I am in the process of getting quotes for supplying and fitting a new central heating boiler. We live in a largeish 4-bed detached house (i.e. not a one-bed flat) with 14 radiators in all.
I know that I have to have a condensing boiler (none of the legal letouts apply), but I am unsure about whether to have a conventional condensing boiler or a combi.
Can anyone summarise the pros and cons of combis, or point me to sites that may help. One supplier insists I should go combi, another says conventional.
Assume that I know next to nothing about central heating.
I'm interested in the practicalities of the different types of boiler, not the theoretical aspects. For example, for combi/conventional:
What kind of servicing/maintenance/supervision is required throughout the year? (E.g. top-up, pressure check) -- with my present old clunker of a boiler, I don't think about it from one year's end to the next, except for the annual "service".
What do I do if the radiators go cold at the top (should this ever happen in a sealed system?)
If the boiler is installed in the loft, must I have a permanent ladder/steps, and electric lighting in the loft?
What happens if the incoming water pressure drops?
Oh dear... a google back on this group might be worthwhile, but you have just asked one of those questions that non unlike boilers may generate more heat than light!
No different. Annual service. Assuming you are going for a sealed system, then the occasional check on pressure. Ed's sealed system FAQ has lots of good information and is well worth a read:
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2. What do I do if the radiators go cold at the top (should this ever
Bleed them, as you would now. You may need to top up the pressure a little afterwards. If the system has no leaks, and is running with corrosion inhibitor in it, then you should not get any accumulation of gas in the system once any air introduced with the fill water has all been purged.
You don't need a permanent ladder or steps. You do need a floored section, lighting, and guard rails round the loft hatch to prevent anyone falling down it.
Pressure is less important than flow rate - although one will have a knock on on the other. You need to check you have 20lpm available from the rising main - any less would be getting marginal. If you have inadequate flow then a combi would be a poor choice.
Assuming the storage system is well lagged etc then they should be comparable. Note that some of the cheaper combis will not run in condensing mode when supplying hot water, so these would prove less efficient. (the HW supply is not included in the SEDBUK efficiency rating and hence you would see no difference here).
Loads probably... there will be a flame war along shortly to enlighten and confuse you ;-)
You will find that there are strong proponents of both storage systems and "instant" systems, who are incapable of seeing an alternative point of view. You will need to take a balanced view as to what is most suitable for your needs however.
Most of the arguments about stored Vs Combi stem around the delivery of hot water. In general a stored system if correctly designed and implemented will supply hot water faster - however whether this matters to you will depend on your usage patterns.
While there are some that will claim "high flow rates are available" you need to keep in mind that the laws of physics are not relaxed for boilers of any sort. A stored system can deliver hot water at a rate that would require an instantaneous heating rate well in excess of 100kW to achieve with a combi. You gas supply is unlikely to be rated for more than 60kW shared between all appliances, so go do the maths!
I think that most of the issues you've raised apply equally to combis and conventional boilers, so don't really affect the choice. For example, if you go for a conventional boiler, it would make sense to have a sealed rather than vented system - so the issues of bleeding rads and topping up the pressure are the same. The only real issue is the mains water supply - which needs to be really good (like over 20 lites per minute) to be viable with a combi.
The main practical difference is the issue of stored vs 'instant' hot water. Stored hot water takes up space, but can fill a bath very quickly - and provides a slightly warm area (even when well lagged) for airing clothes. A combi will give you mains-pressure showers, but instant hot water can only be produced at the lower of the rate at which the mains can deliver cold water and the rate at which the combi can heat it. Even a high powered combi will take longer to fill a bath than using stored hot water - particularly in the winter when the incoming water is very cold.
There *is* a way in which you can have the best of both worlds if you wish. Although a combi is designed to produce instant hot water and to heat your radiators, there's no reason why the central heating side can't be zoned to provide space heating *and* stored hot water. So you could use the HW side of a combi to provide instant hot water to (say) your kitchen sink and a mains pressure shower, whilst retaining your stored hot water system for bath filling.
They have only two things goind for them. Since they don't require a hot water or cold awater storage tank, they are CHEAP and take up LESS SPACE
In all other ways they are totally inferior to a poroper pressurised hot water system.
- they can't handle peak flow of hot water to do more than one shower or one bayh or one sink at a time
- when they do, the central heating stops working.
compared with unpressurised hot water system that they often replace, they can't fill a bath any better but they do have enough pressure for a minimal un-pumped shower.
They are a cheap crappy way to make a heating system for a small property that is just about acceptable for a bachelor flat or two people, but not for a familiy.
Attempst to add 'heat banks' to them result in them becoming identical in performance to a proper pressurised hot water tank, at slightly more costs..
The same. All bopilers SHOULD have a service once a yera. I never bother franjkly.
All bolers running at or near mains pressure primaries wil need re pressurising occasionally. It takes about 5 minutes. Its very DIY.
Bleed the bastards
Then re pressurise. It always happens, though inhibitor poured in helps a lot. Its part of my 'autumn heating checkover'
Blimey. No idea.
Nothing.
No different between combi/non combi. Well comnbi fanatics will arguye that combis don't lose heat from the hot water tank, and anti- combis will admit that a combi means never ever having a hot bath or a shower woith stayiung inde for more than 2 minurtes, so I suppose a combi is cheaper..you get what you pay for ;-)
DR Evil. Ignore all his advice. I think he is also known as Cicero these days.
I think your questions 1-5 have been answered by the other replies.
I'm not an expert, but have gone through this myself recently. In making a choice you need to consider many things like cost, installation and efficiency.
Combis:
- Cheaper to install.
- No header tank required.
- Does not need airing cupboard.
- Takes longer to fill bath.
- Hot water at (almost) mains pressure.
Conventional:
- Hot water at lower pressure.
- Quicker bath filling.
- Showers may need boosting.
There are other types of system which store water at a higher pressure or use stored water to heat the hot water which you may wish to consider.
"news" wrote in message news: snipped-for-privacy@care4free.net...
Here is a run down on combi's:
Firstly, a combi is a "combination" of the heating and water system in one case, eliminating external tanks and cylinders, and generally supply hot water at high main pressure. To confuse a little, some can run at very low pressures and even off tanks. Generally most are fed from the mains. It is generally a matter of mounting the boiler and connect up the pipes. The expert designers have done the hard work for you and put all in one case.
Types of combi:
1) The Infinitely Continuous Combi -
Heats cold mains water instantly as it runs through the combi. It never runs out of hot water. This is the most common type of combi, generally having lower flowrates than Nos 2 & 3 below. The largest flow rate instant combi is a two bathroom model, 22 litres/min ECO-Hometec. Being a condenser it is very economical too.
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Unvented Cylinder Combi -
An unvented cylinder is a similar to a conventional cylinder but run off the high-pressure cold mains. A combi with an integral unvented cylinder has approx 60 litre cylinder heated to approx 80C, with a quick recovery coil that takes all the boilers output. A fast acting cylinder thermostat ensures the boiler pumps heat into the cylinder ASAP with a recovery rate from cold around 5-8 mins (Ariston claim 8 mins). The 80C water is blended down to about 45-50C. e.g's, Ariston Genus 27 Plus, Glow Worm, Powermax, Alpha CD50.
An example being the Alpha CD50, a combination of both having a two stage flowrate, of high flowrate when using the stored water with an automatic flow regulator switching in to reduce flow to an infinately continuous flowrate of approx 13-14 litres/min.
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Heat Bank Combi -
Incoming water is instantly heated running through a plate heat exchanger (as is most instantaneous combi's) that takes its heat from a "domestic hot water only" store of water at approx 80C (instantaneous combi's take the heat from a heat-exchanger heater via the burner). A fast acting thermostat ensures the boiler pumps all of its heat into the store ASAP with a recovery rate about 5-8 mins from cold. The 80C water is blended down to about
45-50C. They are generally two stage flow rates, in that when the thermal store is exhausted it reverts to what the burner can produce, which is approx 11-12 litre/minute. e.g. Vokera & Worcester floor standing models (standard washing machine sizes).
N.B. The heat bank is a variation of a thermal store, but is "not" a thermal store in the conventional sense in that a coil carrying cold mains water runs though a store of hot water kept at about 80C. Heat-banks are far more efficient and give higher flowrates than conventional coiled thermal stores. The stainless steel plate heat-exchangers do not scale up so easily.
5) Combined Primary Storage Unit
(Not classed as a combi, but a derivative of a combi, but still a one box solution, so still in the same family)
These are a combination of a large thermal store, or heat bank, and boiler in one casing. The units are large (larger than standard washing machine size) and floor mounted. The heating is taken off the thermal store, which in many cases the DHW taken off the store using a plate heat-exchanger (heat-bank). Unlike the Heat-bank in 3) above the thermal store supplies heating "and" DHW, giving the "combined" to the title. They are available from 1 to 2.5 bathroom models. Gledhill do an excellent condensing version, the Gulfsream 2000.
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2), 3), 4) & 5) have high flowrates. No. 1 "generally" has low flowrates but there are always exceptions and some can be high - e.g. the ECO-Hometec infinitely continuous combi, actually has a very high flowrate. Nos 2), 3), 4) & 5) use stored water, but in different ways. Unlike No. 1 "some" versions will eventually run cold, but that takes quite a time, hence some are referred to as "two bathroom" models, having the ability to fill two baths with very fast recovery rates. As hot water is being drawn off the high rating burner is also reheating. Very rare do these combi's run out of hot water in average use. When taking one shower the burner may be re-heating faster than what can be drawn-off. No. 3) above uses stored water but will not run out of hot water (high and low flowrates). Most versions of No. 4) above are two stage flowrate models (high and low flowrates) and will also not run out of hot water.
There are combi models that give hot water and heating simultaneously as Combined Primary Storage Units do. Most don't as they are hot water priority.
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