How do I find one of these
NTC with a resistance of 15k at 25C Beta value ~4000

Hi
Check this link.
E
https://www.amazon.co.uk/s?k=NTC+with+a+resistance+of+15k+at+25C+Beta+value+~4000&crid=2F6R8F9T5J838&sprefix=ntc+with+a+resistance+of+15k+at+25c+beta+value+4000,aps,170&ref=nb_sb_noss

and

https://www.google.com/search?q=sup...ehAsIHCTAuMS4yLjAuMcgHLg&sclient=gws-wiz-serp

 

If I use a 10k resistor what would that give me.

 

If I use a 10k resistor what would that give me

Install a 10kΩ resistor and tell us what temperature the furnace indicates.

If it shows 25°C (77°F) we know that the replacement thermistor is also 10kΩ.

If it does not show 25°C (77°F), change the value of the resistor (with fixed or variable resistor) until it reads 25°C (77°F).
Then measure the value of the resistor.

 

I didnt have a variable resister but had a test box lets me dial up different set resistors.
Does this work if not Ill go buy a variable
4700 = 118F
6800 = 104F
10,000 = 82F
15,000 = 66F
33,000 = 36F

 

I didnt have a variable resister but had a test box lets me dial up different set resistors.
Does this work if not Ill go buy a variable
4700 = 118F
6800 = 104F
10,000 = 82F
15,000 = 66F
33,000 = 36F

That's perfect.
Give us some time and we will try to find you a replacement 12 kΩ thermistor.

 

Here are two from two different manufacturers:
https://www.digikey.ca/en/products/...aloric-bc-components/NTCLE100E3123JB0/2230729
https://www.digikey.ca/en/products/detail/epcos-tdk-electronics/B57871S0123F000/1284571

 

My math matches yours for 12kΩ B=3740K. These parts are 2-5% tolerance. I did see some 12kΩ thermistors used in HVAC.
Mouser search 12kΩ stock pretty much penny parts.

 

For liability and safety concerns, I would buy the part from the manufacture or replacement parts vendor.

 

Thanks Guys
Royyyyy not sure y that would be a concern. Its just to tell the boiler the temp outside. The manufacture wants $30 for it.

 

Thanks Guys
Royyyyy not sure y that would be a concern. Its just to tell the boiler the temp outside. The manufacture wants $30 for it.

but then you know that the part is tested. Can't boilers explode?

 

"For liability and safety concerns, I would buy the part from the manufacture or replacement parts vendor."

That's a bit ridiculous unless you are made of money.
You would pay $45 for the $4.50 thermistor? (Actually $0.20 if you can find MF52 or MF58 or MF11-123 but B=4050K, would not work). Point is, these really are penny parts and paying 10-100x their worth... hurts.

Even better, call up an HVAC company and watch the sleazy tradesman take it to two hours labour and maybe replace the control board as well, to make a nice $600 invoice.
I've seen them do this countless times.

The boiler's outdoor temperature sensor is not a safety issue. It changes boiler temperature a bit, nothing to cause an explosion lol. Even the boiler's main temperature sensor cannot cause a boom, there is a high-limit safety switch, a pressure relief valve etc.

The only hazard I have ever seen (with outdoor temp sensor) is the Warm Weather Shutdown WWSD with a falsely high reading temp sensor. A boiler controller can think it's summer, but during winter. Unoccupied building, danger of pipe freezing.

 

I agree with "P" in post #31! An additional first suggestion, quite a bit late, is to verify that the connections for the thermistor wiring are all correct. That is not always the case. At best, sensing the outdoor temperature may adjust the burner heat output. On my gas-fired furnace the gas control valve has two selections, one for the rated BTU heat output and the other for about 2/3 of the rated BTU heat output.