replacing c/heating boiler

[Ed Sirett]

To parody Fowler on the subject of split infinitives:

The combi questioner can be divided in several groups.

Firstly, those who neither know nor care. These are the happiest of all especially if someone else pays the bills.

Secondly, those who don't know and do care. They post to uk.d-i-y.

Thirdly, those who know and always approve. We all know who he is.

Fourthly those who know and always condemn. To those of you who are in this group I beg you to read on. I suspect that this group is larger than it might otherwise be on account of two factors. Firstly there are a proportion of professional installers who for their own reasons prefer to always fit a combi regardless of the needs of the customer. Secondly those in the third group whose style of advocacy could be described as abrasive.

Fifthly those who know and discrimate. There are quite a number of criteria which have to be considered. User's infomred preferences. User's previous experience. User's prejudice. Size of dwelling: Space v. cylinder benfits. User's HW usage patterns. Instant Availability v. Storage heat losses. Number and size of simultaneous or back-to-back HW demands (Showers v. baths.) Quality of mains supply. Pre-Existing arrangements and cost implications. ...

[Set Square]

I'm not sure into which category I'm deemed to fall, having questioned the wisdom of converting to a combi!

Although I have a bias against combis in principle, there are circumstances where they are appropriate - when starting with a clean sheet. For example, my 95-year-old father-in-law lives in a bungalow which is heated by electric storage heaters, with the hot water being heated by an immersion heater. I would desperately like him to ditch the storage heaters and go for a decent gas-fired heating and hot water system. [Gas *is* available, even though he doesn't currently use it]. A combi would suit him down to the ground.

However, in cases where a system already exists - using a conventional boiler and stored hot water - there would, in my view, have to be some pretty exceptional circumstances to warrant converting to a combi-based system.

[Ed Sirett]

A typical dwelling where a combi might well be the best approach is:

A 1 bed flat built in 1975 which has a Arcopack (sp?) and a life expired conventional boiler. The Arcopack unit is pre-plumbed steel contraption with a steel CW cistern and an insulated copper HW tank/cylinder) in the lower section. Typically a small F&E header tank is provided inside the upper part of the cistern for the primary circuit. Flow rates to the bath are moderate and there is no (I don't mean little) pressure for a shower.

The mains is plastic but only 1/2"/20mm. Good enough for a combi but debatable for unvented.

The cupboard space is relatively more valuable in a small flat. The maximum occupation is likely to be a couple + very small kid.

You will not likely rent out any such flat in the middle market private sector without a shower.

The combi will only be a little larger than the old boiler.

Booster pumps can cause noise problems leading to serious conflict between neighbours.

The case for a combi is fairly strong in the above scenario.

[Set Square]

Agreed - there will be specific (dare I say "exceptional"?) circumstances where a combi is the appropriate solution.

However, *in general*, I believe that replacing a conventional boiler with a combi *doesn't* make sense.

[Ed Sirett]

You may well be right. Perhaps I see a somewhat skewed picture of UK housing since a big part of my work is middle to up market private rented flats.

[Mike Mitchell]

Okay, it's time for a complete CH novice to butt in here and ask a few questions, as gas heating boilers may well be of interest to me soon.

What is a combi boiler and how does it differ from a conventional boiler?

I have an airing cupboard, a hot water tank and my boiler has a pump. How/why does the combi boiler get rid of both the latter?

What are the fundamental differences in two properties' heating systems, one with a conventional boiler, and the other with a combi boiler?

MM

[Christian McArdle]

The difference between the (instantaneous) combi and the non-combi boiler is that the combi boiler has a heat exchanger that heats the incoming cold water as you need it. It does this without any tanks or hot or cold water storage. Advantages are that it uses the full mains pressure, so you get excellent showers (assuming a reasonable water supply). Unfortunately, it will heat only so much water at a time, so bath filling tends to be slow. They are a good choice for houses where the bath is used infrequently, as they are cheaper, smaller and more efficient than a storage based solution.

It gets rid of the external pump, because they almost invariably have an internal one, making the external one redundant. Even most non-combi boilers have a built in pump these days.

Christian.

[Mike Mitchell]

Ah, right. I couldn't do without my baths, though! I lived with a shower for 12 years (i.e. no bath) and the joy of a relaxing, piping hot bath, especially on a cold winter's morning, is irreplaceable.

MM

[Christian McArdle]

Don't get me wrong. They will still fill a bath. It's just that if you have one daily, you may find the wait too long, unless you develop a habit, such as getting breakfast whilst it is filling.

Christian.

[Stephen Gower]

I know Christian knows this, but to help clarify: They are a good choice for houses where the bath is used infrequently, or where bath-taking is seen as a slow, relaxing affair and the 10-15 minutes to fill one can be taken to prepare oneself for having a bath (this seems to take Mrs Socks about 25-30 minutes, so 10-15 minutes of that time simultaniously having the bath filling isn't a problem!).

[Andy Hall]

.. and using it after lunch :-)

.andy

To email, substitute .nospam with .gl

[Ed Sirett]

See main FAQ. And the other ones too.

[Ed Sirett]

If the bath is a small swimming pool. :-)

[John Stumbles]

Or other small pool occasionally found indoors ;-)

[timegoesby]

IMM did an excellent post on the differnt types of combi's. Do a Google. I did a Google on Alpha CB50 and it came up. Read this first before you go any futher.

[timegoesby]

There are higher water flow combi's around. The Alpha CB50 I mentioned in my last post is one. The larger combi's fill baths as fast as any storage system and are far easier to fit and get rid of tanks. I know of a few higher water flow combi's and the owners would not have anything else. Don't buy the small flow rate combi's, get a large flow model if you want fast filling baths.

[Dave Plowman (News)]

You are IMM and I claim my 5 pounds.

[MBQ]

Combis are like that other favourute, hot air heating. It's so easy to end up with a bad installation. I have yet to come across a combi (thankfully I have only had experience of 5) which performs well either in terms of flow rate or temperature stabilty when other taps are opened/closed.

MBQ

[Fred]

But this is just not true for most decent combis. Here, I live in a tank system house. The hot water is a hot dribble really, the pump is only used for the shower and sounds as if I have a Moto Guzzi in the house. While I have seen combis in various locations in the UK where the hot water at mains pressure. This results in a high flow at least in areas suitable for combis, not to true for other areas where combis should not be installed - ask your plumber. I usually take a bath after doing the dishes in the evening, my bloody tank is not big enough and with the heating on, it takes forever and a bit to fill the tank. Not an issue with the combi, where hot water is on demand.

No guys, where combis can be done (depending on mains pressure), they usually beat a comparable tank system flat out. Well, to be honest, I have never seen a new tank system, but plenty of new combi systems here. And the thing with the cold water tanks in the loft dribbling through your house makes you feel as if your buddy has a chronic prostate problem ;-) Oh and for hygienic apostles amongst us all, take a look in the cold water tank, then think about the hot water tank, where hot water stays for days. And then count the bacteria. A combi basically serves up drinking water (well, I would still not use it for tea though ;-))

Right. Final issue: water temperature. The combi's performance depends on the distance between boiler and bath. The closer the hotter the water. The tank has the same issue (hence its usual location in the bathroom) but, BUT, a mildly old tank system will loose heat = the hot water is not of the same temperature. Kind of sucks when you keep your hot water at a low temperature only to find out that your bath is lukewarm because the water was sitting in the tank for a day or so.

I know it comes down to personal preference, but don't sell combis as having problems to fill a bath. With any decent combi that just is not the issue.

Fred

[Dave Plowman (News)]

You've not seen a reasonably well done storage system either. Like everything, it requires care and skill to install - more than an equivalent high pressure system since poor pipework will restrict the flow more.