Flow temperature discrepancy

[Simon BBB]

Morning all, I have a Worcester Bosch 8000 life 40kw boiler on system with 7 radiators and the boiler is located in the loft of a 1907 end of terrace house. It has been range rated down to 50% and I currently have the flow temperature set to 64, due to slow warm up.

I recently purchased a digital thermometer with pipe clips to try and help balance the system as I had issues getting some of the radiators (all new) getting hot towards the middle bottom. I thought I would check the temperature of the flow and return at the boiler and found a difference of 4.5 degrees between the set flow on the boiler and the temperature of the flow pipe, directly under the boiler.

Is this normal (assuming the thermometer and clips are reasonably accurate)?

Also does the loft location or the range rating effect the performance of the pump in any way?

Many thanks in advance.

 

[John.g]

Are you sure the digital thermometer is matched to the probes? or that the 20kw range rating is sufficient for 7 rads which it should be. What is the boiler flow/return dT?.

 

[Simon BBB]

Many thanks for your reply and sorry, It didn't even occur to me that might be an issue. What I have done is put both clips on the same pipe to test for any large variation. I have also put one of the original supplied wire probes (came with the unit) against the clips I purchased separately and again they both registered more or less the same. I have attached a picture of the reading which was showing an 11 degree drop after being on for over an hour. I've found it impossible to get much of a drop (10 degree) on any of the Rads.

 

[John.g]

I have the same one with just the K type probes and its seems to be quite accurate on the few occasions I've used it so can't think of any reason for a discrepancy, it's more than likely that the actual temperature leaving the boiler heat exchanger is 59.6C.
"I've found it impossible to get much of a drop (10 degree) on any of the Rads." are you saying that the rad(s) dT (drop) is greater than 10C or less than 10C?. The less the dT for any given inlet temperature the greater the rad output, at that flow temperature, you would generally aim for 15C to 20C dT to ensure greater condensing (more boiler efficiency) but it affects the rad outputs, if all your rads, above had a dT of that 10.8C with flow/returns of 59.6C/48.8C then the rad outputs will be 61% of their rated output.

 

[gpbeck]

A 10 degree drop across the boiler and across the rads was the normal design until 10 years ago. If flow temp 64 return 54 you probably have only a little condensation in the boiler but to increase efficiency you will need bigger rads. This allows a drop in flow temperature and a bigger drop across rads so a lower return temp. Usually easiest to direct swap rads for 22 doubles the same width so no pipework changes. You could do this incrementally room by room.

A pump in the loft works fine.

 

[Simon BBB]

I have the same one with just the K type probes and its seems to be quite accurate on the few occasions I've used it so can't think of any reason for a discrepancy, it's more than likely that the actual temperature leaving the boiler heat exchanger is 59.6C.
"I've found it impossible to get much of a drop (10 degree) on any of the Rads." are you saying that the rad(s) dT (drop) is greater than 10C or less than 10C?. The less the dT for any given inlet temperature the greater the rad output, at that flow temperature, you would generally aim for 15C to 20C dT to ensure greater condensing (more boiler efficiency) but it affects the rad outputs, if all your rads, above had a dT of that 10.8C with flow/returns of 59.6C/48.8C then the rad outputs will be 61% of their rated output.

Many thanks again for getting back to me. Just to confirm, I was only getting a couple of degrees drop (2 to 3) between the flow and return on most of the radiators.

Also any thoughts as to why the flow temperature leaving the heat exchanger is lower than the set temperature? The boiler is 3 years old and under warranty with Worcester bosch and so I am trying to confirm if this is normal before I bother contacting them.

 

[John.g]

Many thanks again for getting back to me. Just to confirm, I was only getting a couple of degrees drop (2 to 3) between the flow and return on most of the radiators.

Just shut the Lockshield valve fully, some need a bit of effort to fully tighten down, then just open it a 1/16th of a turn or so to give a very large dT, then progressively open it to get the required dT.

Also any thoughts as to why the flow temperature leaving the heat exchanger is lower than the set temperature? The boiler is 3 years old and under warranty with Worcester bosch and so I am trying to confirm if this is normal before I bother contacting them.

Re below, a accuracy of + or - 1.5% is printed on the back.
The K type has a range of -200C to +1372C, a span of 1572C, 1.5% of 1572 = 23.58C!
The T type has a range of -250c to + 400C, a span of 650C, 1.5% of 655 = 9.75C!.

My K type probes give a reading of 97.5C when immersed in a pot of boiling water today.
Edit: Which is quite accurate as the boiling point of water at todays 970Mbar is 98.78C

I presume the indicated Flowtemp is the same as the set point?