Not necessarily.
This boiler is particularly susceptible to sludge and blockages causing problems because of the way the diverter valve on it works. The diverter needs positive pressure as well as voltage to work, which comes from the pump via a little narrow weep tube. You'll see it between pump and diverter, it's about 8mm diameter copper and bent like a swan's neck.
Drain the system down, strip out related components and give everything a good clean - you'll soon get a clue if things are all sludgy. This tube in particular gets blocked up because it's so narrow - without it, the diverter can't work properly. It leads to a lot of diverter changes that aren't always necessary.
The other spot to check is a small filter in the top manifold, acessible by removing a brass plug that's sort of up and into the manifold (it points to the base of the boiler). It's a bitch to get out but if that filter is clogged it puts a back pressure on the valve again, preventing correct operation. Early models did not have it.
The other thing that can cause back pressures (believe it or not) is a blocked plate heat exchanger.
Please take care, the diverter on the original HF400 is 240 volts.
Bunslinger, April 2009
