Hardware hacking an electric fan heater without burning the house down

I'm assuming the switch on the heater is a 4-way 2 pole switch, similar to this more robust version: https://uk.farnell.com/schneider-electric/k1f013qch/rotary-switch-2-pole-5-6a-230v/dp/3108333

As such, I started looking at 4 way double pole power relays and I found that there are a few options available, such as this one:

https://uk.farnell.com/schneider-electric/rxm4ab2jd/relay-4pdt-250vac-6a/dp/2056435

Looking at the photo of the fan switch, it looks like there are two ratings one at 6A and the other at 15A. Now, if I went for a 4PDT relay rated at 15A, the price shoots up for that rated current and for the price of one 4PDT relay I could buy two fans.

So I am wondering if I could craft my own hybrid relay circuit using a suitably rated DPDT or SPST power relay, where one is for 15A and the other one is for 6A. This could saves quite a few bob but the circuit wiring will need thought as I wouldn't want a case of two relays turning on at the same time.

Any suggestions?

I've worked out a possible logic table for the fan and the two heater elements.

So I think 2 x SPST (NO) 16A relays (not sure if this could be combined somehow) and then 1 x SPST(NO) 6A relay would work in this context.

Would this be correct?

It's hard to say, I'm not sure I understand the existing wiring unfortunately : (

It might be two separate elements, but I've never taken apart a modern fan heater.

Heaters are quite high-power, so might need some care if modifying, since wires could get hot.

This is quite an advanced mod, not sure how it will be implemented, will the relays and circuitry be installed inside the heater? It could he hard to install everything in there.

It could be way easier to use a remote controlled mains socket adapter, but it may take some of the fun out of the project too, if the aim was to modify the heater product itself.

So, not really sure what to suggest : (

I am still unpacking and tearing it down myself to understand the wiring and the rest.

I've yet to sketch out a wiring diagram and it's quite hard to capture the key elements in photos.

It looks like two separate elements to me based on the wiring.

shabaz  wrote:

 

Heaters are quite high-power, so might need some care if modifying, since wires could get hot.

This is quite an advanced mod, not sure how it will be implemented, will the relays and circuitry be installed inside the heater? It could he hard to install everything in there. Will the relays and circuitry be installed inside the heater?

No the idea is to remove this top covering and then have the control hardware outside of the enclosure. Not sure just yet as to where to place. As you say, the wires can get quite hot.

It's actually quite neatly designed, in my opinion.

When I started reading you post I thought, why didn't I think of that. My initial response is how fail safe the mains using solid state components like triac verses relays. A logic device that monitors the environment controls the triac. The logic device gives me the advantage of creating doors to options that I can come back to and use when I need them. Unfortunately this type of design is not cost efficient and you wind up with project like a car block heater being controlled with a Raspberry Pi. It is cool but way to expensive.

"It is cool but way to expensive."

A common trait amongst many entrepreneurs is "naive optimism". They set off on a quest believing they can make a difference without knowing the full picture. Then when looking back, years later having done it, they realise that if they knew what they know now they would never have begun that journey in the first place.

In this case it's beginning to look like it's going to be pricey before the journey has even started. Still, I had forgotten about triac switches. So, thanks for that. It may just be the jolt I'm looking for to spark further ideas...

The part that has me perplexed is the mechanism behind the dial:

What is this component called and how does it work. It looks to me like a beefy potentiometer with an on-off switching mechanism, but not sure has cannot confirm via google (and how did I survive before googling).

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Hi Colin,

I was looking at your photos earlier, and think that's a bimetallic strip (it bends when it gets hot, and self-disconnects). the rotary control just adjusts the point it occurs by moving it closer or further away by a fraction.

These were popular in older thermostats (nowadays it will be a semiconductor or perhaps a thermistor, and an electronic circuit, but the older (but quite reliable) designs had the bimetallic strip.

Very interesting teardown.  Always fun figuring out these electromechanical gadgets.  You've almost inspired me to hack an old thermostatically controlled window fan but summer is here and I use it all the time .  Plus have too many other projects - never enough time.

I'll be watching to see what you come up with....   Have fun hacking.

Also meant to ask - what is the meaning of the symbol in the picture?

Thanks. Yes, not sure how far I'll get by July 14 for the Project 14 hardware hacking deadline as this project requires a cautious "spark free" approach. I believe that symbol indicates that you should not hang clothes over the fan heater.