Ravenheat RSF84ET DHW temperature stability

[Slicstudio]

I have a Ravenheat RSF84ET boiler which I installed about 3 years ago and whilst the CH side is fine, the DHW does not control as well as I would like. Incoming water temperature is about 15C at the moment and if I open a tap to a flow rate of 8 L/min then with the thermostat set to max the hot water temp settles at 51C and with the thermostat set half way the temp is 47C. So far so good. However if I set the thermostat to min then the gas valve does not modulate sufficiently to achieve a stable temperature before the gas is cut off. After a few seconds the burner relights and the controller tries again to stabilize and again fails. This results in a fluctuating water temp of 32C - 43C. If the flow rate is reduced to 4 L/min then no stable temp is possible and the temp ranges are 47C-59C, 41C-53C and 33C-43C respectively for the 3 thermostat setting.
If I check the voltage across the modulator coil I find that it starts at 15.5V at first and then gradually reduces to 7.3V and it sits there unable to go any lower. This corresponds to a burner pressure of about 4 mbar and this is not low enough to avoid overtemp. (Note that the pressure does not begin to fall below 8 mbar until the voltage is below 12V)
Additional investigation shows that the modulator can adjust the burner pressure from max pressure (8 mbar) down to min pressure (1.5 mbar) if the input voltage range is about 4 to 12 volts and if this was the case then I am sure that a stable water temp. would be possible over a range of water flow rates and a range of thermostat settings.

Does anyone know what the design spec. is for the modulator voltage output on the controller board ?

I suspect the controller but it's an expensive part to replace if it is not the source of the problem.
My next move might be to draw the circuit diagram of the controller circuit board and see if I can identify any faulty component which might be producing an output voltage which is too high.

Sorry about the long post but I felt that someone out there might find these observations useful.

 

[weareleeds]

Budget boiler should have got a better one

 

[Slicstudio]

Budget boiler should have got a better one

You obviously didn't find the info useful and hence the unhelpful response. Incidentally, I have a colleague whose boiler cost 4 times as much as mine and in less than 2 years he has had to pay more than £300 for a new controller board so price isn't everything. In the main I'm reasonably happy with my £300 quid boiler and I'm posting on the off chance that somebody might be able to answer my question.

Perhaps you should be more supportive of a Yorkshire company. Budget boiler 'yes' but very good value all the same.

 

[Rob]

Seen loads that dont modulate at low temp setting.

Try the NTC's.

 

[Slicstudio]

Seen loads that dont modulate at low temp setting.

Try the NTC's.

The NTC's seem OK but thanks for the input.
If you have seen a lot of these boilers with this sort of problem then perhaps I am heading for a redesign of the controller that they could all benefit from.

 

[Agile]

Have you done the obvious first check that any competent CORGI would do?

The modulation depends on the gas valve settings being correct. They are set up for a minimum and a maximum !

Not only are these setting beyond many CORGIs but virtually beyond all DIYers. Perhaps just as well as safety issues abound.

Tony

 

[Slicstudio]

Have you done the obvious first check that any competent CORGI would do?

The modulation depends on the gas valve settings being correct. They are set up for a minimum and a maximum !

Not only are these setting beyond many CORGIs but virtually beyond all DIYers. Perhaps just as well as safety issues abound.

Tony

Yes, they were the first things I checked and they are indeed correct, but thanks for the input.
I am interested in your comment about these settings being beyond many CORGIs and find it quite concerning if it is true.

Incidentally, I am not a CORGI, I am a 61 year old electronics and control engineer.

 

[Agile]

Yes, I have realised that otherwise I would not have given you advice on gas topics. I also come from an electronics background !

A comon cause of poor DHW regulation is a deposit on the NTC wet side. I know that it does not seem likely but even a small deposit will slow the response time. It must be clean and shiny.

Boiler designers work with new and clean components!

Another aspect is the gas supply. This must be stable and within 2 mB from min to max boiler demand.

Tony

 

[Slicstudio]

Yes, I have realised that otherwise I would not have given you advice on gas topics. I also come from an electronics background !

A comon cause of poor DHW regulation is a deposit on the NTC wet side. I know that it does not seem likely but even a small deposit will slow the response time. It must be clean and shiny.

Boiler designers work with new and clean components!

Another aspect is the gas supply. This must be stable and within 2 mB from min to max boiler demand.

Tony

Thanks for your continued interest Tony.
The gas supply pressure is stable because I have checked it but it is worth checking the NTC. If its response is slow then it would certainly effect the modulation of the gas pressure. I will check it out and come back with the results.

Ken

 

[ChrisR]

WOrth calling Ravers, they're often very good.
Worth removing and wiping the ntc, but you're likely to need a new washer.

 

[Slicstudio]

WOrth calling Ravers, they're often very good.
Worth removing and wiping the ntc, but you're likely to need a new washer.

A call to Ravers would probably answer my initial question but their technical help is only available to CORGIs. Competent non-CORGIs have to discover things the hard way. I was hoping that this Forum would make things a little easier.

I will try cleaning the NTC though ... might even buy a new one.

Ken

Took out the NTC and it was black in colour although there was no physical build up. 'Cleaned' it with a piece of very fine emery cloth and now it's bright and shiny. Put it back and tried again but alas no better but at least no worse. When I get chance I'll investigate the controller board because I still think it could do a better job with a different modulator output range (say 4-12V). I see that they have used more than one controller board on this model of boiler so perhaps they are just 'off the shelf' boards and they have been searching for the one that does the best job.

When they made mine they hadn't found it yet.

Ken