Combi boiler - not modulating

[Richard Walker]

Hi all,

I've been lurking here for quite a while, but feel it's time to jump in!

I've been in this house two and a half years, and for the last year my Ravenheat RSF84ET boiler has been playing up. Now I know Ravenheat get a slating all over the 'net, but this was working great, and I cannot justify replacing the whole boiler when I'm not planning to hang around in this house for much longer. It was fitted a few years ago, by the previous owner (a spark who was training to be a plumber and gas fitter). The whole house was re-plumbed and has modern radiators.

The central heating works very well. About a year ago, the hot water started misbehaving. When opening a hot tap, the boiler senses, lights on a low flame, increases to a full flame, provides hot water for anything up to a minute, then clicks off. The flames vanish completely. After a moment, it re-lights and the cycle begins again.

Initially, I found that lowering the DHW potentiometer to about two thirds helped reduce chances of the problem occurring (although the water still arrived to the tap piping hot). This doesn't happen every time I use the DHW, but it does most times. Also, if the CH is on, it is much less likely to happen.

A recommended Corgi engineer tried to sell me a brand new boiler (1350ukp!) and finally decided that the gas valve wasn't modulating. He reckoned it must be the gas valve or the PCB, but wasn't sure which to replace!

I am 99% convinced by the gas valve not modulating theory, as when it malfunctions, the flames are either max or min (nothing in-between). However, I don't know if the PCB is telling the gas valve to switch off, or the gas valve modulator is busted.

I put a multimeter across the two modulator terminals on the gas valve (which connect to the PCB) and it reads anywhere between 0V (no flame),

3V (ignition flame) and 16V (full flame). Turning down the DHW potentiometer reduces the max voltage slightly. From my crude experimentation, the voltage seems to drop to 0, then the flames go out, so I'm starting to wonder if the gas valve is OK and the PCB is shot.

The same multimeter also shows me the resistances across the DHW and CH potentiometers changing when I turn them. And the resistances across the DHW and CH thermistors change as the pipes heat and cool.

Can anyone help with this? Does it sound as though I'm on the right lines? Is there any way I can definitely work out if it is the gas valve (which I'll have to get my Corgi to replace) or the PCB? (which I can do) Or am I barking up the wrong tree completely?!

Many thanks, Richard. (apologies if Google Groups mangles this, but my post to alt.test came out OK!)

[fred]

In article , Richard Walker writes

Sounds like you're going about the fault finding in a logical manner but you really need an installer manual, they're here:

should guide you through the process to narrow the fault down but the varying voltage on the modulator coil suggest that the pcb is doing its job. If it hasn't been looked at in a while then it may be needing a good service. Re: The flames are either max or min, do you know that it has continuously variable modulation? I've just looked at a Vokera where it was either min or max set by a solenoid coil on the gas valve and that was it, crude but it is modulation.

[Andy]

Sounds like you're doing OK Richard. I can't fault your approach. My Vaillant has an LCD diagnostic display, and one of the parameters you can display is the gas valve current. Current mind you, not voltage, but if the gas valve is essentially a resistive load then voltage and current should vary proportionately.

When setting up my Vaillant combi in the beginning I had to measure the gas pressures as modulated by the gas valve: I did this with a u-tube manometer, and when I opened a hot tap ( there was one right next to the boiler, which was convenient ) the gas pressure would shoot up to the maximum modulation level for a short time then come down in a series of steps as the modulator ( which presumably sensed water flow ) adjusted itself to the flow of hot water demanded. The valve current as displayed on the LCD varied in sympathy.

This was a fairly rapid process, only a few seconds to modulate down. You have determined that your gas valve operates, as it'll turn fully on or fully off. You have also not seen much in the way of modulation from the valve input voltage. I would have expected to see the input voltage to the gas valve come down in a series of steps, especially if you varied the water flow ( have you tried this? ).

So, your conclusion that its probably not the gas valve sounds correct. Presumably after a minute at full chat, with insufficient water flow to justify that heat output, either the DHW or the primary loop thermistor hit their limits and cause a safety lockout. My Vaillant limits the DHW to 63C max. It doesn't actually use the same thermistor to sense operating temperature(?), only whether it exceeds 63C.

Its difficult to suggest how you might test this easily. Of course, if you had the right kit you could unplug the gas valve input and operate it via a variable voltage but that'd be a bit hairy unless you removed it from the boiler. At the risk of getting the usual flak from safety first types, if a variable resistor in the 0 - 5 kOhm region ( bit of a guess that, based on your valve voltages and my gas valve currents ) were inserted inline with the gas valve input, you could modulate the gas valve yourself by turning a potentiometer ( configured as a variable resistor, i.e. using the central terminal and one end terminal ) during the one minute periods when it goes full on. Subject to not electrocuting yourself and not blowing anything up of course! If the gas valve modulates when you twiddle the pot, then it the pcb at fault, not the valve.

Fault-finding the pcb will be a thankless and impossible task unless something is very obviously dead. Check for loose connections, blown fuses, cracked solder joints ( they can be subtle ).

I would repeat your experiments with measuring the voltage to the gas valve, and varying the DHW flow. If it doesn't modulate over at least half-a-dozen steps then I guess its the pcb.

I suppose there is another possibility: when you demand full flow hot water with the temperature dial at max, I imagine the gas valve will go to max modulation at first. The question is, what method does the combi use to determine when it needs to modulate down? Do the thermistors sense the delta T between the rising main and the DHW and flag to the pcb when the required temperature rise has been met? I'm not sure that it does work this way, but my point is that if it did then the fault could be the inputs to the pcb being wrong. The thermistors are probably standard ones, from a range ( try YSI + thermistors in Google and you'll get a list that you can pick a likely match from ). That way, you can check out your thermistors resistance at cold ( probably 2200 Ohm ? ) and at hot and see if they're OK. It still sounds like the pcb though.

Bit of a ramble, sorry,

Andy.

[Richard Walker]

Hi Fred,

Thanks for your comments.

fred wrote:

fault finding section is particularly poor. For my fault, It only leads to 'check/replace DHW sensor'.

Yes, that's what I thought.

I don't think ever been looked at. :(

I'm terrified of 'a service' consisting of a wipe with a cloth, and a

100ukp bill.

According to the manual, it can and should modulate.

Cheers, Richard.

[Richard Walker]

Ah, yes. The Corgi engineer who had a look did something like this. When I first explained the problem, he said the valve min/max was probably set up incorrectly. After checking the valve and the documentation, he said they were incorrect, so adjusted them to where they should be. However, it still doesn't modulate correctly.

Yes. I tried the HW tap at various points between a trickle and a fire hose.

The voltage measured did change: if the tap was fully open, the voltage approx. 15 V, and if it was barely open it was more like 5V.

That sounds feasible. Just before it clicks off, something makes a bit of a 'puffing' noise - I think it's the fan going up a gear. Perhaps because it's getting a bit too hot? A few seconds after the noise starts, the flame is shut off completely.

Hmm... That does sound iffy!

Now that's interesting. Has anyone done anything like this before? I can't see it blowing up in my face, and it would only be a quick test. It's not like I'd leave it wired up that way!

I was thinking that if I can find a cheap replacement PCB, then I could always try it. Unfortunately, they are always rather expensive! :(

I did that before (probably didn't mention it in my post, though) and the voltage does vary as I would expect. The thing I couldn't work out is if the valve cuts out itself, or the PCB is telling it to cut out. I suppose your potentiometer theory will help with this...

Yes. However, If I unplug the two wires which go between the modulator and PCB, then open the hot tap, the flames light (low) but never rise to max.

There are thermistors sticking into the copper pipes for the HW output and CH flow which are cabled back to the PCB, so I assume it monitors them. Their resistance values appear to change as the pipes change in temperature.

Hmm. I'm edging towards trying to find a cheap replacement. In fact, they seem like trivial microcontroller systems, so why on earth can't we buy off-the-shelf generic ones which you program via a web page?! I suppose I'd better not start on that! :)

No worries. I've realised that my post also looks like a ramble! :)

Cheers, Richard.