C/H - Combi, condensing or conventional?

Oh fount of all knowledge,
With my back boiler dead, I would like to replace the complete C/H system here. I would prefer to put the boiler (and storage tank, if necessary,) in the loft space out of the way. What, in the groups view, is the best boiler type these days? I have googled around for a while and remain as confused as ever! Are condensing units still unreliable? How do purchase costs compare between the a combi and a condenser (when comparing like for like in terms of thermal output)? Are there any advantages of buying a system from British Gas or would I be better off just getting a local Corgi man in?
Any recommendations for installers in the Gloucestershire area?
Any/all advice most welcomed!
HNY & TIA
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Mark


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wrote:

No they are not. Only some of the early generation UK designed and manufactured ones were. Generally German and Dutch manufactured ones have been good anyway, and now most of the UK manufacturers are as well. The important thing is to buy a good quality product which will typically have a stainless steel heat exchanger. Don't buy on price at the bottom of the market.
The UK heating industry is conservative and some installers still will not supply them. This is largely based on old wives tales that they hear at the merchants and if they are not familiar with how to install a condensing boiler - this is not rocket science however. The only additional item required is a drain for the condensate.

The economic equation can be worked out approximately from looking at www.sedbuk.com.
Your existing boiler is likely to have an seasonal efficiency of around 65%. A new conventional boiler will have one of 78-80% typically - there is a legal minimum of 78%. Condensing boilers have an efficiency on the same scale of around 90-91%. I have done an exchange from a 65% to a 90% efficient boiler and the gas savings have been around 25-30%.
The cost difference is becoming less and less but typically around 10-20% more for an equivalent condensing boiler of the same output *and* build quality. Don't make the mistake of comparing a cheap conventional boiler with a good condensing one like a Vaillant.
You need to look at the cost equation for the period of time that you intend to keep the house. Probably if you only plan to be there for two years a condensing boiler will not pay back its cost. If you plan to be there for a while then it is worth making the investment in a better quality condensing model.

None at all. They simply charge a lot of money for a system and there is no guarantee of a better job. They sweeten it by giving two years of "maintenance", which in practice is not worth having.

Probably. Look for Heating Engineers though and then as part of the selection process ask for references.

.andy
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use for council housing.
They should have filtered out the cowboys and have a reliable, fairly efficient company. Whether you can get a good deal, and whether their response time would be acceptable when they have a significant work load, I don't know, try it and find out. It must at least be better than picking a company who probably have their pet source of references.
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geoff

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Then choose anybody else.

Splorf.
Ask other people who've had similar work done. Or find out who your local factoring agents use.
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Niall


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"Niall" wrote | geoff wrote: | >Just a thought, phone up your local area authority and find out | >who they use for council housing. | Then choose anybody else.
*anybody* else. Even if they don't have a CORGI card :-) | >They should have filtered out the cowboys and have a reliable, | >fairly efficient company. | Splorf.
Something like that. | > Whether you can get a good deal, and whether their | > response time would be acceptable when they have a | > significant work load, I don't know, try it and | > find out. It must at least be better than picking a | > company who probably have their pet source of references.
Instead of picking a company run by some councillor's relative ...
Owain
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geoff wrote:

What an excellent tip! Thanks Geoff.
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No.
You may be getting confused. The "combiness" of a boiler has no relationship to the "condensingness" of the boiler.
A condensing boiler is more efficient than a non-condensing type, but produces a certain amount of steam plume, particularly when the flue is short. With a loft mounting, this plume is likely to be much less of a nuisance. The emissions from the boiler are actually much cleaner than a traditional boiler. It is just that the temperature is much lower, so it is more visible that people get annoyed. Non-condensing boilers will shortly be banned.
A combi boiler replaces your hot water cylinder with instantaneously heated water at mains pressure. This gives excellent shower performance, recovers space that would be taken by a cylinder, is more energy efficient and cheaper to install.
However, there are major disadvantages too. It takes longer to get hot water at the tap (some boilers have systems to remove this disadvantage at the expense of some energy efficiency). It produces much lower potential flow rates, leading to slow bath filling. It will only work if there is good pressure and flow rate available from your mains water supply.
If you decide that the slow bath filling is an issue, then you should install a storage system. When designing a system from scratch, you should really install a mains pressure system (provided that the mains is up to it). You have a choice of installing a mains unvented cylinder, or a heat bank. Both will provide oodles of hot water at mains pressure, with many times the flow rate of a combi.
An unvented cylinder is likely to be easier to get hold of. A larger proportion of installers will have heard of them and know how to install them. They provide the highest flow rate.
A heat bank has the advantage of inherent safety and doesn't need the elaborate precautions and regular servicing that an unvented cylinder needs. This is because it doesn't store the hot water at mains pressure, but instantaneously heats the incoming water from the stored zero pressure water. This makes an explosion much less likely, so there is no need to use an approved installer, or have regular servicing for the safety valves and vessels.
Some boiler manufacturers produce boilers with integral unvented cylinders or heat banks. If you see a "combi" quoted with more than 40kW, there is a good chance that it is not actually a combi at all, but a preplumbed combined heat bank and boiler system relabelled at a combi for marketing reasons.

So, you need to answer the following questions:
1. How many baths and showers do you have? 2. How many people live in the house? 3. Does anyone prefer baths to showers? 4. What is the maximum flow rate at your kitchen sink? (use a bucket and stopwatch to measure). 5. How happy were you with your old system when it worked? (I presume it was probably a gravity hot water cylinder fed by gravity circulated water from the back boiler).
Christian.
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Christian McArdle wrote:

1 bath, 3 showers

5
Yes, I prefer a bath, the wife, a shower. The kids prefer neither ;-)

150 galls/hr

I wasn't. Cold rads, and it took a long time for hot water to reach the kitchen.
Thanks for all the info!
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Mark


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Well, you definitely wouldn't get away with a non storage combi with that lot.

I'm worried that this is a bit low. I make this 11.4lpm. This basically isn't enough for a decent mains pressure system. However, it may just be that you have a tap that can't pass more water than that (many modern taps have narrow passages and are flow rate limited) and that the supply isn't actually that bad. You'll certainly want to investigate further to ensure that the real potential flow rate is higher. If you can get at least 20lpm, then mains pressure will be OK. 40lpm and it will be good.
You may need to turn off your stopcock, and remove a section of pipe and measure flow rate from there to get the real story. Although a bit drastic, it is better than spending thousands of pounds on a system that turns out to be inappropriate.

If you intend to install a new boiler in the kitchen, and the mains pressure turns out to be OK after all, then I'd recommend installing the heat bank/unvented cylinder where it is now, presumably near the bathroom. Then install a combi boiler in the kitchen and run just the kitchen hot tap from there. As there won't be loads of pipework between the source and the tap, the hot water should come quicker. A combi version of a boiler is usually only a few quid more than the system equivalent, so there would be little additional cost.
If the flow rate really is bad, I'd recommend replacing the boiler with a modern condensing system type. Replace the old cylinder with a modern Part L, and convert to fully pumped Honeywell S plan. Fit a quality brass bodied heavy duty shower pump (such as Stuart Turner) off a flange at the cylinder and feed this to all the showers and the bath. Other outlets should use the unpumped hot to avoid running a noisy pump in the early hours when washing hands (or clothes). This may require additional pipework.
Christian.
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wrote:

If these are run concurrently you would have a high demand on the system.

This suggests concurrent use or will do as time goes on.

They will when they are teenagers.

I make this about 11.4 litres/minute. (there being 4.55 litres to an imperial gallon).
You really need at least twice that for any mains driven system or the results are going to be disappointing. You may need to consider getting the service pipe upgraded from the road.

.andy
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Check all the stopcocks/isolators in the line are fully opened up. I measured a disappointing 12 l/min in my kitchen after installing a new combi in a new location, but this increased to 19 l/min after I opened up the brass isolating tap that fed that spur by a few more turns. It had been opened only by a couple of turns during my initial tests.
Andy.
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andrewpreece wrote:

Thanks Andy, I was going to check this over the weekend anyway, (particularly after Christians comments yesterday) but a good tip nonetheless.
Cheers!
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Mark


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Put a condensing Potterton Powermax in the loft. All one unit and will do CH and DHW with high tap flow rates. http://www.powermax.co.uk
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