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Old 25th Aug 2020, 7:35 pm   #1
chris.oates
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Default Miniature heating coil for Danfoss RHO controller

Miniature heating coil from obsolete Danfoss BHO 11-12 oil burner control box (see photos).

Does anyone know where I may get one of these, at low cost, either by salvaging from an alternative use or as an individual component? It is 230v and seems to have about 3k9 resistance cold, measured on a complete one in situ, it seems to operate in the 300-400C range based on my experiments with a soldering iron.

This failed on our oil boiler burner, the fan ran but the ignitor and oil supply failed to switch on. This is because this heating coil heats a bimetallic strip that toggles the various contacts. In the photo you can see the recess it sits in on the end of the switch block.

I have a supply of salvaged burners including their BHO controllers so probably have sufficient to last for many years but it is a challenge to fix this one. The exploded Rifa capacitor came from one of my working spares that only ran for a few minutes, I had the cover off at the time and the sparks were impressive, it took a 39v Zener out. I’ve replaced this now with a different type and this spare is working again.

The nearest I can find for the heater is a small PTC heater plate that gets to 270C so I’ve ordered one from China to see if it works, I thought I could mount it directly on the flat of the bimetallic strip if I make a shaped aluminium spacer.

I wondered if these small heaters were used in other household appliances as old as this controller. In case anyone recommends it I know I can get modern electronic replacements that will do the job but I’m not short of controllers would just like to fix this one.

Chris
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Old 25th Aug 2020, 7:39 pm   #2
McMurdo
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Default Re: Miniature heating coil for Danfoss RHO controller

as it's a safety device, the best bet is to replace the box. The heater is critical to the ignition sequence timing and lockout threshold. Not worth risking a boiler fire.
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Old 26th Aug 2020, 8:19 am   #3
trsomian
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Default Re: Miniature heating coil for Danfoss RHO controller

That all sounds very familiar. Ours failed at the start of the season a few years ago, capacitor expired, and as with yours took out the fusible resistor, the zener, and one transistor. I repaired it to get the boiler going, and then ordered a later style replacement, which is uC based rather than thermal I still have the old thermal unit as a spare should I need it. I can measure the resistance of the heater if you need me to
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Old 26th Aug 2020, 8:48 am   #4
trsomian
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Default Re: Miniature heating coil for Danfoss RHO controller

A further thought is that if the original heater is nominally of fixed resistance (I know it will always have a temperature coefficient but not like a PTC) the temperature versus time characteristic will be very different from that of a PTC; the PTC will get hotter quicker, then level off as its resistance increases
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Old 26th Aug 2020, 9:25 am   #5
McMurdo
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Default Re: Miniature heating coil for Danfoss RHO controller

Put it this way, if the heater is not correct it could potentially get hot enough to activate the ignition sequence but fail to get hot enough to trip the lockout mechanism if the boiler fails to light. Under fault conditions this could allow the boiler to pump oil into the combustion chamber for an indefinite period with the igniter sparking away.

Manufacturers of flame failure devices have this all worked out, which is why if the heater fails, it prevents either the ignitor or the oil valve from ever operating at all. The safety aspect does not take into account what might happen if the wrong heater is fitted, why should it!?
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Old 26th Aug 2020, 12:09 pm   #6
AC/HL
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Default Re: Miniature heating coil for Danfoss RHO controller

Quote:
Originally Posted by chris.oates View Post
In case anyone recommends it I know I can get modern electronic replacements that will do the job but I’m not short of controllers would just like to fix this one.
In view of the safety implications, and the fact that such equipment is off topic for the forum we feel that we can't continue with this thread.
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