Flow and return differential

4 Jan 2004
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United Kingdom
System: Baxi 100HE (control knob set about 2/3 of max), 9 rads with Drayton TRV4 valves, plus 1 rad (in airing cupboard) on bypass loop with lockshield valves. Grunfoss pump set to position 2. Range Tribune sealed HW system. Whole lot is brand new (except boiler, which is 18 months old). Everything works fine and heats up amazingly fast, except....

Impossible to achieve more than 6deg temperature differential between flow and return (often less, when all the rad valves are open, and boiler is going full bore), and I've read somewhere (on this forum, I thought, but I can't find posting) that for condensing boilers to work at max efficiency, the temp diff should be 10-12deg or more.

Flow and return temps are approx 51deg and 45deg respectively, both measured simultaneously with pipe-mounted bi-metallic dial thermometers.

I've tried closing the bypass rad valves right down (to reduce flow through this rad and force water around the system and hopefully cool it down a bit) but makes little difference.

Should I worry about this, or just leave it? Or is there a magic solution?
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Handyman Wrote

Flow and return temps are approx 51deg and 45deg respectively, both measured simultaneously with pipe-mounted bi-metallic dial

If your return is 45 degrees then the temperature drop across the rad is irrelevant as the boiler should be in steamer mode anyway.
Sider, I think I understand what you mean, but I thought that if too much water was passing from the flow to the return via the bypass rad, then the temperature would build up until there was no differential. By forcing the water around the other 9 rads, it would lose some its heat, and thus bring the return temp down.........But it doesn't.

By 'steamer mode', if you are referring to a nice white plume, then 'yes' I'm getting that. And the condensate drain fills and flows nicely.

My question still stands: Should I worry about it - if everything is working OK? Alright, the boiler might not be working to max efficiency, but if the system works OK, is it worth more hassle just to improve efficiency by a relatively small percentage??
If the products of combustion are condensing it is running efficiently.

The theory that you should dissipate more heat in order to make the thing efficient is a contradiction of terms.

With modern heating you only get the full 20 degree drop while the system is warming up the rads, once the house is warm it would be foolish to look for a 20 degree drop.

The benchmark log is nowadays a farce. In fact I think I'll get a stamp made up for it.
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Thanks, Paul B.

Over the last couple of hours, I monitored the temp gauges. With the system running smoothly, all rooms controlling nicely, with the room stat set a degree higher than normal (to make sure the boiler kept running, and relying on the TRVs to control the temp), the flow/ret temp differential varied between nothing and 2 deg (i.e. 51 flow 49 ret). And yes, condensate trap full and draining nicely, and the semblance of a plume (it's about 12deg outside temp). So, all seems to work perfectly.

At 50deg, the system certainly chucks out the heat, and I'm not sure that I'd want my rads hotter than 50deg (for safety reasons), so I'm not going to adjust the boiler control knob any more.

Unless someone tells me that my boiler will explode, or burn out in 6 months, or that the boiler efficiency police will be knocking on my door, I won't concern myself any more.

Thank you, Paul B.

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