No heat, outdoor sensor reads 80-130 degrees (it's about 25 outside )

dhw_probs

I have a lochinvar cadet boiler and woke up to no heat.

When I checked on the unit, the problem seems to be that the outdoor temp sensor jumps around from 80 to 130 degrees, seemingly at random.

I think I know which side of my house the sensor wire goes to, but I have more than a foot of snow outside and don't really know what I'm looking for out there to find it.

I tried disconnecting the sensor to get heat temporarily until a tech might check it out. It gives an error about operating without a sensor, then the unit just fires up for a few minutes & shuts down with an anti-cycling error message.

Thanks for any help,

Big Ed_4

You need to reprogram the control to run without the sensor until you replace the sensor . Lochinvar tech support number 800-722-2101

dhw_probs

Thanks-- Support directed me to some wiring I didn't want to mess with, but I was very fortunate and soon after my service company had a cancellation & was just able to come 2 days earlier than expected. He disconnected to run w/o the sensor.

Odd though, the tech said the sensor reads fine in terms of the expected resistance Ohm value, and it appears the problem is with the connection board (not the main board, thankfully).

Just curious, on a a high 95% efficiency unit like this, how much efficiency is lost by running without an outdoor sensor? I'm sure the specifics depend on a few variables, just wondering about the ballpark-- 10%? 20%? Probably less when it's cold and would have been running close to 100% anyway.

Jamie Hall

The outdoor sensor does affect the efficiency, but indirectly. The efficiency of the boiler itself is determined -- beyond the construction of the boiler and the accuracy of the burner settings, of course -- almost entirely by the temperature of the returning water. The lower that is, the higher the efficiency will be, ranging from perhaps 85% with water much over 140 to as high as 96%, if the return water is below about 100 or so.

The outdoor sensor helps to keep the return water temperature low by keeping the boiler output water temperature low. There are, however, other ways to do that.

dhw_probs

@Jamie Hall

Thanks-- under normal circumstances my return water is almost always about 15 degrees lower than outlet. If outlet is 175, I'm getting return around 155 to 160. If Outlet is 160, I'm getting around 145.

Jamie Hall

dhw_probs said:

@Jamie Hall

Thanks-- under normal circumstances my return water is almost always about 15 degrees lower than outlet. If outlet is 175, I'm getting return around 155 to 160. If Outlet is 160, I'm getting around 145.

Just what you want. Design normally shoots for 20 degrees difference, but 15 is just fine.

So you can see that if the outdoor sensor reduces the outlet temperature through modulation, the return temperature will drop correspondingly -- and you efficiency will go up.

HomerJSmith

Sometimes the insulation on the ODR wire interferes with the conduction from the sensor to the control board. Check and make sure that the wire insulation isn't pinched in the connectors on the board or sensor. Of course the thermistor in the sensor may be bad.

A temperature-resistance chart measurement would verify the integrity of the sensor thermistor. I guess you are reading the temp off the display on the boiler.