Radiators not heating up is it the pump or a blockage?

[D_Hailsham]

They are Drayton TRV4 which can go on either end.

Then it does not matter if some rads have the valve on the flow and others have it on the return. You still adjust the LS vave to give the correct drop.

The rads that were cold are now heating up (so the system was out of balance) but the hall isn't getting above 18deg when it used to get to 21deg easily (I know this because the room stat is in there).

Any idea why?

Still not properly balanced. It can be a time consuming job to get it right. Did you follow the method in the Wiki? Balancing Procedure

Looking at the boiler spec it shows a line for the pump at a maximum of 350mbar lift giving 1000l/hr flow. As our boiler is in the loft it has about a 5m lift. Could this be causing the problem as 350mbar is only 3.5m?

The graphs are misleading. The vertical axis, labelled "Lift", is actually the pressure in the system, which is determined by pipe sizes and lengths, number and size of rads etc. The fact that your boiler is in the loft and about 16 ft off the ground is irrelevant.

Two things might be worth investigating:

If you boiler output is set low (d.0), the pump should be set to two speed operation. It's explained on page 44 of the installation manual - option d.19.

Check the setting of the auto bypass. It's on page 32, item5.8. Try increasing the pressure slightly. This means they bypass will open so soon.

 

[webbm00]

Still not properly balanced. It can be a time consuming job to get it right. Did you follow the method in the Wiki? Balancing Procedure

Yes but will go round and make sure all the rads still have the correct drop

If you boiler output is set low (d.0), the pump should be set to two speed operation. It's explained on page 44 of the installation manual - option d.19.

The factory setting already does this. I've tried setting it to the low and high speed setting and it makes little difference

Check the setting of the auto bypass. It's on page 32, item5.8. Try increasing the pressure slightly. This means the[] bypass will [not ]open so soon.

Until I read the installation manual in detail I didn't know the boiler had an internal bypass (I've fitted an external one). I will adjust it and see if it helps.

The one thing I still don't understand is why the flow is getting to the temperature when the boiler shuts down rather than the return? I always thought that the boiler runs and heats the water, it's dissipated by the rads and comes back cooler. This continues until the rads can't dissipate enough of the heat (rooms warm), the return water warms up and the boiler switches off. Is this not the case?

 

[D_Hailsham]

The one thing I still don't understand is why the flow is getting to the temperature when the boiler shuts down rather than the return? I always thought that the boiler runs and heats the water, it's dissipated by the rads and comes back cooler. This continues until the rads can't dissipate enough of the heat (rooms warm), the return water warms up and the boiler switches off. Is this not the case?

It will depend on what the boiler is monitoring. If it's the flow temperature, the boiler will turn off when the flow temp reaches the set valve, similarly for return, though this is not adjustable by the user. It could measure the differential and when this gets too below a certain value, i.e return is nearly the same as flow, the boiler will turn off.

If you have an external bypass as well as the internal that could cause problems. I might be a good idea to disable the external bypass (set it to max) and try adjusting just the internal bypass.