Vokera Mynute 20 locks out with code 20 or 79

[cjcoffin]

Woke up yesterday to find house cold and boiler locked out with code 20 (limit thermostat). Reset it and it ran OK for the rest of the day. Flow temp display shows 63C.

This morning the same thing happened but the fault code was 79 (flow temp or temp differential too high). Again reset it and OK so far.

Does this sound like a fault with the flow temp thermistor? Can anyone tell me what resistance reading I should get with a multimeter?

If that is what it is, they seem readily available (Vokera p/n 20081926) and I can probably replace it myself as it doesn't involve breaking any gas connections (I am a chartered engineer).

Grateful for any other suggestions!

TIA

 

[cjcoffin]

Would love to hear from a Vokera specialist, surely there is someone out there?

 

[ianmcd]

The boiler you have is a Mynute 20 VHE there are loads of Mynutes you have to make sure which model you have, it sounds like either the pump is sticking or you have a flow problem.bleed the highest radiator and see if the water flows free for a few mins if not you may have a blocked cold feed, dont over think about the Thermistor it is very unlikely to be that and the resistance changes depending on the temperature that is how it works so no point in giving you a resistance

 

[picasso]

Is your boiler in a garage ? I had the same boiler and the frost protection used to start the boiler but the bypass could not handle the flow and it would lock out.

 

[cjcoffin]

Is your boiler in a garage ? I had the same boiler and the frost protection used to start the boiler but the bypass could not handle the flow and it would lock out.

Thanks but it is in the bathroom. I will put a max/min thermometer there tonight but I doubt the frost protection is being activated.

The boiler you have is a Mynute 20 VHE there are loads of Mynutes you have to make sure which model you have

Yes it is a Mynute 20HE, installed in 2009. It has not required any attention since then, though this fault has occurred once or twice before. I have turned the weather compensation gain down from 2.5 to 2.0 and this seems to prevent the problem. The heat output is then not quite enough in extremely cold weather (as we are expecting this week(!)) but better than waking to a cold bedroom.

it sounds like either the pump is sticking or you have a flow problem.bleed the highest radiator and see if the water flows free for a few mins if not you may have a blocked cold feed, dont over think about the Thermistor it is very unlikely to be that and the resistance changes depending on the temperature that is how it works so no point in giving you a resistance

There is good flow to all the radiators in the house so I don't think the pump is suspect. It's a sealed system pressurised to 1.1 bar so I don't understand your comment about the cold feed. And I meant the room temperature resistance of the thermistors. Knowing that I can at least check for gross failure. Since however the two are the same part no. I can always check they have the same resistance when cold, also I can swap them over to see if that makes a difference.

Am reluctant to call a techician out when it is an intermittent fault in case there is no fault found or he changes parts which make no difference (I used to be a consultant to Baxi and a common "fix" tried in the field was apparently to change the pcb but when re-tested in the factory most of them had no faults at all).

 

[NBarnes]

Woke up yesterday to find house cold and boiler locked out with code 20 (limit thermostat). Reset it and it ran OK for the rest of the day. Flow temp display shows 63C.

This morning the same thing happened but the fault code was 79 (flow temp or temp differential too high). Again reset it and OK so far.

Does this sound like a fault with the flow temp thermistor? Can anyone tell me what resistance reading I should get with a multimeter?

If that is what it is, they seem readily available (Vokera p/n 20081926) and I can probably replace it myself as it doesn't involve breaking any gas connections (I am a chartered engineer).

Grateful for any other suggestions!

TIA

Did you get this sorted out? I have the same boiler and the same problem and I'm just thinking of replacing the thermistors.