Poor Heating Commissioning?

9 Nov 2009
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United Kingdom
Before the season turns much colder, I'm going to balance my heating system properly - the 12C drop way rather than twiddle lockshields and hope for the best.

Now I've made the classic DIYer error of reading the boiler manual.

"Balance the system so that the required temperature difference across the central heating flow and return pipes is obtained"

Manual says 12C so I interpret the passage to mean each rad drops 12C.

"Adjust the central heating by-pass valve until the same temperature difference is obtained. This should be carried out with only a single radiator operating." and there's a handy diagram showing where to put the screw driver to access the by-pass.

Ok, so that's adjust until the boiler pipes are 12C apart.

Just out of curiosity, I decided to measure the temperature drop at the boiler. It was a pretty consistent 20C.

Should I infer the system wasn't set up properly when it was installed or am I missing something here?
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If the temperature drop across the bypass valve (with only one rad open) is 20C, then you should increase the flow through the bypass until it is 12C. It's a tricky measurement because at such a low load the boiler will probably be on minimum rate and the flow temperature still rising. For an auto bypass (and I suspect you are too embarrassed to say that you don't care about our kid's jobs, and bought a Vaillant) once you increase the number of rads in use the pumping pressure will reduce, and with it the flow through the bypass.
Each rad, as you state, should have the same temperature difference across it in a two pipe system, that is they are hydraulically all in parallel.
It's increasingly common to see heating systems where the balancing has not been done thoroughly, the installer relying on the TRVs to do that for him/her. That's a pity because a well balanced system will heat up faster and give a comfortable environment more quickly.
You've not told us what boiler you have. A modern condensing boiler should have a differential of 20°C between flow & return, with a maximum return temperature of 52°C to achieve best efficiency
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Sorry, should have said.

Worcester Bosch 35CDi II modulating combi boiler. I'd guess installed around 2005 before we bought the house.

The 20C differential was measured while all rads were set as normal, not just one working.

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