Worcester 28CDi - not modulating?

[Rory]

My daughter had just bought her first house (despite my protestations that it's not the right time etc etc) and it has the above combi. The house is a 1969 built 3 bed "modern" (ie pretty small and poorly built) terrace. It has 7 single panel rads. I've had an initial stab at balancing the rads.

The boiler is struggling to heat the house although it does get there eventually. It appears to operating in either on or off - boiler fires and the flow pipe get almost instantly too hot to touch, runs for a few mins then shuts down again. It stays shutdown for a period (I think I read somewhere it won't restart for 3mins) and in that time the circulating water has cooled considerably.

Then it re-fires and the process repeats. Eventually the the whole system gets warm but I would have expected the boiler to run until that happens.

Either it isn't moduating, or the system can't dissipate the heat fast enough even with the burner on its minimum setting. Is there any way an amatuer can check if the boiler is modulating?

[geoff]

In message , Rory writes

There are two possibilities, water not getting out of the heat exchanger fast enough, and electronic - problem with the feedback circuit

Is she in a hard water area and is there a leak in the system requiring it to be re-pressurised on a regular basis ? Is the pump working properly?

When you say "amatuer(sic)", do you have a meter that you can test resistance and dc volts with ?

The sensor should be around 10k at 20degrees dropping down to about 3k at 70-80 degrees

Is the modulation coil open circuit?

What happens to the modulation coil supply from the pcb as the temperature rises?

[Rory]

Thanks - the water in the house is extremly soft and the system holds pressure perfectly (I've been checking as we had a couple of the rads off).

I could do the electrical tests (I used to be an electrician and have some electronics capability and I have a couple of DMMs) but I'm way out of trade and it's one thing doing stuff for yourself and quite another when it's someone else so I'd be loathe to delve inside the casing.

What I'd like to avoid is for her get someone out only to get the "they all do that, miss" response.

[geoff]

In message , Rory writes

Hardly what one would call rocker science

Measure the resistance of the modureg coil (- at the pcb connector, then you are measuring the whole circuit that the pcb sees) - it will either be finite (several kohms, I would expect or open circuit)

Measure the temperature sensor (- at the pcb) should be 10k going down to about 3k at temperature

monitor the dc volts to the modureg at the pcb

WHat else do you expect to be able to do? You need to make a couple of basic measurements

So, get your meter out then

[cynic]

If the water flow rate through the radiator circuit is too low the flow temperature will rise rapidly whilst the cooler return water flow will be too little to ensure the boiler has work to do. There should be a bypass route but more importantly the rads should be taking sufficient flow to load the boiler. Have you closed down each radiator too much in your attempts to balance the system or are there a number of seized TRVs in a closed or almost closed position? During this short cycling is the return temperature low or is it close to the flow temperature?

[Rory]

Hmmm...maybe. :) I was hoping for some easy way to tell if it was modulating or not. :)

[geoff]

Get a man in ...

[Rory]

the system or are there a number

I don't think so - eventually the rads all get stinking hot.

The return temp is low at first (cold even) and gradually warms up. But even once hot it drops noticeably when the boiler is off (as I mentioned earlier, I think it can't relight for 3mins even if it otherwise wanted to).

It seems to me that either is can't be modulating or the minimum burner pressure (it starts off at min and ramps up) is still too much for the system. I think what surprised me is how fast the flow gets hot - within a few seconds it's untouchable. Perhaps even the 9KW minimum output in more than enough for a little system?

[Pete Zahut]

I know absolutely nothing about boilers so I may well be making a prat of meself here, but have you got a wireless programmer/thermostat? If the batteries in ours get low the system goes into a sort of 'limp home' mode where it fires, runs for a few minutes then goes off and continues to cycle like that until the batteries are replaced.

[Rory]

Thanks - it does have a wireless programmable 'stat, but I'm confident that's working OK. In testing, there's a continuous call for heat (image of a flame in the 'stat window) and the boiler CH lamp is on all the time.

[Mathew Newton]

I think the 28CDi has a flame inspection window once you've removed the front cover (lift up and out jobby) - have a look through that to see what the flame is doing.

I seem to recall that when running for hot water it starts off high and modulates down, whereas when heating the radiators it starts low then ramps up - knowing this may help you spot the difference between high/low and stages in between.

Mathew

[newshound]

Honestly, the easy way is to put a voltmeter (I'd suggest an analogue one) on the solenoid which is part of the gas valve. I don't know Worcesters, but my Halstead had spade terminals, and I made up a pair of leads with the male/female connector on each so that it's very easy to tap on to each terminal and run the volts outside. I had a meter permanently set up outside the casing for a while when I was trying to locate an intermittent fault.

[Rory]

Yes - I've read through the installation manual this evening and you're right, there is window inside the cover.

That's right too - again, reading the manual, it stays low for 2mins (longer than I would have expected) on startup then ramps up over the next minute. On HW it starts on max and then within a few seconds modulates down.

[Mike Tomlinson]

In article , Rory writes

I'm watching this as I have the same boiler. I'd be thinking: 1) How many rads are there? 2) Where's the bypass rad? 3) is the pump working properly?

[Mike Tomlinson]

In article , Rory writes

Yes, I've got the wireless 'stat too. Initially I found that it wasn't causing the boiler to fire when the 'stat flame indicator came on, and the green CH light on the boiler control panel would flash.

Nowhere in the docs could I find out what this meant. Eventually I discovered that the signal from the 'stat was not reaching the boiler; moving it to another location (where it wasn't trying to get through two thick walls) did the trick and it's been fine since.

[brass monkey]

I also have the wireless stat (with a 30CDi). I'd like to know the firing algorithm. I really need to sit there with pen, paper and a watch. When the stat calls for heat, ok, I get heat soon after. Then a while later I notice the stat is still calling for heat but the boiler is off, then later it's on again. It looks as though the temp increases in steps. OR, I'm not running enough rads so the return flow is too warm maybe?

[Rory]

You can download the installation manual from the literature tab on the Worcester site and the sequence flowchart is in there (near the back of the intructions, so it's part 2 if they're split into 2 parts):

It looks as though the temp increases in steps.

Welcome to my world (well, my daughter's anyway).

[brass monkey]

Yes, what I'd expect is the boiler to reach its set point (control knob) and continue to pump that around until the stat is satisfied. I'm only running a few of the rads, maybe 6kW worth. The boiler only modulates down to around

7.7kW, consequently the return temp gets too high too early so it shuts down for a while then repeats. As I see it, if I was running say 15kW worth of rads, the return temp wouldn't get high enough to cause shut-down so it would wait for the stat.

I'm prolly completely wrong :)