Johnson & Starley J50 MAF Series 2 - Advice

Jay Smith

Novice Member
Hi,

I’ve recently come across this forum and I can see a lot of knowledgable people contributing which is fantastic.

I have a Johnson & Starley J50 MAF Series 2 warm air system in my house which has been working absolutely great without any issues for many years. I have noticed this week that it doesn’t seem to heat up as much as it used to be even when on full power (thermostat turned all the way up) for hours.



Couple of things that I have noticed:



- when I turn up the thermostat in the morning I can hear/see main burner firing and working well (the fan is blowing strong as well) but after a while before it has reached the temperature I want the burner just goes off (back to pilot) and the fan continues to blow at a lower speed. It maintains that temperature throughout (~19/20 degrees) the whole day but it just doesn’t heat up more than that (and I know this system is capable of heating up the house to some ridiculous temperatures).



- I think that previously when I turned the thermostat knob clockwise I used to hear a quiet “click” once it goes over setting 4/5 (depending on the room temperature) but this clicking noise seemed to have disappeared now.



- I pulled the thermostat cover and measured the voltage across the two terminals when the the thermostat is set at lowest and highest setting - with a voltage readings of 8.8v and 4.4v respectively. The burner seemed to occasionally start when the readings is 4.4v but it acts sporadically. Could this be an indication of a faulty thermostat?



Can anyone explain what controls (what is the logic behind it) the main burner - I am just trying to understand it myself (hoping that it could be something simple)before calling a gas engineer to diagnose it…



Many thanks in advance,



Jay
 

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Gavtech

Administrator
It may be that the air filter is blocked causing the unit to go into overheat mode and shut down.

You can download the maintenance manual here which includes troubleshooting flow charts and checks that you can perform to aid diagnosis.
 

Jay Smith

Novice Member
Thank you for the advices above - really appreciate your input :)

A bit of an update on the issue:

  • I’ve bought and replaced the thermista stat just so that I can eliminate the easiest and the most common (from what I’ve been reading) source of issues. Unfortunately I am still experiencing the same issues.
  • I’ve sat with my phone next to the heater and timed exactly when the main burner turns ON/OFF. Initially I thought that it does is sporadically but in fact it follows an exact time pattern:
  • - ON for exactly 1 minute 15 seconds
  • - OFF for 2 minutes 45 seconds
  • - The fan seems to work during the ON/OFF cycles.
  • The manual and troubleshooting section talks about a high temperature rise across the heater that could trigger the overheat protection indeed. I’ve measure the temperature rise during the ON cycle and I have a temperature difference between 64 degrees at the beginning of the ON cycle and 84 degrees towards the end of the ON cycle (after just over a minute). The air filter is clean and all air vents are wide open. I have already increased the fan speed to maximum (through the balance screw) so I can’t really drop that temperature from there. The manual also talks about a return air path blockage - where is that/ how can I check it? If no restrictions would this indicate an issue with the gas pressure?

I think that I should call a gas engineer now but if in the meantime any of you had a similar issue or have any other suggestions I’d really appreciate your further advice :)


Many thanks,

Jay
 

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depot

Well-known Member
I don’t like the look of the two stains above where the burner goes in, I think you need to get a gas engineer around, but when you call one, which may not be easy at this time of year, you have to mention that it’s a modairflow wau, they tend to be rare appliances nowadays.

Do you have a carbon monoxide detector, always a good idea with wau’s
 

Jay Smith

Novice Member
I don’t like the look of the two stains above where the burner goes in, I think you need to get a gas engineer around, but when you call one, which may not be easy at this time of year, you have to mention that it’s a modairflow wau, they tend to be rare appliances nowadays.

Do you have a carbon monoxide detector, always a good idea with wau’s
Thanks, I’ll get someone to check/diagnose it (to be honest if I don’t turn if off during the night it does actually maintain a pleasant temperature but it just concerns me that it isn’t as powerful as I used to remember it from the previous winters).

I have a Carbon Monoxide detector yes - it sits where the heater - no issues so far :)

I was a bit concerned about these stains as well but after looking on the web it looks like this is quite common for these old warm air heaters (that area around the bottom is regularly heated up to ~120 degrees, I guess that the paint just gives up after a while)

I’ll post back once I get to the bottom of it (hopefully this year) :)
 

Jay Smith

Novice Member
Hi again,

I’ve made some calls and hopefully I’ll get a gas engineer to call me some time soon but you were right - it is really difficult to get hold of anyone at this time of the year who understands these old systems…

In the meantime I’ve taken some further measurements and I might have potentially found the issue (but not the cause): I measured the voltage at the fan terminals when the thermista-stat is turned all the up and the burner is going for that 1 minute loop I mentioned above and the most voltage I get to the fan is 156v (ac).

Could anyone confirm whether I should be seeing 210 V (ac) there at that point? If so that would probably explain the quick rise of temperature across the heater and despite that it feels that the fan is blowing OK through the vents maybe just isn’t enough to cool down the heat exchanger and it is triggering the overheat protection…

Considering that I’ve just replaced the thermista-stat with a brand new one would the next “suspect” be the electronics module?

Thanks,

Jay
 

Gavtech

Administrator
Do I correctly remember seeing something in the manual about disconnecting the yellow wire to determine if overheat was the problem?
 

Fergal82

Distinguished Member
Have you rung Johnson & Starley? Think their technical department consists of one guy sat on the phone taking calls so you might have to leave a message for him to call you back but they’re usually pretty good.
 

depot

Well-known Member
@Jay Smith if i remember correctly the fan speed is variable, so the voltage would be as well, @Fergal82 if it’s the same guy I spoke to many years ago, he is brilliant but nowadays I would expect that he has to ask for your gas safe ID number before he could help anyone, most technical helplines are like that now, probably for a liability reasons.
 
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depot

Well-known Member
British Gas have a fix for free offer online only (which is a pee take name) costs £25 per month for 12 months cover, might be worth considering, it’s must be best part of 10 years since I spoke to the Johnson and Starley tech guy ,( he’s very good) and he diagnosed a faulty pcb the part cost over £200 back then.

Another thought if he can’t help you, he might know of companies that do work on J&S wau’s
 

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