Ariston 23 MFFI combi Hot Start Problem

[parky]

I've read through a lot of problems with the 23 MFFI but not found any like mine yet and wondered if anyone had suffered the same or knew what the problem is

Recently moved in and this 4year old combi was already there, and seemed to work fine, though noticed primary water pressure was near 0 (previous owner had neglected to top up). I have since added water to 1.5bar and bled the air from the rads.

A few weeks later, from cold, heating and DHW fire the boiler up perfectly, as soon as a tap is turned the burners are alight.

However, if there is any residual heat in the boiler (say after heating just switched off, or hot tap recently used), when a tap is then switched on (or heating requested), the pump and fan seem to start up straight away, but the burners do not light. Eventually they light but often up to a minute after the water has started running cold. Once alight, everything seems fine, water is the right temp etc.

The only thing I have changed is adding an external thermostat but that is working fine.

I have checked that the diverter valve is working, and it is definately striking the micro switch (as that is causing the pump and fan to start), so guessing problem is in the primary circuit somewhere (as this effects both heating and DHW)

What else is it checking before lighting the burners, that could be effected by residual heat. I have not checked the air pressure switch yet, but could not really see how that would be effected by heat???

 

[ArtfulBodger]

It sounds to me (not that I am an expert) that you may have a partially blocked secondary heat exchanger if the problem only occurs when calling for DHW.

 

[parky]

It does affect the central heating as well, but this is not so noticeable, or as much of an issue as the heating only switches in and out a couple of times during the evening on the room thermostat, and a one minute start up is not even noticed unless you happen to be stood by the boiler when the heating is requested, whereas the hot water is a pain, as you will get 30 seconds of hot water from the residual heat in the boiler followed but 60 seconds of cold water before the boiler fires up - hey presto, a cold sink of washing up water.

I only discovered it affected heating as well when trying to diagnose the problem and manually requested the heating with the boiler cold (fired up straight away) and boiler hot (took around a minute to fire up)

 

[ArtfulBodger]

You could always try setting the pump to overrun by setting the jumper on the PCB. The installation instructions will tell you how. This would help dissipate heat which might aleviate the symptoms but not cure the problem. Worth a go anyway.

 

[parky]

Thanks, I can see how that might help. I will give it a go (after I test the air pressure switch - just in case!!!!). Sadly even though I have the installation and servicing manuals for this boiler, they are pretty useless at the level of info they provide. But the jumpers might be marked on the PCB.

 

[ArtfulBodger]

For what it's worth I can give you the main causes of overheating in a combi:-

(1) Defective pump
(2) Scale or sludge build up
(3) Fan/pump overrun defective
(4) Temp sensors defective
(5) Bypass inadequate
(6) Overheat device defective
(7) Wiring fault
8 Excessive gas rate

Some of these are easy to eliminate, the most likely ones whould be, IMHO, 2, 3 or 4

Let us know how you got on.

 

[kevplumb]

or it could be anti cycling control

 

[PEDANTICVINDICTIVEMAN]

What about the spin cycle Kev

 

[kevplumb]

aristons dont have a spin cycle they condense the moisture (bottywarm)

 

[parky]

For what it's worth I can give you the main causes of overheating in a combi:-

(1) Defective pump
(2) Scale or sludge build up
(3) Fan/pump overrun defective
(4) Temp sensors defective
(5) Bypass inadequate
(6) Overheat device defective
(7) Wiring fault
8 Excessive gas rate

Some of these are easy to eliminate, the most likely ones whould be, IMHO, 2, 3 or 4

Let us know how you got on.

Thanks for all the responses

I don't think I am overheating. Its just the burners don't seem to fire up on demand when the boiler is hot. The overheating lamp is not coming on.

in order (based on artfulbodgers list)
1) I can feel the pump running on demand, so believe it is ok
2) Scale of Sludge, could be, had a service 12months ago, so could use another one, where would this be? is it common?
3) There seems to be no overun on the fan or pump, when a tap or room stat is switched off, everything shuts down straight away. Is this right? There are some jumpers on the LV PCB but they are not mentioned in any manual
4) Temp sens, if this is feeding the LED bar graph then it is prob OK as that does seem to relate quite well to the temp at the taps
5)By pass - no idea
6)I'm not getting any boiler overheat alarm or temp guage is not indicating overheat - so suspect I am ok there
7)Wiring fault (maybe - could manifest itself when hot - that will be easy to find!!!!!!!!!!)
8) Gas rate - I'm not messing with that, so lets hope not

There is a pot on the LV PCB for the anti cycling (manual reports it should be factory set to 1minute). What does this do?

I'm going to have a play this weekend, but just thinking about what the boiler needs to know before it lights up.

1. Demand (I'm getting that)
2. Fan running - Air pressure switch contact (Need to check that)
3. Pump running (it is running and there doesn't seem to be a check for that in the wiring diagram)
4. Primary water pressure (is ok and no alarm lamps)
5. Gas pressure (is ok, well it works)
6. Ignitors - they seem to work when finally asked to fireup
7. Overheat OK - the overheat alarm is off

Am i missing anything else - The only thing on this list I have not checked is the air pressure switch - but still cannot see why this would be affected by heat.

Anyway, I'll play this weekend and let you know, but I interested to learn more about fan/pump overun and the anti cycling

 

[kevplumb]

the anti cycling stops the boiler firing for a set time
hence the name

 

[ChrisR]

ANti cycling only operates on CH though. Never seen it on HW anyway.

Don't play withthe fan pressure switch - it wouldn't light at all if that wasn't OK. Or it would light and go out and light etc.
You need to find out what the overrun handling stuff is - fraid I don't know on this boiler. That would be fairly eaasy to check , but the tap water still shulddn't run cool.
Do you see the flames at 2 different levels - it has just hi-lo control. If the lo one isn't working that could cause your problem. It should be operating with the tap half on, for some of the tme.

If that is ok then the temp sensor is suspect, I would say. It's the sort of thing to check with Ariston, though they would ask for a corgi number.

 

[parky]

I was going to ask if anticycling was only on CH, otherwise it would mean always having to wait one minute between running taps. So unlikely the problem.

When running, the boiler seems to have two burner levels and you can hear it clicking in and out between the two, so that is OK

Is it likely to be the same temperature sensor that is used to drive the temp display (in my case a series of LEDs from 40 to 80 degrees) that will also be used to tell the boiler to switch on? As I said earlier, the temp display seems OK and all the LEDs extuinquish before the boiler will fire up again (after approx 1 minute).

So if the display is OK but still takes a while to decide whether to fire up could this be a PCB fault?

Also, this does appear a bit random, this morning the boiler came on within a few seconds of running cold (though guess running cold is still not correct operation)

 

[parky]

After a bit of fault finding, I put an electrical meter on the return from the air pressure switch, and found that when the boiler was delayed in starting it was due to delay in the air pressure switch in making contact. As soon as my meter indicated the the contact was made, the boiler fired up.

Which leads me to one of three options I guess
1. Faulty air pressure switch sticking open when warm.
2. Fan not running effectively so air pressure is actually low
3. Flue blockage

Haven't got any further than that as I could not get the cover off the combustion chamber where the fan and switch are located. I removed the 4 bolts as instructed but it still wasn't shifting. A new challenge

Out of the three options above which is more likely on a 5 year old ariston which has been serviced annually? Being a year since its last service

 

[ArtfulBodger]

1. Faulty air pressure switch sticking open when warm.
2. Fan not running effectively so air pressure is actually low
3. Flue blockage

(1) My favourite
(2) I would have thought that you would be able to detect a difference in noise if it was running differently when warm. Only way to really tell is to use a manometer, but unless the MI's tell you what it should be then what are you actually checking?
(3) Easy enough to check - go outside and look down it