Condensing-combi boilers

Sorry if this is off topic for the group but several people in cam.misc suggested you would be able to give better advice that they could. I do a lot of my own DIY (which means I'll probably keep this group subscribed now I've found it) but putting a new boiler in is too big a job for me (I wouldn't have a clue as to what needs connecting to what) so I'll be getting someone in to do it.
I'm about to have a new boiler in and the plumber is trying to persuade me to put in a condensing-combi boiler. Apparently they heat up the water on demand and are quite efficient but I've heard that they can be slow to get going. Anyone know if that is a problem?
Currently we are looking at Vaillant.co.uk boilers:
http://tinyurl.com/o1er
Has anyone got one of these (or something similar) and if so then is it any good?
Another thing I don't understand is the difference between the "high" efficiency (ecoMAX) and the "standard" efficiency ("turboMAX"). More efficient looks better on the face of it but what's the disadvantage?
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On Sun, 21 Sep 2003 01:16:20 +0100, Sapient Fridge

Do a search in Google Groups for Vaillant in this news group and you will find that people have good experiences of their products.
There are some general points if you are changing from a system with a tank in the roof and a hot water cylinder to a combi.
- These do heat the water on demand and save the space of the tanks. If you have a small house this can be an advantage.
- Because you are using water directly from the mains, there is the potential for greater water flow and pressure and being able to have showers without pumps.
However there are some issues to check.
- Make sure that the mains water supply is adequate. Do this by timing how long it takes to fill a container of known size at the kitchen cold tap. It is not so much the pressure when everything is turned off that matters, but the combination of pressure and flow under running conditions. If you are getting less than about 20 litres/minute then the supply may need to be upgraded from the main in the road. This can be inexpensive, can be costly, but you will have to pay. Bear in mind, that for a decent shower you need to have 15 litres/minute, and that if somebody else turns something on, the supply has to deal with that. There have been posts here where people have done this change, not checked the water supply and been very disappointed. It is even more important if you have a larger house and/or 2 or more bathrooms in simultaneous use.
- Make sure that the boiler is adequately sized to give the hot water flow required. Since the water is heated instantaneously, a lot of heat is required. Combi boilers are specified for hot water by a flow rate for a certain temperature rise - normally 35 degrees. For most of the time this is reasonable. However, in the winter, the mains water supply can drop below 7 or 8 degrees. If you consider that a shower is normally run at about 40 degrees, and the boiler will be producing water at 43 at its specified flow rate, very little cold will be being mixed in and you will effectively be limited by what the boiler can do. Tank and cylinder systems don't have this issue since the water is heated over a longer period and stored normally at 60 degrees. This is not necessarily a problem, but if you go for a small boiler at 11 litres/min, that may be all you will get in the winter, and it will also take a very long time to fill baths. The type of boiler that you mention varies its heat output for central heating purposes, so you can use a boiler that is larger than the heating requires in order to get the hot water performance needed. Again, consider the usage requirements in the house. If it's 2 bathrooms, this will need a lot more output.

There are explanations of this in the UK.D-I-Y FAQ as well as on numerous internet web sites.
There is a system used in the UK for measuring this which takes account of the climate and pattern of use. You can look up the SEDBUK web site for more details, but it involves more factors than just measuring energy in and energy out.
Older types of boiler of 10 or more years ago had efficiencies in the 50-65% range using heat exchangers of various technologies. Various improvements have been made to the designs, such as larger heat exchangers, fan operation and so on, which have taken the efficiency of conventional (meaning not condensing) boilers up to around the 80+% mark. There is now a requirement as a result of Building Regulations that requires all new boilers being fitted to have an efficiency of at least 78%
Condensing boiler technology achieves even greater efficiencies by controlling the boiler operating temperatures where it can to run at lower temperatures - principally for the central heating. By doing this, they deliberately arrange for the water (effectively in the form of steam) in the flue gases to condense inside the boiler, either to water or to water vapour (remember that what you see from the kettle is water vapour - steam is invisible). At any rate, the change of state from steam to water/water vapour cause a release of energy (the latent heat of condensation). This energy is effectively added back to what the boiler is producing. Thus, you get an even greater efficiency - of the order of 90% on the SEDBUK scale. There is a table of different boilers on the SEDBUK web site. Fairly soon, there will be changes to the minimum requirements in the Building Regulations and condensing boilers will effectively be mandated anyway.
The UK boiler manufacturers, through conservatism in the industry in general, were slow to produce condensing boilers. They have been in use in the rest of Europe for 15 years at least. It is necessary to design the boiler with heat exchangers and other components able to handle the condensate water produced. This is mildly acidic and the early attempts of UK manufacturers were not good, leading to reliability issues. You do find a lot of installers not wanting to recommend condensing boilers because a) they don't understand them and b) might have had bad experiences in the past, or more likely heard old wives tales from the other old women at the plumber's merchants. Manufacturers from the rest of Europe have been making these for a lot longer and are on their 3rd or 4th generation, while the UK manufacturers are typically on their 2nd. Therefore, in that sense, purchase of a German product like Vaillant is a good move.
There are a couple of issues to be aware of with condensing boilers.
- A condensate drain has to be provided. This is usually plastic pipe (e.g. overflow pipe) led to a drain inside or outside the house. It should go to a drain and not just onto a patio.
- When the boiler is in condensing mode, which won't be all the time, there is a plume of water vapour from the flue. This needs to be directed sensibly away from the building. Sometimes it is referred to as a "nuisance plume". Personally, I haven't found it to be an issue. In any case, the flue gases from conventional boilers, when the weather is really cold and they are running at full blast will condense to form water vapour as soon as they hit the cold air. The condensing boiler will do it even on a summer afternoon.
.andy
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"Andy Hall" wrote | Sapient Fridge wrote: | >I'm about to have a new boiler in and the plumber is trying to persuade | >me to put in a condensing-combi boiler. | These do heat the water on demand and save the space of the tanks. | If you have a small house this can be an advantage. | However there are some issues to check. | - Make sure that the boiler is adequately sized to give the hot water | flow required. ... Tank and cylinder systems don't have this issue since | the water is heated over a longer period and stored normally at 60 | degrees.
Tank and cylinder systems also offer the option of a back-up immersion heater at negligible extra cost for if the boiler ever breaks down. And if there's a power cut there's at least a hope you have a cylinder of stored warm water to tide you over.
Owain
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If back up is really an issue then feed the output of the combi into a small instantaneous electric heater. These are not expensive. They will supply one tap or one shower. OK for backup until the combi is back up. see: <http://www.tlc-direct.co.uk/Products/TTTW10I.html
Screwfix now do one.
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"IMM" wrote | "Owain" wrote | > Tank and cylinder systems also offer the option of a back-up immersion | > heater at negligible extra cost for if the boiler ever breaks down. | > And if there's a power cut there's at least a hope you have a cylinder | > of stored warm water to tide you over. | If back up is really an issue then feed the output of the combi into a small | instantaneous electric heater. These are not expensive. They will supply | one tap or one shower. OK for backup until the combi is back up. see: | <http://www.tlc-direct.co.uk/Products/TTTW10I.html
110 + VAT plus 10mm cable at 1.60/metre is expensive compared to about 12 for a 3kW 36" immersion heater + thermostat on a cylinder.
And the unit you suggest is only suitable for handwashing purposes or low fill rates to a sink. It cannot supply two taps simultaneously, and is also not suitable for basin/sink mixer taps, shower mixer valves, or bath taps. And 9.5kW is only going to give a max of 4 l/min at below 40C output temp under winter ambient conditions. (All according to the product manual.)
More importantly, it will give you nothing if there is a power cut. Depending on water use and reheating patterns, a cylinder can have a day's or more warm water stored. It may not be a big issue for the OP but it is worth considering.
Owain
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"> > Currently we are looking at Vaillant.co.uk boilers:

Potterton have made one or two shockers (Suprima springs to mind). Vaillant do have a good name. I've been very satisfied with some Glowworm & Ravenheat combis that I've used. BTW the usual experience on this group with plumbers seems to be a reticence to recommend/install condensing combis because of a lack of knowledge - it would appear that you have found one of the few who has taken the time/trouble to keep abreast with current technology. The rest will all have to follow suit shortly as new energy efficiency requirements will effectively compel the use of condensing technology.
Neil
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Avoid Raventheat combi's. Their condensing CSI condensing boilers are fine.
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Andy covered most, except, as usual, that high flowrate combi's can be had. The most common 10-11 litres/min are fine for a one bath house that uses showers a lot. If you want a zippo bath fillup then 16 litres/min and above is what you go for. Alas, they are usually more expensive.
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I actually did say "make sure that the boiler is adequately sized" and then went on to point out that a modulating burner for CH allows a larger boiler than might intuitively chosen for the house to be used.

10 l/min for a shower? Pretty poor shower in my view.

.andy
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Not in the winter you don't.
Do the sums. If the water temperature is 5 degrees and the boiler provides an uplift of 35 degrees then you won't be able to add any cold when using a shower. A more typical average winter water temperature is only about 8 degrees or so, and you won't be adding much then.

Not in my view. 15-20 is more reasonable.

Environmentalists are concerned about a lot of things - not all of them having any merit.

That really depends on your definition of necessary. Nobody said that the shower had to be run for a long period of time or even at the same rate all the time.
The reason for hose pipe bans and most of the other deficiencies of the water supply system have to do with decades of lack of investment in the infrastructure.

.andy
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wrote:

That is true. Friends of the Earth are front for large landowners, with the aim of them keeping their lucrative acres at our expense.
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Like, for example, this summer?
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Sapient Fridge wrote:

The turbomax is a good non-condensing version of the Ecomax. Vaillant unfortunately have a HUGE difference in the price between the condensing and non-condensing versions. I for one would be prepared to 'branch out' to another make if I had to fit acondensing combi. I guess that I'd go for the Glow Worm or maybe the Ideal.
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I looked into the Big Green Boiler scheme and the cost of a Vaillant condensing boiler is not that much more under that scheme than for a non-condensing boiler. Very nearly went for it, but I wanted a rear exit flue, so had to get the non-condensing version. Incidentally, I think that was a big mistake. If I was doing this again, I would get the top flue version. (I can explain why if anyone is interested.)
Geoff

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Very nearly went for it, but I wanted a rear exit

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around, to understate the point
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writes

Baxi and Potterton are separate ranges Baxi bought Potterton and inherited the crap. In doing so they tarnished their decent models.
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