I'm planning to fit a new boiler to an existing CH system soon. Currently it is gravity fed DHW and pumped, vented CH but I will go fully pumped and pressurised adding the expansion vessel etc. Currently the pump is on the return side and it will be a whole lot easier to leave it there. However the boiler manual shows the pump in the flow.
Does this make much difference??
TIA
Bob
I think the logic is that by increasing rather than decreasing the pressure at the boiler you reduce the chance of local boiling. What the pressure change *actually* is in the boiler depends on all the resistances elsewhere in the circuit. Might be interesting to put a pressure gauge near the current boiler and check the effect of switching the pump on and off.
with a pressurised system it makes virtually no difference at all.
the pressure gauge on my boiler doesn't change perceptibly when the pump runs.
In days of yore it was put in the return because the motor had better cooling. Also less likely to cause pumping over into the header tank. Also less wear/cavition in the pump with cooler water However if there is no header tank, no longer applies. Also system pressures are higher nowadays so cavitation is lessened You would be better to keep the header tank if it is in good condition, much simpler and less to go wrong. And think about having a constant pressure pump.
You may have to convert the DHW to pumped, some modern boilers don't work well on gravity, the internal resistance of the waterways is too high.
Good point
In a fully pumped system it is usual to have the pump on the flow side, but it would probably work on the return. [I vaguely remember reading installation instructions for a zone valve which said that *that* must be on the flow side. Can't remember whether it said anything about the pump].
The thought occurs to me that converting from a gravity HW water system to fully pumped is going to require some non-trivial plumbing mods anyway, so putting the pump on the flow side shouldn't result in too much extra work. Since you're changing the boiler at the same time, you'll have to re-do the connections to that (as well as reducing four to two, or whatever) so you may be able to leave the pump where it is, but turn it round so that the flow goes the other way.
The boiler can be pressurized by the pump if pump is the return between the the 'feed and expansion' and the boiler. I can't see it pressurising the boiler if it's in the flow side!
Personally I see no reason why it can't be left where it is, subject to siting of the feed and expansion pipes to ensure pup-over doesn't occur.
Thanks for all the replies.
Turning the pump round is an interesting thought but as I have a number of TRVs installed which tend to be directional or at least are marked as such, that might cause a problem.
Not a huge amount, but it can depend on the zone vale arrangement.
If going for a sealed system, you could just fit a system boiler and remove the existing pump.
I prefer a sealed system any day... less system corrosion, easy to fill with no worries about airlocks etc, no worries that a leak will flood the house. No pesky float valves either.
Won't be a problem if he is converting to a sealed system.
Besides the other comments, if it's a sealed system, the pressure vessel should ideally be connected near to the pump inlet, which helps reduce cavitation.
Which bit of "Currently it is gravity fed DHW and pumped, vented CH but I will go fully pumped" are you having difficulty with?
IME TRVs (especially cheap ones) have a limited life; could be a reason to swap them all to Draytons, the modern ones are bi-directional.
Yes.
Pump away from the point of no pressure change, the connection point of the expansion vessel. Pump on the flow. Sealed system, every time.
Look up some manufacturers' schematics for system boilers (with pump & expansion vessel within boiler) and see how they do it.
I'd suggest you check for leaks first though, you should be able to shut off the cold water feed to the F&E tank and have no perceptible change in the cold water level over several weeks/months.
An F&E tank will keep a system with a small leak topped up, but the limescale and oxygen contamination will cause irrepairable damage to the system over a few years.
and that leak may keep dry rot fed nicely as well, somewhere you can't see!
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