Boiler Fan Control Board

[andrew_c]

Hey Guys,

My boiler has a fan control board which reduces the output voltage to get it's A efficiency rating. The board does this by flicking a relay and passing mains through what kinda looks like a capacitive voltage divider.

It would appear that the manufacturer designed this board to fail. I have a stack of these failed control boards thanks to my uncle (boiler engineer) - all with crapped out capacitors. I replaced some of these caps with Panasonic FC's but unfortunately, one of them has returned.. again with caps destroyed.

I'm happy to replace the caps again and send it back out but I was wondering if anyone had a recommendation to prevent this from reoccurring? The caps are 275V rated - could spikes be killing them?

Pins 2/3 appear come from the negative of the fan, 1 going back to ground.


Thanks,
Andy

[Seekonk]

I find it hard to believe that is the schematic with caps that size in parallel.  I could see the two 1uF in parallel and switching in the 47uF.  the 47uF would have a tolerance that would make the two other caps meaningless. How about a picture of this board to get a physical size of everything.  The weakest part of a cap is the welds.  I assume the failure is open. What voltage is this operating on?

[andrew_c]

My apologies, there is meant to be a point before the 47. So it's 0.47uF

Your wish is my command







Thanks, Andy

[tpowell1830]

You didn't mention the operating voltage. But, if it is 240VAC then the caps are highly underrated for the working voltage. The peaks of 240 VAC is 240 X sqrRt of 2 which is 339 volts p-p. The caps should be rated for 400VDC for safety.

[grumpydoc]

If the relay kicks in at the peak of the mains cycle then the caps are going to be put under a fair bit of strain, if the relay kicks in/out a lot it will eventually fail.

As tpowell1830 says they could probably do with being 400V rated

BTW is the PCB side picture the same board? it looks completely different.

[Paul Moir]

Just to be clear here, it was the X2 275VAC rated caps that failed (big square ones)?  And you replaced them with Panasonic FC (polarized electrolytic)?  I'm sure we've got a misunderstanding going on here.
From the schematic, the 1uFs are being a snubber while the .47uF is being the dropper.

[andrew_c]

Hi guys,

Some clarity the operating voltage is 240V.

The relay kicks in for around the first 10s every time there is a call for heat (so quite often).

The PCB is the same, just single side construction.

Again my mistake on the caps, they are actually Panasonic ECQU(L) series film capacitors I replaced them with. FC caps are on my mind from another project.

The board top left is a new design with the same cruddy capacitors.


When my own board first failed on me I simply replaced all the caps as a matter of course without even checking the old ones and it resolved the issue. I did the same for a number of my uncles boards (all like for like) which worked fine.

I had never even thought to check the peak voltage against the cap - I would like to the manufacturer was a little more competent.

I'm happy to hear this might be a pretty simple fix. Higher rated caps. Unless there is a better long-term solution?



Edit: to be fair, I've not checked which of the caps have failed. I would just assume all three at this time and replace.

[Paul Moir]

Also look for capacitors that are suited to "Motor Run" applications.  Some will mention "High current" in the datasheet.  The ECQL(L) series is geared towards EMI suppression rather than passing motor current loads.  I see no current handling spec in the datasheet.

I misread the schematic:  I see the 2 1uF capacitors are in series and the .47 is doing the suppression.

[Seekonk]

Those caps are perfectly fine for that voltage.  X2 caps are double caps, basically two caps in series. I wouldn't think twice about using them at 1,000V DC.  Current wise they are likely marginal and the welds are opening up.  A better solution might be to bridge another .47-1uf across them if you have the space. A X2 cap with long leads could fold over the board on one side and through the spare hole for the longer cap.  This would drop the current on the other two caps a little.  The motor wouldn't care.  Ideal would be to run two wires out to a 2uF oil cap designed for motors.  If it was my heater that is what I would do.

On the plus side you have a job for life.

All those caps are in parallel making a 2.5uF.  I guarantee you all those ECQL caps are at the very negative end of their uF tolerance, every ECQL cap I've ever tested was.  Hot melt another one of those on the end.  Trust me.

[andrew_c]

Just like to say thanks for all of the replies! You guys are great.

From what I have read on the subject of X2 capacitors they should be more than capacble of handling the peak-to-peak voltage of 230VAC.

The tolerance of these capacitors is around 20% which seems rather shocking! I actually found some spare new caps from the last replacement and tested them.

.47uF = .417 = 11.28%
1uf = .867 & .858 = 13.7%

So yeah definitely towards the low end.

Oil filled capacitors feel like a no-go. Maybe the next time my own board fails I might look into it - just a tad clunky.

Does anyone actually "understand" what is going on in the schematic above? Is anyone able to explain it to me - I really would love to learn.

I see two potential circuit paths.



In the scenario on the left I can only assume full AC is provided to the fan and that the rest of the circuit is left in limbo?

If my assumption is correct, how does the scenario on the right reduce the speed of the fan? Is it simply using capacitive resistance?

I tried calculating the capacitive reactance using the follwing XC=1/(2?fC) = 1326.96? but I guess since I don't know the current draw this is pretty useless?

Wouldn't adding more capacitance to the citcuit in parallel cause the motor to run slower?

Also, where does the 10K resistor come into this?

[tpowell1830]

Hi Andrew_c, the AC motor efficiency has to do with the phase angle of the current in relation to the phase angle of the voltage. If these are changing the speed of the fan motors, then my question would be what type of motor do you have on the fan? A reduction of voltage in an induction motor would cause a current spike and heat to build up and do nothing for efficiencies or speed.

As you can see from the above information, without a schematic and more info on the motor, it is difficult to tell you what this circuit is doing.  I would only guess that the phase angle is adjusted for a short period while the motor starts. 10 seconds sounds like too long for starting, is the time (10 seconds) something that is easily adjusted by the technician?

[andrew_c]

Hey, thanks for the reply.

This board is not used to start the motor, just attain a higher efficiency rating. When there is a demand for hot water (from a tap) this circuit provides the full 230V direct to the fan. When there is a call for heat (central heating), the board initially provides the full 230V to the fan, then after 10 seconds clicks over the relay reducing the voltage to around 150V (I might recheck this, along with the motor current).

Edit: When I say efficiency, I don't mean motor efficiency. Instead I mean boiler efficiency - the motor is not required to run at full speed so it reduces it to save electricity.

[Seekonk]

So, more caps and it runs slightly faster with lower current on each cap. 

It is interesting if you are not careful with this method.  I had some nice 230V muffin fans I wanted to use on 120V.  I added a cap in series getting near LC resonance.  The voltage on the motor increased to 195V, I wanted to run it a little slower anyway.  Don't think you are any where near resonance.

[Armadillo]

The board is a Power Factor Correction Circuit which is a legislation requirements as far as my country is concerned. This is to correct the power factor of AC inductance machine so that the energy meter from the grid can be read correctly [more]. There are formula to size the capacitors for PF correction but 275v cannot do the job.

[kaz911]

It would appear that the manufacturer designed this board to fail. I have a stack of these failed control boards thanks to my uncle (boiler engineer) - all with crapped out capacitors. I replaced some of these caps with Panasonic FC's but unfortunately, one of them has returned.. again with caps destroyed.

You can add Baxi Solo's to the list of Capacitor repair's you can do. I have done about 12 boards for my neighbours now. Baxi was kind enough to put in 2000 hour 85 deg semi-no-name capacitors on their boards. Temperature under the boiler where the board sits is actually close to 70 when the boiler operates. So lifetime for each board is about 3-5 years..... Designed to fail right after warranty runs out... Replaced them with some 5000 hour 105 deg capacitors from Panasonic and so far no returns. I was hoping for some 10000 hour 125 deg's but they wore not in stock at the time. But boiler models are now so old they will need replacement anyway soon.

[JFJ]

Does anyone actually "understand" what is going on in the schematic above?

If you simplify the diagram (by combining the paralleled capacitors into one, and ignoring the resistor), its function becomes apparent:



In one position, the relay bypasses the capacitor - applying the mains directly to the fan (for full speed). In the other position, the fan is supplied via the series capacitor (which reduces the motor's speed).

When your boiler is operating as an instantaneous water heater, the gas rate will be at the boiler's set maximum. The air supply also has to be at its maximum (full fan speed) to achieve complete combustion. When starting from cold in central heating mode, the gas rate is initially at the maximum (to heat the system up quickly), so the fan has to operate at full speed. Once the set water flow temperature is reached, the modulating gas valve reduces the gas rate to a level sufficient to maintain the flow temperature. At this reduced gas rate, less air is required for combustion. Hence, the fan's speed can be reduced. The benefit of reducing the flow rate through the flue is that it enables the boiler's heat exchanger recover more latent heat energy from the flue gases. Thereby, improving the boiler's energy efficiency rating.

Also, where does the 10K resistor come into this?

In so far as supplying power to the fan, the resistor contributes nothing. The resistor is bypassed in both relay switch positions (that's why I greyed it out in the diagram). The resistor in series with the capacitors does look like a snubber circuit. However, the values of the RC combination gives a time constant of 24.7ms, which is greater than the period of a mains cycle. It would not, therefore, be of much use as a snubber. The resistor is only, effectively, in the power circuit for a brief moment, as the relay switches over. Some tap changers on high power transformers switch in resistors, between taps, to reduce arcing. Perhaps, the resistor's function is something similar or, more likely, it's simply a bleed resistor - connected across the capacitor while the fan is at full speed.

The 275V rating of your class X2 capacitors is the AC rating (RMS, not peak), which should be adequate for use at UK mains voltage. So, for those capacitors to fail prematurely, they must be being exposed to high voltage transients, high temperatures or both. Excessive voltage transients might be generated if the relay is switching its inductive load too slowly or its contacts are chattering. That could be caused by failing electrolytic capacitors in relay coil driver circuit (the relay being energised with a high ripple current, instead of clean DC).

[Seekonk]

The resistor is indeed there to discharge the capacitor.  It would be better served if that resistor was around 22 ohms to quench arcing when the relay opened. 

[Seekonk]

I just happened to be looking at the data sheet a KEMET R46 series X2 capacitor.  This line just jumped out.

Intended for use in situations where capacitor
failure would not result in exposure to electric shock. Not for use
in "series with mains" type applications.

While I would like to take that at face value,  "series with mains" type applications sounds like voltage dropping power supplies where significant current passes through.  I don't see how this is any worse than putting a cap right across the line.  What good is a X2 cap if it has high esr or is likely to open. Could this be a hint to all X2 cap use?

[SeanB]

How about a MOV across the capacitors, to limit the voltage they have to withstand. A 275V MOV ( probably a 15mm one) will at least limit the amount of self heating, and preferably use a thermal fuse shrunk to the VDR to disconnect the power if the MOV goes short circuit. MOV and thermal fuse are mains rated devices.

[Seekonk]

Well, this could be the real story on why those boiler capacitors are failing after a while. It also debunks the myth that only X2 capacitors should be used in capacitive dropping supplies. I contacted James Lewis, KEMET, about why the data sheet not to use X2 capacitors in series with mains applications.  The following is his response:

"The actual issue is related to how typical X2 Capacitors lose capacitance over time. A typical film capacitor loses capacitance, over time, because of two effects: 1) corona and 2) corrosion. The voltage applied to most X2 capacitors tends to be in the 120-277 range. Corona occurs around 300 Vac and higher. So, while some capacitance might be lost because of corona effects, this isn’t the main concern. The real concern is that moisture will eventually egress into the package and oxidize the metallization. Some plastics, like polyester, more readily absorb moisture and accelerate this effect.

In typical X2 applications where the capacitor is across-the-line, the capacitance loss isn’t a concern. First, the capacitance value isn’t critical and second, most people don’t care if the end-product passes EMI tests 4-5 years into its life.

When used in a series-coupling or series supply application, however, the loss can be critical. Even a 10% loss in capacitance can result in not enough current for the supply.

This comes down to (and this applies across the industry), most X2s were not designed with “stable capacitance” in mind. So the R46, our most popular, can easily lose up to 30% of capacitance in a high humidity environment after only a couple of years. Which is why we made series with “better” epoxies, thicker metallization, and series-electrode constructions. Their capacitance loss will be much less than a typical X2, making them more suitable."