Pump over run and Vaillant 418 install.

[Harry Bloomfield]

The proposed installer for my 418 boiler has pointed out that my present boiler does not support a pump over run output, nor is my present system wired for one. He did mentioned an extra bit of loom he could source, which would allow the pump to run, without the extra external wiring - I cannot guess how this might work. At the boiler, at present it would just have a L + N + E, then a switched live to tell the boiler to run. How might it work? My guess is that it would need to back feed via the switched live.

With such an extra wire going from boiler to wiring centre joint box, in the airing cupboard - I would assume the wire needs to connect to the live to the pump. Is my assumption correct, just a single extra wire - boiler to the pump please?

[Andrew Gabriel]

If your current boiler doesn't support it and presumably doesn't need it, why do you want one?

Any new boiler that needs pump overrun (and I think all will) will support it in the electronics. I don't think many new boilers rely on external pumps at all anymore, in part because they now all control the pump speed dynamically to get the flow rate to give the right temperature drop for condensing operation whenever possible.

If you want to add pump overrun to a boiler that doesn't support it, the circuit is exactly the same as a fan overrun circuit for an extractor fan, although a pump draws more current.

[Harry Bloomfield]

Andrew Gabriel brought next idea :

My understanding at the moment, is that the 418 does not have an internal pump - it expects one be external.

What the installer said, was that the boiler would generate the signal to the pump, to overrun. Idea being to cool the last of the heat from the heat exchanger.

My pump is just a dumb pump, but with an electronic speed adjuster on its side. Yes I appreciate I could cobble together a timer and relay circuit, to provide the pump with a local overrun circuit in the airing cupboard, but the boiler can better control it. I understand the boiler measures its heat exchanger temperatures and keeps the pump running, while ever there is still heat to be drawn out after it has stopped its burn cycle.

[John Rumm]

While changing the boiler, would this not be a good opportunity to convert to a sealed system, and ditch the external pump? (fitting a system boiler rather than than a heat only boiler)

[Harry Bloomfield]

John Rumm laid this down on his screen :

It could maybe double the cost and I much prefer the instant output of stored hot water. If I go for a bath, I just press the one hour override for HW and the used HW is replenished while I am taking the bath.

A stored system can also be around 1/3 less costly to run on the HW side than a combi, if the house is mostly occupied anyway.

A combi is fine, where use of HW is very irregular, HW can be sat uneeded for many hours.

Combis waste gas and water whilst they get the water to temperature, then waste even more gas, at the end of the water draw session, leaving the boiler still hot.

[Richard]

John was not suggesting a combi boiler! System boilers are designed to work with hot water cylinders. The option exists for a sealed primary circuit and an open vented copper HW cylinder. Your choice whether you opt for a Mains pressure cylinder etc

[John Rumm]

You would still have stored hot water - A system boiler is just a normal heat only boiler that also includes all the necessary gubbins for sealed system operation and its own pump.

I was not suggesting a combi.

Conversion to a system boiler would mean taking the existing pump out of circuit, disconnecting and capping off the feed & expansion connections to the CH header tank (and removal if you so wish).

You would end up with a system that is much easier to fill and bleed, and far less corrosion, plus much less likely damage should anything spring a leak. You also get a much wider choice of boilers, and things like the ability to do more sophisticated things with the pump come as standard.

[Roger Mills]

No. You probably already have that, in effect. That is to say that the pump live feed and the switched live to the boiler are probably

*already* connected together within the wiring centre. If you are to implement pump over-run, the boiler needs to control the pump at all times. In particular, it needs to be able to continue to run the pump after the switched live has been switched off. Boilers which are equipped for this will invariable have extra connection terminals. Typically you will have L, N, E mains supply, Switched L, and L, N, E output to the pump. You may be able to achieve pump over-run with just one additional wire because the pump will already have acceptable neutral and earth connections. Disconnect the current live feed to the pump, and replace it with a live from the boiler's pump output - *not* from the switched live.

[tabbypurr]

An overrun timer is fitted to the existing pump. The pump runs during call for heat plus 1 or 2 minutes after that due to the overrun timer. In the en d it does the same job the 4 series boiler does, presumably an overrun time r is cheaper than adding a wire to feed power from boiler to pump.

NT

[Harry Bloomfield]

Oh, right! I had missed that point. It still offers little advantage, other than doing away with the boiler's header tank and today I fitted a new one. The old one appeared to be quite rusty inside, so I thought I might as well swap it out now, rather that further down the line. I needn't have worried it was perfectly sound.

I was also a bit concerned about the galv steel CW storage tank, which would be a much more tricky swap, as it is not so accessible, hidden in a plaster boarded wing of the loft. As it went in at the same time as the other tank and I am not unhappy about that, I will leave as is.

I still haven't got to the bottom of this pump wiring issue. I have downloaded an installation manual for the 418, but it make no mention I could see of the pump. I am wondering if the idea is that the boiler takes full control of the pump, just a switched live and neutral from boiler to pump, boiler is triggered to run, boiler runs pump, fires, call for heat is satisfied, then pump continues to run until the heat left in the boiler is exhausted.

[Harry Bloomfield]

Roger Mills presented the following explanation :

Thanks... That is the conclusion I had sort of arrived at, just before reading your confirmation of how it works. I was just confused by the boiler installer mentioning that a additional loom could be bought, which achieved the same function, without any external to the boiler wiring.

Plan now is to see if I can get that extra wire, from boiler in the kitchen, to the airing cupboard where the pump is.

[Harry Bloomfield]

Harry Bloomfield formulated the question :

I may have found what I have been seeking, in the 418 installation manual...

X14 marked as item 8, page 34 of the 418 manual. It shows a L + N + E terminal marked as 'Heating Pump'. I cannot find any mention of what it is other than that, input or output.

[tabbypurr]

output to power the 240v pump. I have a 4 series. As far as I can tell it doesn't modulate pump speed, despite claiming it does, so you still set the pump speed the old way.

NT

[Harry Bloomfield]

[e-mail] expressed precisely :

Thanks !

There seems to be a mass of options in the menu system, some of it confusing with not much proper explanation. Maybe the variable speed for the pump is only via the E-bus interface thingummy?

[tabbypurr]

That would make sense, I gather there are various options with these boilers that you typically don't get with it. I had to go through mine changing them to get it to work reasonably, so you likely do need to understand them :)

NT

[John Rumm]

Note the 400 series had a radical revamp a couple of years back. I fitted one a few years ago for a friend, and the current range look nothing like that one internally.

[John Rumm]

One of the things I found difficult with the Vaillant documentation is the lack of "big picture" type information to tell you *why* you might want some particular option rather than how to set it.

They often support hot water circulation pumps, and hot water charge pumps, in addition to other things.

There is a smart wiring centre option that is driven by the boiler via ebus, and that in turn controls the old style valves, and connects to the NTC sensor on the cylinder. (it can also IIUC, control blending valves for UFH etc). Much talk of "Controlled" and "uncontrolled" zones (where "uncontrolled seems to be what we think of as a zone controlled by thermostat and normal mains zone valves).

[Harry Bloomfield]

It happens that John Rumm formulated :

My intended Vaillant installer suggests they are much better/easier to repair and service with the new internal layout.