Heatslave 12/18 Poor DHW Performance

[JST]

Hi all,



We have an external Worcester Bosch heatslave 12/18 oil combi which is causing much frustration on the DHW side only, CH is completely reliable.



DHW always starts off hot and is ok for a quick shower but will not fill our very small bath - turns cold after around 50 litres (cold tap off throughout). Turn the water off, wait 5/10 mins and hot water is available again.



Thermal store set point is to max (problem is worse if set any lower) and as the thermal store current temperature depletes (from 75 degrees to 30 degrees in around 5 mins), so does DHW flow temp. Flow temp is always approx 8 degrees below thermal store current temp (kind of what I’d expect, allowing for some inefficiency in the plate exchanger).



Regardless of flow rate (normally set around 8l/min filling the bath), the boiler does not seem capable of DHW on demand - it all seems to revolve around the thermal store temp.



If the hot tap is left running indefinitely, DHW flow temp ends up around 15 degrees (mains in of 6 degrees at the moment).



Diverter valve motor appears to be operating correctly - most often sat in the DHW position when I have been messing around trying to diagnose things (appears to be in DHW position when thermal store is replenishing?). With no DHW demand, it moves to CH position when CH demand is initiated. With the motor removed and hot tap running, I have physically moved the lever towards CH position which causes DHW flow temp to drop rapidly to a level similar to mains in temp. This suggests to me that the valve is also operating correctly internally.



There does not appear to be any variation in this behaviour, other than the fact that it’s less bothersome in the summer due to higher mains in temperatures.



Boiler is always firing at the time when the DHW drops below a satisfactory temperature - presumably replenishing the thermal store (which does increase by 0.5 of a degree every 10 seconds or so, after the hot tap has been turned off).



The main gap in my understanding is how the DHW is heated once the thermal store is depleted. Does the secondary heat exchanger take over or am I reliant of having a thermal store temperature in excess of 50 degrees, in order to get hot water at the tap?





I’m perfectly competent with a multimeter if that helps with suggestions of tests which I can undertake.



We have underfloor heating so it would be difficult to detect whether CH was being heated as a byproduct of DHW demand.



Our firstborn is due in 5 weeks and in an effort to decrease tensions in the household, I’d love to get this working correctly! I do understand that I cannot expect scalding water at 20l/min, particularly in winter, however, 38 degrees at 8l/min should be easily achievable.



Many thanks in advance!

 

[Johntheo5]

If it was a storeless (gas fired only) Combi then 18kw will give a DHW flowrate of ( 18*860/60/(45-6), 6.62LPM, from mains at 6C to hot at 45C, a stored (all oil fired have stored primary water) ) Combi will give high flowrates but only until the store is exhausted, for example a 50L store at 75C will give 88L at 45C, say ~ 6minutes at 15LPM until the store runs out, I imagine the output should then return to 6.62LPM after that, the store should then be recharged when DHW demand ceases, so burner should keep firing for another ~ 8 minutes or so to recharge the store.

My only experience with any combi was a Grant which, I think, stored the HW at 80C, this was then reduced to 60C on leaving the boiler (for safety) via a TMV. I replaced the DHW pump in one for a neighbour but the attahed one has a diverter valve which is probably like your one?.

 

[Exedon]

Flow switch working correctly? Should doing burner in almost immediately there is a demand.

 

[Johntheo5]

If the flow switch is working correctly, should you get HW fairly rapidy even with a cold store?.

 

[JST]

John, thanks for the numbers, gives me more of an idea of how it should behave when working correctly.

Exedon, yes, flow switch was replaced a couple of years ago when the boiler was producing no DHW. Also, DHW flow rate displayed on boiler LCD does appear to be accurate, based on measurements taken with a jug and stopwatch.

 

[Johntheo5]

John, thanks for the numbers, gives me more of an idea of how it should behave when working correctly.

Exedon, yes, flow switch was replaced a couple of years ago when the boiler was producing no DHW. Also, DHW flow rate displayed on boiler LCD does appear to be accurate, based on measurements taken with a jug and stopwatch.

If you run the hot tap at say 5/6LPM from a cold start does the HW temperature still only rise to 15C?>

 

[Johntheo5]

Also, when recharging the store, does it run continuously or cycle on/off to get the store up to temperature?,

 

[JST]

John, when all thermal energy has been bled from the store, then yes, that is the case. I guess it’s ambient temperature and warmth within the boiler casing, rather than the water being heated to 15 degrees ish.

When the store temperature is down the boiler runs continuously and does heat the store back up reasonably quickly.

 

[Johntheo5]

So, when the DHW temp falls to say 35C, its minimum usable temperature, if the DHW flow was then reduced to 6.62LPM then both the store temperature and the the DHW temperature should gradually recover since the DHW demand is 6.62*60*(35-6)/860, 13.39kw, the remaining 4.61kw should slowly (and even more slowly as the store/DHW temp rises) recharge the store.
If the DHW demand (with a fully charged store) matches the boiler output from the very start then no problems?