can find the right manual and we can go from there.

 

[Marcopf]

Thanks, here you go.

 

[SimonG]

Is the thermal store tank hot all the way down?

Could be the head not kicking in the pump.


One thing to try, open the hot tap, and tap the top of the boiler near the clock, see if it does anything. Not very scientific but loads of problems with them clocks.

 

[SJB060685]

Thanks, here you go. View attachment 45954

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.

 

[SimonG]

With the plate heat exchanger I always found it to restrict the flow through hot tap. Had three holiday cottages all need hex's within 3 weeks of each other. The bore hole had failed and filled the system with peat.

 

[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.
[automerge]1603911401[/automerge]
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