Baxi Solo 3: The £2 Microswitch That Cost Me £65

Reading Time: 8 minutes

Somewhere, etched into the very fabric of the universe, it says “your boiler will only fail if it’s subzero outside”.

Somewhere else it also says “if you start a blog about software development, your most popular articles will be about home maintenance”. Nevertheless, I figure I must be doing something right, so when my boiler went weird again it was time to put a colander on my head and pray once again that it was something simple and cheap.

This house has a Baxi Solo 3 (mains gas) system boiler. The advantage of these old system boilers is that they’re as simple as a boiler can be. You put cold water in one side, it does fire and contributes to global heating, you get hot water out of the other. All the rest is just making sure it doesn’t blow up or fill your house full of gas or emissions.

My boiler was being extra safe. It did fire up, but it also did an awful lot of sitting around doing nothing whilst the demand light beamed longingly at me.

The Solo 3 follows a pretty basic sequence:

  1. If there’s demand, it checks the thermistor on the output pipe.
  2. If the thermistor temperature is less than the configured position on the heat adjuster knob, it checks the air pressure (via the air pressure switch) in the flue.
  3. If the air pressure is good, it tries to start the fan.
  4. If the fan starts, it will cause a change in the air pressure in the flue. If the air pressure switch changes, it will try to operate the gas valve.
  5. If the gas vale operates and all else is well, it will fire the boiler.

If you’ve popped over to the Baxi website and have downloaded the manual this might surprise you, because it differs from the “Fault Finding” flow chart in section 11. The flow chart omits the fact that it checks the air pressure switch before it attempts to start the fan.

If any of the conditions change whilst it’s running, it shuts down the burner and returns to stage 1. This is quite normal with the thermistor, it heats the water until it hits the configured temperature, then it shuts down. The water in the system cools as it goes through the radiators, when it drops (significantly) below the configured temperate, the boiler starts again, moving to stage 2.

It’s also worth mentioning that there is an overheat detector which can also shut everything down. It has its own light on the left hand side of the control panel. That can be eliminated from enquiries very easily.

My boiler had demand for heat, but the fan wasn’t starting – or at least it wasn’t starting reliably.

The first thing to eliminate is the thermistor. The manual tells you how to test it with a multimeter, but that means removing the cover and you can get a good indication without doing that:

  1. Set the temperature control knob to minimum
  2. When the boiler does fire up, wait until it stops again
  3. Turn the temperature control knob up

If the boiler fires up again, it’s a relatively sure sign that the thermistor is working. It’s not 100%, but it’s a good indication. I was lucky, the test worked, so I could be fairly confident it wasn’t the thermistor.

I knew the fan could work, but was there a fault with it? The first thing I needed to know is if it was getting power. That did mean opening the boiler up.

At this point I have to mention two things.

  1. In the UK there are regulations about working on gas installations. There’s certain work can only be carried out by Registered Gas Installers.
  2. I’m not a Registered Gas Installer, but I am an trained and apprenticed electronic technician with experience in working on mains power control circuits.

I asked a friend of mine, who is a Registered Gas Installer, what his opinion on the regulations was. This is rather a tricky area, not only do opinions between Registered Gas Installers genuinely differ, they are also professionals and have to consider their professional responsibilities. There are Registered Gas Installers who will tell you that you can’t even take the outer cover off a boiler. I can see some justification for this; a little carelessness with a screwdriver can have fatal consequences.

In my friend’s opinion what I did complied with the regulations, because I didn’t change the installation or touch anything to do with the gas or control of the gas. Basically this is the disclaimer: if you’re following what I did from here, don’t blame me if you end up getting dragged in front of the beak and I’m certainly not accepting any responsibility if you gas yourself or anyone else or cause any explosions.

This is gas, it’s poisonous on the way in, poisonous on the way out and if you get it wrong it goes bang and can take out your whole house and a neighbour or too with it. Always err on the side of caution and if you’re unsure about anything call a Registered Gas Installer.

To test the fan I needed to take the covers off. The procedure is laid out quite clearly in the manual.

What the manual doesn’t make clear is that to remove the outer case you have to hitch it up a bit to get the rear flange out of two slots at either side of the top. Be very careful removing the outer case, it’s cheaply produced, there are gaps between folds and potentially sharp flanges that could snag and damage cables, etc.

The fan is obvious and easy enough to test with a multimeter. I made sure everything was clear and there were no trailing cables or exposed contacts that I was likely to touch or get caught up in, then turned the power back on – permanent live first then switched live. The demand light came on, but the fan wasn’t getting any volts. Since I knew it worked sometimes, that made it rather unlikely it was a fault with the fan itself.

I then turned the power off. It’s only my cats that are likely to get electrocuted, but that’s not the point, exposed mains connectors are a bad idea.

The next thing to consider was the air pressure switch. That’s obvious too. On its left, staring me in the face with 3 wires attached, was a microswitch. It had 3 terminals, meaning it was probably a SPDT type – a common terminal would be connected to one of the other two depending if the plunger was depressed.

The problem was that I didn’t know which terminals should be connected or not connected in either state. There’s a general stereotype for these, but you can never be 100% sure without either the manufacturer’s data sheet or testing it. Fortunately it had quite a lot of information printed on it and after digging through a few online archives of Chinese data sheets I was able to find the information I needed.

The common terminal is the one on its own, on the bottom of the switch (left in the picture above). Depending on the position of the plunger, it’s connected to one of the other terminals, referred to as “Normally Closed” (connected when the plunger is out) and “Normally Open” (connected when the plunger is pushed in).

There was another problem, however. Was the plunger pressed or not? Visually, it was difficult to tell. Nevertheless, it wouldn’t hurt to know what state the switch was in, so I checked the contacts. It was “NC”, normally closed.

This is where a little experience comes in useful. It’s a harsh environment and the switch was clearly old. Sometimes the contacts wear, sometimes the switch action gets a bit sticky. I gave the switch a few smart taps with the back of an electrical screwdriver and tested it again. It had changed to “NO”, normally open.

I popped the inner cover back on, turned the power back on and the boiler fired up right away. I repeated this experiment a few times and got exactly the same result every time.

It was probably the microswitch, but I couldn’t rule out the diaphragm, not without taking the whole air pressure switch apart. The most sensible thing to do was to order a complete new pressure switch and replace it.

There’s a note of caution here, the manual lists the part as 246054 but you’ll find that difficult to get hold of. Baxi has discontinued that part. I was able to find an updated part number from the Baxi web site and get it ordered from a major (and reputable) parts supplier. The manual has enough information in it to make replacing the air pressure switch a straightforward job. I don’t have anything to add, other than the fact that – on my boiler – someone had stuck the tray that contains the fan and the air pressure switch in place with a blob of silicone sealant so it was tricky to move.

I now have a properly working boiler and I certainly saved upwards of £60, maybe even over £100, that calling out a plumber would have cost.

It annoyed me though, with VAT and delivery the air pressure switch still cost me nearly £65 and I know the microswitch alone would have been less than £5. I hate unanswered questions. If I didn’t find out, it was going to nag at me. Could I really have fixed the boiler for less than £5?

The microswitch was held in by a plastic rivets. If you’re careful, you can gently heat an old pair of pliers and squash the rivet heads inwards, then extract the microswitch.

Um… yeah, that will have been the problem; the switch was so old and worn that when I got it separated from the mount the plunger simply fell out.

It seems that Toneluck don’t produce that exact microswitch any more, but you can get switches with the same form factor, similar operating force, the same temperature range and electrical ratings for about £2.50.

In conclusion, yes I could have fixed my boiler for less than £5. I’m not unhappy with the outcome, however. There are two reasons for this.

The first is simple, I now have a brand new, updated part in my boiler. I shouldn’t have to worry about the air pressure switch for some years to come.

The second is a little more subjective, it’s opportunity cost. Let’s assume that a Registered Gas Installer would correctly identify the problem and fix it for £100 in labour (plus £65 for the part). Instead, I spent a few hours (all in all) getting to the bottom of it, ordering the part and very carefully making the replacement, with lots of reading of manuals and double-checking because, you know, gas go boom. If I’d spent that time working, I would have made more than £100.

I could say that I did it because I enjoy the challenge and there’s a little bit of truth to that. The greater truth is that I did it because of instinct. I grew up in a world with very different economics, where spending an entire weekend saving a call out charge would be considered time well spent.

I’m very conscious that I came from that world: it actually means a lot to me. I’m also conscious that there are an awful lot of people still in that world and that, right now, that number is growing every day – as Jack Monroe’s now infamous twitter thread so adeptly explains:

Ed: Unfortunately Space Karen has fucked up twitter so, at the moment, I can’t like directly to Jack’s Tweet. So here’s a screenshot linked to the text. Please give Jack Monroe a follow, however. She’s always interesting.

Woke up this morning to the radio talking about the cost of living rising a
further 5%. It infuriates me the index that they use for this calculation, 
which grossly underestimates the real cost of inflation as it happens to 
people with the least. Allow me to briefly explain.

This time last year, the cheapest pasta in my local supermarket (one of the 
Big Four), was 29p for 500g. Today it's 70p. That's a 141% price increase 
as it hits the poorest and most vulnerable households.

This time last year, the cheapest rice at the same supermarket was 45p for a 
kilogram bag. Today it's GBP1 for 500g. That's a 344% price increase as it 
hits the poorest and most vulnerable households.

Baked beans: were 22p, now 32p. A 45% price increase year on year.

Canned spaghetti. Was 13p, now 35p. A price increase of 169%.

Bread. Was 45p, now 58p. A price increase of 29%.

Curry sauce. Was 30p, now 89p. A price increase of 196%.

A bag of small apples. Was 59p, now 89p (and the apples are even smaller!) A 
price increase of 51%.

Mushrooms were 59p for 400g. They're now 57p for 250g. A price increase of 
56%. (This practise, of making products smaller while keeping them the same 
price, is known in the retail industry as ‘shrinkflation' and its insidious 
as hell because it's harder to immediately spot.)

Peanut butter. Was 62p, now GBP1.50. A price increase of 142%.

These are just the ones that I know off the top of my head - there will be 
many many more examples! When I started writing my recipe blog ten years ago, 
I could feed myself and my son on GBP10 a week. (I'll find the original 
shopping list later and price it up for today's prices.)

The system by which we measure the impact of inflation is fundamentally 
flawed - it completely ignores the reality and the REAL price rises for 
people on minimum wages, zero hour contracts, food bank clients, and millions 
more.

But I guess when the vast majority of our media were privately educated and 
came from the same handful of elite universities, nobody thinks to actually 
check in with anyone out here in the world to see how we're doing. (Fucking 
terribly, thanks for asking.)

Every time there's a news bulletin on the rising cost of living, I hope 
that today might be the day that that some real journalism happens, and 
someone stops to consider those of us outside of the bubble. Maybe today 
might finally be that day.
(But seeing I've been banging on about this for a decade now, it's probably 
not going to be. Thanks for reading anyway, I appreciate it.)

And just to add:
 - an upmarket ready meal range was GBP7.50 ten years ago, and is still GBP7.50 
   today.
 - a high-end stores 'Dine In For Two For GBP10' has been GBP10 for as long as 
   I can remember.
 - my local supermarket had 400+ items in their value range, it's now 91 
   (and counting down)

The margins are always, always calculated to squeeze the belts of those who 
can least afford it, and massage the profits of those who have money to spare. 
And nothing demonstrates that inequality quite so starkly as tracking the 
prices of 'luxury' food vs 'actual essentials'. 

To return to the luxury ready meal example, if the price of that had risen at 
the same rate as the cheapest rice in the supermarket, that GBP7.50 lasagne 
would now cost GBP25.80.

Dine In For GBP10 would be GBP34.40.

We're either all in this together, or we aren't.

(Spoiler: we aren't)

Now, picture if you will, the demographic of the voter who has kept the 
current Party in power for the last 11 years. Imagine the Chancellor having 
to explain to them that their precious microwave dinner now cost almost four 
times what it did yesterday.

Yeah, didn't think so.
I mean of all the things, the Prime Minister claiming that he's cutting the 
cost of living while the price of basic food products shoot up by THREE 
HUNDRED AND FORTY FOUR PERCENT is the one I'm properly angry enough to 
riot over.

If my experience helps a few people not have to make the choice between heat and food, then all of my time, including the time it took me to write the article, was – without question – worth it.