Pumping Over on HW Only - Modulating Boiler

[Ragworm The Abominable]

Hi,

Quick question for the Panel.

CH & HW work perfectly from a heating perspective. There are no hotspots/cold radiators and HW heats up very quickly.

When the boiler runs HW only, pumping over occurs.

I have reduced the level in the expansion tank to a minimum.

There is no way to control the pump speed as the pump is intergrated into the boiler and modulates depending on the demand for heat.

The Boiler is on the ground floor, the HW cylinder on the 1st floor, the expansion tank in the loft.

The pumping over on HW is more of a 'trickle' than a gushing, could extending the loop/overflow thingy by a couple of feet fix the problem?

Cheers,

[Set Square]

It would appear that there's more resistance to flow when just HW is on, causing the pump to run at a slightly higher pressure. Is there a gate valve anywhere in the HW circuit (probably near the cylinder) to balance the HW/CH flow? If so, you could try opening it a bit - assuming that it's not already fully open. [If you have independent timing of HW and CH - maybe by having a programmable room stat for CH - you may be able to set it up so that they're not usually on at the same time - so balance is less important].

Raising the level of the top of the inverted J of the vent pipe would also help.

Have a careful look at how the fill and vent pipes are connected into the flow pipe (or maybe the return pipe?). The normal rule is to put them both in the flow pipe on the suction side of the pump - but you can't do that if the pump is integral with the boiler - hence the thought that they should perhaps be in the return pipe. The important thing is that they shouldn't be more than a few cm apart in order to ensure that there's no significant flow-induced pressure differential between the two connection points.

[Ed Sirett]

Essentially this is an installation fault.

The vent and feed pipes are too far apart on the circuit and/or there is a partial blockage between them.

What you do next will depend on many circumstances. Doing nothing is not a good idea.

[Ragworm The Abominable]

I don't plan to do nothing :))

HW has been working fine the past three years - I am not sure what has caused the problem.

A gas installer replaced the shoe on the 3-way value quite recently (took him two attempts to fix it). I wonder if he dropped/lost something down into the pipe / or replaced it with an incompatable shoe.

The HW does not have a restriction valve on it - but I have opened up the rad in the bathroom to full to see if this helps. Bathroom rad comes on with both HW & CH.

I am pretty sure there that gunge is not an issue as I gave the pipes a good clear out when moving into the house.

I have 4 meters of 22mm pipe in the garage - any estimates on how much I need? Or just go for broke and use as much as possible? It's more of a splash of water every second or so rather than a continuious stream of water.

Many thanks for all received replies, and any future ones :)

[Ed Sirett]

It's possible that the 3 port valve did not go fully to HW only before it was repaired?

There are several things you can do each with strengths and weaknesses.

The best by far if you can afford it and the boiler supports it is to go for a sealed system. See FAQ.

You could try putting a higher loop on the vent pipe so that it requires a bigger pressure difference to start pumping over.

You could route the feed into the vent pipe - this will make refilling the system harder but definitely fix the problem.

You could try a lower pump speed and provided it still actually heats the house and HW. It might also cause the boiler to kettle.

[Ragworm The Abominable]

Hello Ed, thank you for you reply.

The spring went west, the motor got tired, and the shoe got gummed up, I believe that the three way at this time is functioning from a diversion point of view as it should.

I am considering this as a long term fix - it's a Keston Celsius 25 and I really don't think this boiler is happy unless it's sealed.

Planning to do this at the weekend - the better half got me the raw materials, all I need to do is ensure I don't set fire to the loft. Any 'guesstimate's on whether I should make the loop as tall as the loft roof, or go for something less>

I could put a T-Junction join in the loft between the vent and the overflow tank - or would the join need to be lower down in the airing cupboard?

Unfortunately the pump is modulated directly by the boiler.

Many thanks,

[Andy Hall]

To be honest, by the time you've mucked around with the other vent options, you can have disconnected the loft tank and capped off the pipes with air vent fittings and have fitted a sealed system kit with filling loop somewhere. It's really not difficult and the kits are inexpensive.

[Ian_m]

Seen this type of thing before, the pump pumping over into the header tank. This happened many years after a new boiler was fitted. Basically probably due to the bodged way the water feed and vent pipe were connected to the pump just via a jumble of soldered 15/22mm T fittings. Anyway a plumber fitted a thing like

item number 11334 (Aerjec water de-aerator) before the pump (I thing) and generally tidied up the pipe work and the overflowing stopped. Something to do with the connecting distance of feed pipe and vent pipe and pipe size, I think

[Ed Sirett]

So that's the probable cause.

My own KC25 is sealed, but I'v down a couple of vented installs no problem.

I don't think it will make much difference nearby is simpler and may lead to less air locks.

Now I know it's a KC25 indeed so. It will always use full powre and pump when the primary circuit is cold.

[Ragworm The Abominable]

All, thank you for the replies to date :)

I have some more information - on opening the bathroom rad all the way, no more pumping over. This is not ideal as it warm's up the bathroom a little more than one would really like(!) - but it's a good temporary solution given my current 'broken list' *grin*

Still, the plan is to extend the existing vent, and if that fails to put a pipe between the vent and the expansion tank, with a full bore valve to allow easy emptying/refilling of the system.

My Keston hasn't got a pressure valve - would an expansion kit come with a seperate valve or would I need to purchase the Keston item?

Cheers!

[Set Square]

I assume you're talking about the possibility of converting it to an unvented system? In that case, something like

should provide everything you need.

[Andy Hall]

To be more explicit - this is a generic kit for creating a sealed system. The Keston one is functionally the same but a cosmetic convenience.

[Ed Sirett]

Keston do a complete kit with all the bit in a unit which fits under the boiler. However my boiler is uite near the floor and was not practical todo this so I 'rolled my own'.

[Ragworm The Abominable]

Popped* a extra 80cm of hight onto the 'inverted J', turned the bathroom rad off, popped a bit more water into the header tank, and wound the temp up on the HW cylinder, turned the output on boiler to max, and waited until boiler modulation went down to zero.

Results so far, no over pumping, but will keep an ear out.

(*my first time soldering so hope I didn't mess up..!)

Many thanks all.