Quartz and Digistat problems WB 35CDi Mk1

[gwiseman]

I've got two problems with my boiiler:

1) Shower goes from hot to cold and back again rather than staying stable (argggggggggh). This happens intermittently. (Everything was previously OK for years; the Quartz processor is the correct one).

2) Wireless digistat thermostat and receiver board on boiler randomly losing synch - the red 'signal' light illuminates steady to indicate loss of comms link atfer a seemingly random period of time.

Re (1) - I have dialed the DHW temp to min, and this (usually) fixes the problem... but what's causing it, and can I fix it permanently so the Quartz processor doesn't do this oscillation game ? Sometimes a shower will be fine, and sometimes it won't. The wife ain't too impressed, I can tell you ! (We have two Quartz units - they are both doing this - and I doubt they have failed at the same time. I have tested the DHW temps off the boiler when the knob is on min and max and (at least when I tested) I got the correct 50 and 60 degrees at both 1/4 and full flow).

A WB engineer replaced the air switch, PRV and EV and this seemed to fix the problem for a few months, but as it has occured again I am not sure this actually cured things...

I replaced the PCB prior to calling WB out (I thought it was causing the faults). Erm... it wasn't.

Re (2) The boiler can be fine four hours, heating the house nicely, and then (seemingly for no reason) the heating will cease responding to the 'stats heating demand. Often we go to bed cosy but wake up cold... Can you tell I'm getting an earful ?

(I have to hit '-' or '+' to attempt to 'wake up' the system when this happens, or else power down the boiler or else remove the battery trays and do a 'Learn' operation etc. etc.). I have inserted new batteries to no avail. This occurs whether the transmitter is 1 or 5 metres from the boiler. They are correctly paired.

Dazed and confused. Any ideas, guys ?

 

[Nickthedentist]

As for the digistat losing synch, have you or your immediate neighbours recently acquired any RF/wireless home gadgets e.g. DECT phone, wireless modem/router etc.?

Nick.

 

[gwiseman]

Hi Nick,

Thanks for the reply. Funnily enough....

I came back from holiday and I noticed two new wireless routers show up on my Mac... good for free Internet access in emergencies etc... but are you saying BAD for digistats ?

I thought they were playing in different radio frequency spaces ?

But you might be onto something because when I put the digistat very close to the boiler (say 1 - 2m away) things seem to respond more or less perfectly, but when the transmitter is further away (say 4 - 5m) I lose the integrity of the link.

Is this a well-known problem or just an interesting hypothesis ?

 

[breezer]

i would say its a well known problem.

although its on a differnt frequency it will be "blinding" the receiver

 

[gwiseman]

Thanks breezer - this fault was beginning to drive me nuts.

I notice that the newer (I think post 2000 / 2001) Drayton Digistats operate on 433MHz instead of 418. Possibly they suffer less from RF interference ?

Before I go out and buy a wired programmer, I am thinking of fudging a long length of insulated wire onto the SCR receiver's little aerial terminal to improve reception (it's only about 15cm long, in a square shape around the receiver board... genius or madman ?

 

[radio_engineer]

Hi

Antennas are one thing I know about...

If you simply add piece of wire to your existing antenna you may actually reduce it's performance. This is because each antenna has to oscillate at correct frequency in order to transmit or receive the radio wave, the correct electrical lenght of antenna is therefore crucial for correct operation. Term used is main or centre oscillating frequency.

In practise however addition of length of wire to antenna may also improve the performance, but you may not know the outcome unless you can measure or at least calculate correct length. So I would advise you to concentrate on keeping the units closer, remove any source of interference like WiFi point, cordless phone etc, or shield your units from interference source, by for example making sure there is solid wall between your Wifi router and units.

If you still need better antenna then add the wire but be prepared to experiment by reducing the size. First calculate wavelenght by dividing operating frequency in Mhz / speed of light Km (300.000 Km/s). result is in meters. 418/300 000. You would need 1/4 or 1/2 of calculated result as you total length of antenna including any existing cable.

 

[gwiseman]

Hi radio_eng,

Not sure that works though as:

418/300000 = 0.00139 (metres you say)

X 100 to get into cm = 0.139cm

which is a seriously short length of wire, LOL, especially if I divide it down further !

What am I doing wrong ?

 

[radio_engineer]

hi,

sorry my mistake it should read:

speed of light (km)/ fequency HZ so 300,000 Km / 418 000 = .718 m or 71cm so your antenna would be 1/2 or 1/4 of this figure.

some antenna can be using the entire wavelenght or in this case .718 m

It's been while since I had to do this calc, well it's alway good to unstuck the brains!

 

[gwiseman]

fab. Will attempt kludge tomorrow. Thanks for de-rusting the old grey cells !

 

[gwiseman]

Darn.

Either the stat or receiver have malfunctioned as they will no longer pair up. Possibly my helpful circuit prodding didn't help...

I suppose the interference could be sooo overwhelming they can't hear each other, but I doubt this as I've tried to pair them a dozen times or so. I think whatever may have been iffy is now brok.

Question is - would a new (Worcester Bosch / Drayton) wireless stat / receiver suffer exactly the same interference problem because of increased congestion in the band, or was that all a red herring and a new one would likely work just fine ?

I could get WB out to do a 'three components free' repair (about £170 IIRC), and they could maybe address the strange gas modulation I am experiencing which I think is behind the shower hot / cold thingy too... or else leave the DHW temp on minimum, buy a Honeywell CM67-RF-NG wireless stat and forget the Digistat altogether... hmmm....

Any comments anyone ?

 

[gwiseman]

Just reading up on Wifi networks... they operate (usually) in the 2.4GHz range. I don't know much about radio interference but would have thought that wouldn't have too much affect on a Digistat operating at 418MHz, right ? DECT phones, microwave ovens etc. yes... but wireless routers ????

 

[radio_engineer]

Hi,

Radio interference sources are huge in numbers from malfunctionig power supply unit on PC (one of the worst) to all other similar device. Any electrical component may, under some circumstance, create radio interference, hence us radio engineers make living designing protection systems etc. I am not specialist on this type of interference ( I design mobile phone networks) but I can say for certain that there are more sources of RF interference in modern home than ever before. From brushes on your washing machine motor to your Wi-fi. reduction of interference is costly business as it takes time and sophisticated test kit/labs to remove during product design stage so cheap radio gear (such as Chinese made WI-Fi ) could interfere with other devices if not functionig properly or not installed correctly. Even during "correct" operation devices transmit outside intended frequency range (it's simple law of physics but wont go there) and it is for designer and compliance body/test house to determine if this is acceptable. ( FCC regs in USA or CE mark in UK for example)

Have you ever heard your car radio or your desk phone handset make noise from your mobile phone you have placed nearby? These systems work in completely different frequency range but they can interfere under specific conditions. Both devices probably operate within compliance limits but they interfere with each other. Your temp cont units are basic radio receivers and transmitters (mobile phone/car radio example) with some added functionality so there there is no doubt interference is possible but not certain.

I suggest you try to turn off as much gear as you can in your house and see if this helps. make sure you turn power off rather than put them on stand by mode. This includes phone chargers you may have plugged in. This wont cost you and may point you in correct direction



It is impossible to say if you have radio interference problem or not, but it can be caused by other devices and Wi-fi.

 

[gwiseman]

Hiya, and thanks again for this valuable info. Yes, I have heard (and seen) mobile phones do *all* sorts of things to nearby devices, so I can see how my Digistat could be affected by all kinds of nasties in the modern home.

Really, though, I can't leave my DECT / mobile phones off and have no wifi (though I'll turn everything off to do some more testing !) so I guess I'll have to go with either a hard-wired thermostat (digital or mechanical) or else get a Honeywell CM67RF which operates in 830 / 838 (?) MHz but instead of just sending a signal on heat demand / heat satisfied (and an 'I'm still alive' every five minutes) it sends a data burst (with information about load too !) much more regularly (every 10 seconds or thereabouts) so ends up being more reliable IYSWIM.

 

[radio_engineer]

if you could identify source of interference then it's easy to find solution. you need to turn things off/on to identify culprit, if in your house. It's likely to be device always plugged on.

Another problem you may have is that your units are to far apart, or there is to much clutter between. Also depends how these units are powered as interference can get to them even via "dirty" power supply they use. If you have device emitting interference plugged in the same mains power than interference may go down 230V wiring and even affect several houses if on the same main phase. If they use battery then check battery is ok. Another common one to confirm is to make sure your PC is properly closed, there are metal fins on the edge of outer case. These are there to make electrical contact and create enclosed Radio space so any radio interference is contained within. If case is not closed then it is emitted from your PC or similar devices.


Let us know outcome

 

[gwiseman]

Things got worse when the Digistat receiver failed to pair with the transmitter, and as each component costs about £100 each, I got WB in. I figured the system had worked for years without fault so I didn't think the failure was due to RF interference... turns out both the transmitter AND the receiver had failed. Can you believe it ?

The Quartz processors are still doing hot / cold / hot / cold on anything but the lowest DHW temp, but the boiler will correctly modulate output to any temperature at the kitchen sink, so I think an Aqualisa swap out will be needed on the Quartz units. Again, weirdly, two of them seem to have failed at once. Bizarre.

BTW - for anyone searching archives, the Digistat the engineer left with me says 'Digistat Optimiser' (the old one said 'Digistat CD') - and was boxed with the 'new style' receiver (large white plastic blob, tap icon in blue in the middle). It paired perfectly with an 'old style' SCR 'large receiver' (greyish, two LEDs, override / learn).