well that made a difference

Usual practice.

Sounds like the system wasn't balanced.

Convection systems use *very* large pipes.

In message , "Dave Plowman (News)" writes

Not come across that before, guess it saves on using a pressure relief valve.

I'd intended convection to the rads as opposed to the whole system, like the 'single pipe loop' system here: which I think was the original system in the house, which is now plumbed as the 'feed and return' system on the same page. The 'short' was a six foot loop of pipe connecting the flow and return paths on the bottom right rad of the 'feed and return' system.

================= This is an interesting situation in view of comments made in an earlier thread (06/02/2005 - 11:32).

Several contributors to that earlier thread suggested that a loop without a radiator would cause the boiler to over-heat. If those predictions of over-heating were correct then the OP's boiler in this present case should have been constantly over-heating because of the presence of the 22mm (3/4") loop.

In fact it appears to have had the opposite effect since the OP states that the temperature of all the radiators immediately *increased* when he removed the loop. Logically the temperature of the radiators should have fallen when the offending loop was removed because water temperature through the boiler should have been lower when the alleged cause of over-heating was removed.

One explanation is that the effect of the loop was *neutralised* when the system was initially balanced.

Cic.

Don't recall them saying it would overheat, only that it would return hotter water back to the boiler, which may cause its thermostat to cycle off sooner than it might otherwise...

Assuming the boiler is modulating then it would not have made any difference to the flow temperature - the boiler would adjust the power delivery to keep the output at a set point. IMHO, the reason the radiators got hotter in this situation was because the loop was unbalancing the system, and depriving the later radiators of adequate flow.

The message from John Rumm contains these words:

ISTM that if the boiler is controlling the output temperature to a particular figure then the higher the return temperatue the less the actual power output of the boiler and consequentially the cooler the radiators.

What you are saying is correct, although it is a symptom of the situation I described, rather than a cause IYSWIM.

The temperature of the radiator overall will be the average of the input water temperature and output water temperature. The bypass loop will mean the radiator is getting a low flow rate, and hence it will draw a significant amount of heat out of the water flowing throught it. So the exit temperature of the water will be low, and so the average temperature will be as well (with a corresponding low flow of heat to the room). The small flow of cold water from the radiator will then be swamped with the large flow of hot water from the bypass, causing the boiler to see a high return temp and to deduce the load is low. So it will modulate down quickly and perhaps cycle off.