Secondary heat exchanger - flow/return temperatures

[Jennie]

Hi all,

I'm a Gas Engineer, and new to fault diagnosis/repairs. I've measured the temperature on the flow/return pipes across my combi's secondary heat exchanger, with the hot bath tap on full. I have a Vaillant Ecomax 24 (year 2003). The flow was 70c, the return was 43.8c. I'd be interested to know what a good temperature is on the return pipe. No doubt I'll come across a few blocked heat exchangers in my future work. I presume if the return is too hot, that shows the heat transfer to the mains water hasn't taken place - hence a blocked heat exchanger. So ideally, the return temp should be as low as possible. Am I on the right track? Thanks for your advice, Jennie.

 

[joni os]

Temp. on return will never be lower than the water it's heating.Larger the difference in flow and return greater the heat transfer.

 

[Last Plumber]

Hi all,

I'm a Gas Engineer, and new to fault diagnosis/repairs. I've measured the temperature on the flow/return pipes across my combi's secondary heat exchanger, with the hot bath tap on full. I have a Vaillant Ecomax 24 (year 2003). The flow was 70c, the return was 43.8c. I'd be interested to know what a good temperature is on the return pipe. No doubt I'll come across a few blocked heat exchangers in my future work. I presume if the return is too hot, that shows the heat transfer to the mains water hasn't taken place - hence a blocked heat exchanger. So ideally, the return temp should be as low as possible. Am I on the right track? Thanks for your advice, Jennie.

There are a few if's and but's there Jennie.

There will be differences on the primary return temp ( from PHE) dependant upon the flow rate through it on either primary or secondary side of it. This is because it's all about heat transfer as you know.

For example, if you had a high flow rate through the secondary side of the Plate Heat Exchanger, of water at 5 degrees C, you would see a difference between the primary return from PHE than the temperature that a low flow rate of water at say 15 degrees C might give you.
However the boiler would/should modulate to suit so the F+R Temp diff would most likely alter in line with it. To what degree is down to a few different variables.

There are also differences in the size ( internal surface area ) of heat exchangers and this too will give slightly different transfer rates on different boilers.

If the Primary side was partially blocked, you would normally find that the heat from the primary side is lost more into the secondary due to the restriction on flow through the primary side. So I would expect to see a very low temperature on that return in that case.

( due to the flow being so slow, the primary water has lost all its heat to the secondary).

It will only drop as Joni os said to a level above the secondary temperature.

I apologise in advance if you've had to read through this a few times. I hope it answers your question.

 

[Last Plumber]

Hi all,

I'm a Gas Engineer, and new to fault diagnosis/repairs. I've measured the temperature on the flow/return pipes across my combi's secondary heat exchanger, with the hot bath tap on full. I have a Vaillant Ecomax 24 (year 2003). The flow was 70c, the return was 43.8c. I'd be interested to know what a good temperature is on the return pipe. No doubt I'll come across a few blocked heat exchangers in my future work. I presume if the return is too hot, that shows the heat transfer to the mains water hasn't taken place - hence a blocked heat exchanger. So ideally, the return temp should be as low as possible. Am I on the right track? Thanks for your advice, Jennie.

There are a few if's and but's there Jennie.

There will be differences on the primary return temp ( from PHE) dependant upon the flow rate through it on either primary or secondary side of it. This is because it's all about heat transfer as you know.

For example, if you had a high flow rate through the secondary side of the Plate Heat Exchanger, of water at 5 degrees C, you would see a difference between the primary return from PHE than the temperature that a low flow rate of water at say 15 degrees C might give you.
However the boiler would/should modulate to suit so the F+R Temp diff would most likely alter in line with it. To what degree is down to a few different variables.

There are also differences in the size ( internal surface area ) of heat exchangers and this too will give slightly different transfer rates on different boilers.

If the Primary side was partially blocked, you would normally find that the heat from the primary side is lost more into the secondary due to the restriction on flow through the primary side. So I would expect to see a very low temperature on that return in that case.

( due to the flow being so slow, the primary water has lost all its heat to the secondary).

It will only drop as Joni os said to a level above the secondary temperature.

I apologise in advance if you've had to read through this a few times. I hope it answers your question.

 

[Jennie]

Hi all,
Thanks for all your replies.
I've got more info on it, from Valiant technical support. My boiler should achieve roughly a 20c difference between the flow and returns through the secondary heat exchanger. If the temperature difference becomes too great, possibly there'd be a circulation and issue/blockage and eventually the boiler will issue a fault code and cut out.
I'm supposing that for accurate diagnosis, if the heat exchanger is blocked, the heat can't pass through it, so the return temperature will be very low. And the CH water may be filthy. Is it that simple?
I know of someone who had a new secondary heat exchanger fitted. But the system water wasn't cleaned, nor was a magnetic filter fitted. Six months later, her new heat exchanger is playing up.
Thanks again for all your advice,
Jennie

 

[Last Plumber]

Pretty much it !

 

[Last Plumber]

Pretty much it !