Electronics > Repair

Heat Pump Water Temperature goes up at night

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zizebra:
Hi All

My heat pump has a strange problem which i am trying to figure out. I do not have ideas on how to approach the issue. Initially i thought the thermistor for the geyser was faulty so I purchased a new one. The old one simply showed water temperatures between 60 and 80 degrees. Occassionally the temperature would be ok and the heat pump would heat up water. Now with the new sensor, first day it was ok worked perfectly. Then the next day around 7pm, the temperature started creeping towards 60 when the actual water temp was around 40degrees. Switching the settings to use outgoing temperature solves the problem. Water is heated properly as per requirement. But if i use the incoming temperature, the issue resurfaces. The temperature goes up and heat pump wont heat the water as the temp will be above normal. Strangely enough, as soon day the starts when the outside warms up, the heat pump starts reading the temperature correctly and operates normally. Now iam trying to figure possible causes to determine if this is a minor issue  and where i can start and  diagnose or simply get a new board. 

Poroit:
G'day zizebra,

There are lots of variations with thermistor type and resistances.

Do you have any specs on the Thermistor and are you sure you replaced like for like?

zizebra:
Not sure how best to answer your question but I did determine that the thermistor was 10k NTC. how the observation which is worrying me is that both the old and new seemed to be unable to provide the right measure during the night starting after around 7pm up until the next day when the sun rises. This is what is baffling me as I thought the original thermistor had given up. Now it seems there is something else on the board that is probably acting up.

thephil:
The first thing I would do is verifying that the reading is really the sensors fault. I.e. measure the resistance with a multimeter and get the corresponding temperature from a reference table for your sensor. Repeat a few times during normal operation and times where the reading is off. That will tell you where to look further: sensor or control board.

zizebra:
Thank you. I will check on that this evening. My mind was stuck thinking it must be the board or something else.

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