"Vintage" Worcester 240RSF, DHW cycles hot/lukewar

ChrisOxford · 17 Aug 2014

Hello,

My 240RSF has been misbehaving for the past two days. DHW starts hot, then goes rather lukewarm. Sometimes it gets hot again, sometimes not. I did notice the rads getting slightly warm too while the taps were running (as opposed to afterwards because of the pump overrun).

I presume the diverter valve's playing up. Am I right? It had a new synchron motor about 15 years ago, but apart from that it's still the original, 20-year old one.

It's P/N 87161424210, like this one: http://www.spareboilerparts.co.uk/cgi-bin/trolleyed_public.cgi?action=showprod_WOR87161424210

Should I just change it, or can it be repaired (like the ones in the newer 24cdi etc.)?

I know my boiler's living in borrowed time, but it's reliable (a bit like Trigger's broom!) and I'd like to stick with it until parts are NLA or it breaks down too often.

Cheers,

Chris.

Agile · 17 Aug 2014

I don't remember seeing a 240 with that type of diverter valve.

But regardless, your main fault would not normally be caused by a diverter valve.

Tony

ChrisOxford · 17 Aug 2014

Nice to hear from you again, Tony.

That's definitely the type of diverter fitted to mine, which I think is quite an early model (literature dated 1993, installed 1994, model apparently discontinued 1998). It's described in the parts list as "special"

So you've told me that's not the fault, much appreciated. My gut feeling was that if the rads are heating while the CH is switched off but there is demand for DHW, then the diverter is surely suspect. Any suggestions as to where I should look then, please, anyone?

Everything was working 100% fine until yesterday and there are no other obviou signs of anything being wrong (system pressure fine, EV OK, no overheating, etc.).

C.

Agile · 17 Aug 2014

The diverter valve is obviously leaking slightly.

But that would not be causing the how ater to go cold.

A small amount of leakage in the diverter valve is quite common and as long as its slight the only effect is a small reduction in the amount of heat to provide hot water.

Tony

ChrisOxford · 17 Aug 2014

Thanks Tony.

It's definitely not going cold, just a bit less hot than it should be. And I've never noticed the bathroom rad being a bit warm the second I switch off the shower, which it was this morning.

DHW sensor tests ok with an ohmeter.

Do you suspect the Modureg/gas valve, in which case it's obviously time for a GasSafeRegister professional?

C.

Justin Passing · 17 Aug 2014

That diverter is the type the 280RSF used.
You may have some sort of Friday afternoon job

.

Do you suspect the Modureg/gas valve

Unlikely.
You could try turning off all the rad valves then turn the water on to see if some heat was being lost.
(You COULD turn off one of the isolation valves under the boiler for the same test, but they can weep afterwards.)

And you could turn water taps on full and measure the gas rate (how much gas is being used). Though if does measure low, you won't know why.

Unless you have the graph for the NTC thermistor, you can't be sure about that. You could replace it temporarily with a 10k resistor so the boiler thinks it's cold. Or a new one of course.

ChrisOxford · 17 Aug 2014

Thanks, Justin (nice to meet someone other than the infamous Justin Time!).

Friday afternoon job it may be, but it's outlasted almost every other household appliance we bought when we moved here 20 years ago, as well as most of my friends' and relatives' boilers.

I will try isolating the rads as you suggest. I make a point of "exercising" the iso valves a couple of times a year, so although they weep slightly for a few seconds, they still seal perfectly afterwards (for the time being, anyway).

I will also check the gas rate as you suggest.

I have a new NTC DHW thermistor in my "stores" and the one fitted and the spare one agree with one another quite accurately.

Thanks again for your time,

C.

EDIT: Just realised I've been an idiot - it IS a 280RSF as you suggest. Sorry to both of you.

Agile · 17 Aug 2014

Can you set the hot tap to something close to the 11 li/min which the boiler should be able to heat to about 50 C.

Assuming your boiler has a DHW temp knob, then after leaving it to stabilise for a few minutes then what effect does turning the knob a little to either side on :-

a. Flame height?

b. Water temp?

Justin, which 240/280 models used that motor diverter valve compared with the usual Gianoni diverter valve I normally see ?

Tony

ChrisOxford · 17 Aug 2014

Tony,

It is in fact a 280RSF as you suspected all along. Sorry for being thick.

There is no user adjustment for DHW temp on this model. The MI says: "The maximum discharge temperature of hot water to the taps is preset at the factory".

They also state that the DHW output is modulated from 10.5 to 28.0kW, with a gas rate of 1.38 to 3.25 m3/h. Max flow rate is 10l/min.

C.

Justin Passing · 17 Aug 2014

All the 240rsfs were the giannoni with the leaking trombone section and all the 280's were like chrisoxford's, as far as I know.

Chris - measure that gas rate to see if it matches the rating plate +/- 10%.
You could if you wanted measure the voltage on the modulation coil.
I don't know what it is, offhand, but it should change as the BOILER water heats up, even if the tap water doesn't affect it.

I don't know if the tap water temperature modulates the gas valve. It may not. As the heat capacity of the calorifier is large, the gas may just go on and off as the tap water gets to temperature.

It's possible the main heat exchanger or the hot water calorifier (secondary heat exchanger) is partially blocked. They tend not to get blocked on those earlier types, but flakes of oxides can build up.

Agile · 17 Aug 2014

The usual operating voltage on the mod coils is about 6v for minimum and about 15 for maximum.

BUT, on some of the early boilers there was not a full range on modulation and they just cycled between minimum and maximum ( and off ).

Tony

ChrisOxford · 13 Sep 2014

OK, more progress with this.

It turns out that the burner is being shut down completely after a minute or so, which the MI says happens if the primary circuit temperature exceeds 90degC. It comes back on half a minute later, then shuts down again and so on and so on, hence my shower going a bit chilly.

There's also a kettling noise and occasionally, the overheat stat trips and has to be reset manually.

The MI says that in DHW mode, the burner should light at maximum output (with burner pressure of about 14mBar), then should decrease to meet the actual demand.

When it was working properly a few years ago, I did some measurements and found that there was 21v across the modureg coil with a hot tap fully open, which decreased to 9v when the tap was open just enough to trigger demand. Now, it sometimes drops to somewhere near 9v, but usually the burner usually cuts out before it has a chance to do so. The modureg spindle is free to move and does what it should, i.e. if you push it in, the burner flame shrinks in size.

What's everyone thinking? Poor primary circulation? Scaled gas-water HE? I've got a new one lying around (as you do), so is it worth fitting it?

Thanks.

C.

Justin Passing · 13 Sep 2014

As you lied

about it being a 240 we weren't suspecting the secondary (water to water)heat exchanger because it was a different type.

The 280 has a flat plate type though (check) which blocks like they all do. You can take those out to clean them.
You could try running it first with a dose of Sentinel X800 or similar. Put it into the boiler and keep it strong in the boiler by running hot water only at first. That would help the main heat exchanger as well.

No, you can't change the main heat exchanger, you'd need to be Gas Safe for that.

ChrisOxford · 13 Sep 2014

Fantastic, will nip down to Screwfix and get some X800 and report back.

BTW, I changed the plate HE 3 years ago - converted it from the old-style white enameled cannister thing. A pig of a job that I don't want to repeat if possible.

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"Vintage" Worcester 240RSF, DHW cycles hot/lukewar

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