Difference between boiler flow and return temperatures

12 Feb 2017
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United Kingdom
I've just installed 2 digital temperature probes on the boiler flow and return pipes, to try and ensure the boiler is condensing, below the magic number( what ever it is, but that's another discussion). When the system start up from cold in the morning the temperature difference between flow and return is quite large, flow is 50C and the return is about 25C, but after the system has been on for a couple of hours the flow temperature drops a little to 47C and the return rises to above 40C sometimes as high as 45C or 46C. I'm happy that the boiler is condensing, as I did a quick calculation on heating the hot water tank from cold and it turned the boiler off after 50 mins and I worked out how much gas it used and it was over 90% efficiency. My question is should the difference between the flow and return be larger? we have heavy cast iron radiators in all the downstairs rooms 3 of the room have 3 around the bay windows. I'm wondering if slowing the pump speed down will allow more heat to dissipate from the rads before returning to the boiler? Also would a pump which automatically varied the speed be a good investment, as its running for a fair length of time each day during the winter months?
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Slowing the speed down will increase the dT but you may find you starve the system of the required flow.

Any system with TRVs should be using some form of pressure control. Flat speeds are for systems that do not change, like a charging a HW tank.

Aslong as you get accurate room temperatures, the dT matters very little.
The rooms do seem to take a while to heat up normally over an hour in the mornings, But it's an unusual house with lots of single glazed chital leaded light windows and most rooms have at least 3 outside walls. I've just fitted some fancy digital TRV's as the nice antique looking ones didn't seem to be working that well. I have now had a few problems with the room temps going way past the setpoint. When i looked into it the TRV's were shutting but the rads have such a large thermal mass they continue to heat the rooms for an hour raising the temp higher than requested. I just set the digital TRV's about 5C below what i want and it works ok. Not sure how i'm going to set the frost protection on them as i'm not happy setting at -3 as they won't come on in time and could freeze pipework.
If boiler flow is at 50, what is your cylinder thermostat set at? Should be 55.
For condensing boiler I would be looking at each heat load to display 20 degrees drop
If boiler able to output higher temperature for cylinder, I would go for that.
For radiators, external sensor optional but radiator balancing crucial .
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I've just installed 2 digital temperature probes on the boiler flow and return pipes, to try and ensure the boiler is condensing, below the magic number( what ever it is,
Just curious to know
- What is the size of your boiler?
- how long do you use it for heating?
- What is your gas consumption on hourly basis
What boiler model is it? If an external sensor or weather info could bring external data to the boiler you could run it constantly at low temperatures swapping between a comfort and a setback temperature which would even out the swings of heat from the larger high water content radiators.
It a Worster Bosch Greenstar 40 CDi classic regular ErP. I've just noticed the digital display on the front of the boiler is saying 65. I have rechecked the flow pipe temperature directly as it comes out of the boiler case, with an analogue pipe thermometer and a digital thermocouple on my multimeter and they all show around 44-47C, which tally with my flow and return sensors. So either the boiler is reading 20C high or I'm losing 20C in about 8cm of pipe! Up till now we've only been putting it on for an hour in the morning and an hour or two in the evening if its really cold. We've got log burners in the 2 rooms we tend to sit in, and one is kept on most day in the winter unless its really mild. I've been experimenting on running it for longer during the day at a lower temperature. Cylinder stat is set at about 55 but I never trust now accurate they are, I tend to do the touch test, you should just be able to hold your hand under the hot tap for a few seconds but not indefinably.

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