Combi boiler: filter on return necessary?

[JavaEnquirer]

I've recently had a number of quotes for the installation of a new condensing combi boiler and new radiators. Currently, we have an old gravity fed system with 20 year old rads.

Some of the plumbers we've spoken to strongly suggest chemical or power flushing the system and fitting a filter on the return to the combi. However, the plumber we are feeling inclined to go with suggests just letting the boiler run and water flushing. He said that he hadn't heard of people fitting filters on the return.

Is it necessary to have a filter on the return if the system has been water flushed and the radiators are new?

[Andrew Gabriel]

I would suggest it in the case of a combi, otherwise the plate exchanger will act as a filter and unlike hard water scale on the other side of it, the rust debris doesn't clean out easily. If the whole system is new, or it's not a combi boiler (i.e. no plate exchanger to block), then it's probably not worth it.

[JavaEnquirer]

You say that the "rust debris doesn't clean out easily". Is this actually a problem, will it actually cause the boiler to work less efficiently or break down prematurely, or does it just make servicing a little more time consuming?

Some of the quotes I've had have mentioned chemical flushing, filters and inhibitor. However, those quotes are roughly =A32000 pounds more than the one without. I mentioned this to the "cheaper" plumber ( Corgi registered ), but he didn't seem to think not having a filter was a problem.

Could this be down to the actual boiler? Do some have in-built filters? I'm going for a Worcester Combi Junior 28i.

[IAN CAPEL]

With the boiler comes a benchmark certificate which asks was the system cleansed in accordance with the manufacturers requirements, what cleanser was used and what inhibitor was installed. Should the system not be cleansed and you have a problem with the boiler the manufacturer will not honour the guarantee. Also the building control certificate that your decent installer will apply for will require the boiler to be installed to the manufacturers requirements. So the system needs to be cleansed. I do not see why a properly cleansed system would need a filter but others may disagree

Some of the quotes I've had have mentioned chemical flushing, filters and inhibitor. However, those quotes are roughly £2000 pounds more than the one without. I mentioned this to the "cheaper" plumber ( Corgi registered ), but he didn't seem to think not having a filter was a problem.

Could this be down to the actual boiler? Do some have in-built filters? I'm going for a Worcester Combi Junior 28i.

[John Rumm]

The first two... the plate heat exchanger is full of loads of tiny water pathways. This is how it achieves the large surface area required for rapid heat transfer between the central heating water and the incoming fresh water that goes to the taps.

If the primary side fo this gets cloged with rust or other debis then it is often going to need replacement to restore decent hot water performance.

All of these are worth doing. The chimical flush is best done a couple of weeks before you decommision the old boiler. Stick it in the system so it has a good chance to get to work (and it means you need it in there while the heating is actually on as well so it gets to all the rads). The state of the system (level of existing sludge / corrosian etc) will then dictate what other flushing it requires (ranging from a good rinse through with fresh water, to powerflushing or individually taking out each rad and flushing). If lots of pipework changes are happening a flush with a system cleaner to remove flux residue etc can be worthwhile. Finally inhibitor is essential for the long term protection of the system and will hopefully prevent the strainers etc from having much work to do.

Well a simple strainer on the return will go a long way to catch any rust etc before it gets to the boiler and is only a five quid part. Nothing stopping you buying one and asking the plumber to fit it (pref with a couple of full bore valves either side to make servicing easy).

Some may - but an external one is usually simpler to get at and clean.

Out of curiosity, why do so many people fit the Jnr model? Is it that much cheaper? IIUC the Jnr models are very similar to the Si models however they only run in condensing mode for CH, and not for HW. So you are throwing away some of your potential energy efficency gains.

[John Stumbles]

Have you heard of this actually happening on a system that's been flushed (chemically, not power-flushed) and treated with inhibitor in accordance with the boiler manufacturer's instructions & benchmark requirements?

The valves are a must. The strainer will block up with a minute amount of crud and stop effective circulation in the system. (You'll know because the boiler will short cycle.)

AFAIK only the Alphas, which have a cyclonic trap to encourage sediment to precipitate out into a chamber (which can presumably be cleaned out periodically).

They're a lot lighter (advertised as being a one-man lift) so they're attractive to installers working singlehanded. I wasn't aware of (and I'm a bit surprised about) the difference you quote in DHW mode, although I don't think that's a major issue: condensing boilers are still more efficient than non-condensing ones even when they're not condensing, and the amount of energy used providing DHW is generally vastly less than for heating.

[Andy]

My brother fitted a strainer after I mentioned them to him, and he found quite a lot of debris in it after he had run the combi for a week and drained it down to refill with Fernox. Mostly it was solder splats, copper shards and bits of cloth etc. Of course, he did not flush the pipes out before turning on, so it was a lazy way of catching the debris.

Andy.

[John Rumm]

The only one that I have direct experiance of (and hence know in detail what treatment was carried out) is my own... I have certiainly heard of HW heat exchangers getting filled with crud, but don't know enough of the back story to know what steps were taken.

I found on my one (chemical flush for 14 days, and then flushed via continous mains feed before filling and dosing), that the filter has only so far (in two years) collected one solitary grain of "something solid". Having said that, the system prior to that had been treated with inhibitor by me, and also its previous owner.

Yup, I can see that may make a differance...

(then again my boiler was in theory a one man lift, and it still took two of us to get it on the wall!)

In fact looking at (see page 14):

suggests that it is the CDi and not the Si that condenses in both modes. I must admit I am supprised that they claim this to be "one of the few" boilers that does this however.

Oh sure... It will vary a bit depending on household as well no doubt so may be more of an issue for some than others.

[Tony Bryer]

A Magnaclean does the same thing: not cheap bull well thought out.

put one on my mother's system after I got feed up with a Y strainer for ever clogging on next to nothing.

[John Stumbles]

Presumably the new W-B Junior Greenstar unless you're either ignoring the regs or qualify for an exception. These are radically different from the non-condensing (not 'Greenstar') model: apart from the condensing business the old ones had a combination (not sure what you'd call it really) heat exchanger where both primary water and DWH are heated by the main burner, rather than having a plate heat exchanger and diverter valve as in most combis including the Greenstar Junior.

[John Stumbles]

Ooh! a recommendation based on personal experience: how novel! (At least, compared to A Certain Other contributor who's been lauding them recently :-)

Thanks Tony - think I might give one a whirl.

[Andy Hall]

If we all get one, there should be enough reward for him for that one way Eyebyeza trip....... :-)

That does appear to be how they work......