Combi or not combi - help!

[Terry Casey]

I note it says in the wiki that: "I need a new one, what should I get!" comes up in some form almost every day on uk.d-i-y, but couldn't find enough information to clench the deal, one way or the other.

My wife and I are living in a two bedroomed bungalow - there's nobody else living there except the cat! The central heating is old - if the pencilled date on the back of the controller I replaced recently is anything to go by it dates from 1996.

We had a quote recently from a local plumber who advocated a combi - which I've always been convinced from various things I've read (probably here) that they can be problematic - but he said that modern combis are much better than their predecessors and would be highly suited to our needs. He also pointed out that he has a combi in his 4 bedroom property and has no problems with it.

We then got another quote from someone who is clearly prejudiced against combis! Apart from additional strain on the old pipework - which we are well aware of - he said that the increased pressure can damage the non-return valve in dual fill washing machines (ours is currently a dual-fill Hotpoint) but that we wouldn't be able to run hot water from two taps at the same time!

This could be a worry if we couldn't wash our hands (or whatever) every time the washing machine was filling or the other is running a bath. Can this really be true?

Quote No. 1 includes an Ideal Logic+ 30kW boiler whilst No. 2 is for a Worcester Bosch 15ri boiler. No. 1 includes quotes for both combi and non combi options (but does give any indication of which non-combi boiler would be fitted) and No. 2, as might be expected, doesn't have a second option!

How do these boilers compare - assume No. 1 would use a non-combi version of the same boiler?

All comments gratefully received!

Terry

[The Natural Philosopher]

Does it work though?

He has a deal with comnbi supplires.

Well thats bollocks too.

Mate, the combi I have here currently wont even supply enough hot water to drive ONE hot tap, bath or shower.,

Bite the bullet and do the proper job, Mains pressure hot water tank fed from whatever boiler you have and if in a hard water area all fed via a pukka ion exchange softener.

Then never have a hot water shortage ever again

[Davey]

In East Anglia, it is stupid to not have a water softener. Other parts of the country can do perfectly well without them.

[Dave Plowman (News)]

If you already have a storage system which you're happy with, fitting a replacement boiler for that is not only going to save money, but be easier than changing to a combi.

The problem may be finding an installer who does think of your needs and likes, than just his own profit.

[Terry Casey]

In fairness, the first one has also included a quote for a like-for-like (but condensing) replacement which is £350 less than the combi approach.

[ChrisK]

A personal observation going from a 1987 installed conventional system in a (then) new 4 bed detached.

A WB 34KW Greenstar boiler is more than adequate and shows no sign of struggling to meet the thermal demand for HW with multiple showers operating.

A lot of pipework & HW cylinder was removed and the airing cupboard recovered in full as storage. Roof insulation could now be improved as no need to worry about the cisterns freezing.

No issues with leakage from (now) pressurised 30 year old HW/CH system (8mm microbore). This was my biggest worry as the water pressure round here is highish (3 bar).

The thermostatic shower valves and newer taps now operate correctly with the higher pressure. It was a bit hit and miss before getting them started enough to kick in the shower pump.

There is a noticable issue with HW water flow rate upstairs & interaction between showers & filling the bath is a bit slow. This is probably down to re-using the original 15mm pipe work. It would have been better to have replaced this with 22mm.

Reliability OK so far 2 years in but needs paid annual inspection to maintain the 7 year guarantee - the old Potterton Netaheat boiler needed about £200 in DIY repair over 25 years (2 replacement fans), doesn't look possible with this one.

Been a noticable drop in gas consumption but not huge as the property is well insulated and the old boiler was not that bad (rated 69%).

HTH

Chris K

[Max Demian]

Should be OK unless you have hand washing OCD. Combis are adequate for small or medium flats/houses unless you have more than one bathroom.

[DerbyBorn]

Do you have a suitable size gas pipe to the location? I needed to re-pipe in 22mm with minimal bends. A combi needs a lot of gas and a good flow. My earlier conventional boiler took less - for longer and 15mm pipe was ok.

[Tim Watts]

Given you can get combis up to 35-42kW (WB) they should be adequate for

2 bathroom properties. WB even do a floor standing combi with insane DHW flow.

Some combies also have a small water store that they keep hot that helps with occassional demand.

[Jim GM4DHJ ...]

I resisted a new combi for years but they are now quite good...you will save a fortune ....just go for it ....

[Scott]

[snip]

I installed a combi boiler (Worcester Bosch) last year and I am a bit disappointed with it.

The water takes a while to heat up when using Eco mode. I mostly run without Eco mode (pre-heat is capable of being programmed) so I assume I am using extra energy to save water.

The bath takes ages to fill. This may however be related to having an unusual thermostatic mixer tap arrangement that serves the bath and the shower.

The radiators warm up even when the heating is supposed to be off. I understand this is because the boiler uses the heating as a heat sink when it is pre-heating the hot water. Again, I assume this means wasting energy.

I find the extractor system noisy for a domestic application, but this may be common to all condensing systems.

The hot water even at its maximum setting is not hot enough. It's okay for filling sinks and baths in the first place but for a top-up it is fairly ineffective.

The washing machine is cold fill so this is not an issue. I think nearly all washing machines are cold fill, so probably your next one will be. Could you not disable hot fill if this would help?

[The Natural Philosopher]

no gas at all here

[The Natural Philosopher]

a 40kw combi costs a lot more than a 10kw system boiler with a mains pressure tank

[Terry Casey]

The gas pipe does indeed need upgrading for a combi and that is included in the quote we have.

[Andrew]

If your mains pressure and flow rate are adequate then 15mm should be fine. Sounds like you have another problem somewhere.

[Roger Mills]

The first question is do you actually *need* a new boiler? What's wrong with the existing system? There's no point in replacing it just because it's "old" if it works ok and if spares are available. [My boiler is far older than that - and still going strong - although I have updated the controls.]

A new boiler will almost certainly by be more efficient than an old one

- but the payback time until you have broken even is likely to be quite a few years, particularly if the new one require more frequent maintenance. If the controls need updating to make sure that the boiler doesn't waste energy keeping itself hot when there's no heating or hot water demand, you can do that for far less than the cost of a new boiler.

If you are convinced that you *do* need a new boiler, the choice between conventional and combi depends on many factors.

Firstly, do you bath or shower? Even a good combi will take a long time to fill a bath with hot water compared with a stored hot water system.

Are you desperate to get rid rid of your hot water cylinder in order to redeploy the space for some other purpose?

Have you got decent pressure and flow from your cold water mains. I think the figure often quoted is that the mains needs to be able to deliver at least 14 litres per minute at the point of entry - often the kitchen tap.

Is your gas pipework up to the job. A combi will consume gas at a very high rate when heating the hot water (as opposed to the radiators) so the internal gas pipework may need upgrading.

In my view, there *are* some circumstances in which a combi represents a good solution - but they're few and far between. I've stayed in one or two holiday lets in the summer where a combi has been ok, but when I've stayed with friends and relatives who have a combi in a normal household, the hot water supply has always been dire!

[John Rumm]

Not sure this will help any ;-)

Not *that* old then...

Appreciate that some folks are so biased one way or the other, as to be incapable of being objective!

This is not a combi issue, but a sealed system issue. The vast majority of new boilers (combi or other type) are designed for sealed system operation, although there are a few vented options still out there.

IME having converted several systems from vented to sealed, the problems whilst frequently mentioned are somewhat overstated. Every system I have encountered has been fine with the raised pressure. The worst case is that you may need to replace some rad valves. If a rad springs a leak, then it was near its end of life anyway.

Not heard that before. Even if true you may have no actual symptom of a damaged not return valve anyway. If you were really that worried you could fit a pressure reduction valve prior to the WM. (check the makers spec for the model and see what the max pressure is. I would be surprised if its going to be an issue)

That depends entirely on the power of the combi you get. A small one (24kW) will have difficulty delivering more than about 9 lpm in the depths of winter. More powerful ones ( >=35kW) will do much better. The

35kW one I had in my last place would do two concurrent showers with "adequate" performance, or one with excellent performance.

What happens when a combi runs out of power to to heat the flow rate of water being consumed, depends a little on the design. Some include rate limiting that maintains the water at a reasonable temperature, but limit the flow rate. Most will simply allow the temperature to fall, but carry on delivering water at the rate requested.

Internally combi boilers and "system" boilers [1] are very similar and frequently share the same basic platform.

[1] a heating boiler that includes the pump, expansion vessel and most of the components required to build a complete system in one box

Some background things to check. Is your mains flow rate adequate for a combi? (you need a minimum of 15 lpm really, and more is better).

Is the pressure adequate. 3 bar or better is good.

Is the water supply reliable? (if not, some stored water may be handy). Have you though about backup water heating if the boiler were in need of repair?

If looking at stored water systems, have your installers been thinking about using a conventional gravity fed cylinder, or an unvented one? (the latter tend to give better overall system performance if there is adequate mains flow rate availble)

[John Rumm]

The mains water supply pressure has no real bearing on the primary heating side of the boiler - that is pressurised via a filling loop to typically a bit over 1 bar. It then rises somewhat as the water heats up. The only bit of a the boiler that will see the full mains pressure is the secondary heat exchanger that heats the water from the primary side of the boiler.

Choosing pipe sizes carefully can minimise interactions a bit. Although a 15mm pipe will likely be able to supply as much hot water as most combis can produce. Also a smaller pipe will give you less dead leg of water to drawer off at each use.

[John Rumm]

24kW by any chance?

(makes for dismal bath filling - but will do a "twice as good as the best electric" shower)

Not much value in ECO mode really - it only fires fairly infrequently to temper its small store of hot water.

Or an underpowered boiler...

That sounds more like a faulty diversion valve.

Extractor system?

(do you mean a fanned flue? If so, a standard feature of any boiler you can buy today - although some are louder than others)

Depending on how well the plumbing is done you can get temperature interactions even from cold fill only - especially of the WM cold feed is taken from the pipe before the cold feed to the combi. Really it wants to be plumbed with that being the first thing to be taken off from the incoming main.

[Scott]

Yes - I thought this would be okay for a flat.

Marvellous.

It does seem to make a big difference. Or at least if the heating is on the water arrives more quickly. I may not be to do with Eco mode, I suppose but I think it is as I think it is much the same effect even if I turn the heating down to 10 degrees.

Or the hot water not nearly as hot as it was before?

Is this what used to be called the 'motorised valve' when I was a boy?

Flu gas extraction.

Thought so, my neighbour's is even worse.

That's one for the OP. My washing machine is cold water (LG) for which I am grateful given the unpredictable behaviour of the boiler.

As a complete diversion, if I wanted to replace my boiler with a mains water filled storage tank is there a market for second-hand boilers or would economic and regulatory issues consign it direct to the scrap heap?