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Combi boilers have a number of potential drawbacks. Here are a few of them... (quickly cut and pasted from my old bathrooms website to save time).
"Reasons NOT to have a combi:
As you may have sensed from the section above, I don't really like combis! OK they work for many people but they have so many little drawbacks that it's hard to remember them all at once, but people keep asking me what the problems are so here's a quickly compiled list for starters...
1) They are obviously more complex than an ordinary boiler, so when stuff goes wrong, they are more difficult to diagnose and repair. (And they remind me of those TVs with video recorders built in - just a clumsy idea). Fewer heating engineers are prepared to attempt repairs to combis than ordinary boilers, which means it can take longer to repair a broken combi.
2) Water scale. They scale up just like an electric shower does. Then you have to buy a new heat exchanger or pay for a descale every year or three. Several hundred quid either way. (But note! The Worcester CD range of boilers have a 'plate to plate' heat exchanger designed to overcome this problem. Worcester even guarantee the plate heat exchanger against scaling for five years. I look after one of these boilers but I can confirm from personal experience that they DO still scale up. The difference is, Worcester seem happy to send us a free new one every 18 months under their five year guarantee, so if you MUST buy a combi, buy a Worcester!)
3) Latency. When you turn on the kitchen tap, it takes about 45 seconds for the hot water to arrive. Only after you turn on the tap does the boiler fire and start to warm up. It warms very quickly but it's still much slower than having a tank of hot water connected to your hot tap. I find this irritating, but many people do not.
4) They are BIG! Ok if you have a big house and plenty of room, but they are mostly fitted in little houses where they take up valuable space and look ugly.
5) Water pressure. They are great when your water pressure is high, but come the summer evenings when everybody puts their hose pipes on to water the gardens, pressure drops. And your lovely new combi stops working. There's NOTHING you can do about this, except complain to the water company and claim £10 statutory compensation each time it happens. Not much help there then unless you are happy with a tenner instead of a shower. Oh, and you have to produce evidence that it happened. Oh, and you can only claim the tenner once in a financial year.
6) Showers again. Performance of showers connected to combis is pretty average. Not bad, and not brilliant. Problem is, if you want a better shower, you are stuck. You have to fit a bigger combi, and then the performance won't be that much better. Adding a pump just doesn't work. The boiler itself is limiting water flow so it can deliver the temperature your shower needs.
7) Combi boilers have pressurised sealed system central heating circuits. This means that a very slow water leak from your heating system (and these are VERY common) will cause your system pressure to degrade to zero over weeks or months, and these leaks can be fiendishly difficult to identify and fix. The old way of having a header tank in the loft gets around this problem, but combis are not designed for use with them.
8) Redundancy. You need some! With a conventional boiler/hot water cylinder system if the boiler breaks down you can still have hot water by turning on the immersion heater in the hot water cylinder. When a combi breaks down your heating AND hot water are usually both out of action until it is repaired.
9)Nothing else that I can think of for now. But I'll be back with more soon!"
If you understand these potential issues and are happy they don't apply to you, or you are happy to take the risk to obtain the benefits of having no bulky water storage tanks in your home then there is no reason not to buy a combi!
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