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conventional boiler v combi running costs

Discussion in 'Plumbing and Central Heating' started by matz, 7 Aug 2014.

  1. matz

    matz

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    sorry, hope I'm not repeating a much asked question. At home we have a near brand new conventional condensing boiler running hot water cylinder and 11 rads in a 4 bed detached house. Family of 4 consisting mum and and and 2 kids. Sadly I think I made a mistake not installing a combi due to my predjuces against combis! I'm trying to convince myself of the wisdom of ditching the newly installed boiler in favour of a combi. Main reasons being we need a couple of decent showers (gravity fed currently) the hot water cyl needs replacing and the CH/DHW divert isnt working. These issues need money spending on them (showers need pumps) etc so to persuade me I'm looking for cost savings with conventional v combi.

    are there savings to be made per year with having a combi rather than conventional boiler and if so, how much?

    thank you :)
     
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  3. Dan Robinson

    Dan Robinson

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    If you have high hot water usage then a decent cylinder will be a much better option.


    Especially as the kids get older.

    Remember "decent" is the operative word.
     
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  4. matz

    matz

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    hmmm interesting. We all shower every day. Our water bill (metered) is £60pcm. hardly ever have baths. As I say, interesting reply as it points toward a cylinder being better value possibly for our usage....
     
  5. gas112

    gas112

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    Pumped shower can use as much as a bath in a reasonably short time so 4 people cylinder will need a good recovery rate and/or be a fair size.
    4 People using a combi for shower one after another in winter means the heating is off whilst HW is being drawn anything like my kids and thats25 minutes a shower.
    They both have there pitfalls and benefits.

    Savings ? debatable and time frame for payback of cost of changing will considerable especially as your boiler is already A rate efficient .
    As i say to people going for combis its all about convenience forget all about cost savings
     
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  6. Dan Robinson

    Dan Robinson

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    Good points 112... hence my emphasis on "decent". :D.

    The chosen controls and how they are used will have more of a bearing on things.

    The important thing to remember though is that with a combi, there is no backup. Not a problem for a single person or a VERY understanding couple. But for a family?


    However, a combi never runs out of hot water - for single outlet that is running.
     
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  7. CombustionGuy

    CombustionGuy

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    Unvented if water flows and pressures are correct. The offset cost of the installation if anything will take awhile to make any real saving by that point gas prices will be silly high and nobody will save anything.. So I wouldn't worry about it.
     
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  8. matz

    matz

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    thats why I didnt want a combi in the first place... what sort of cylinder constitutes "decent" ??
     
  9. St0rmer66

    St0rmer66

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    Ideally a "high recovery" cylinder then have the hot water timed to be on before the heating and left on until after any demand is likely to have finished; some people leave on 24/7 since they're generally well insulated compared to an old cylinder.
     
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  11. fluxeyfingers

    fluxeyfingers

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    I have a relatively new (vented) cylinder and it will keep hot for days if I don't draw any water off. So the standing heat loss from the cylinder is quite low. Also any heat lost will help warm the house in winter.

    If you get a combi, you'll be cursing it when it breaks down (which it eventually will).

    I inherited a combi when I moved in and couldn't stand the thing so went back to a cylinder.
     
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  12. matz

    matz

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    thanks all :) I think I need to get to grips with cylinder sizes relative to our usage. Presumably if you have a high recovery cyl then you dont need such a big cylinder, there must be a calculation somewhere that gives optimum size.
     
  13. fluxeyfingers

    fluxeyfingers

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    I worked it out in longhand, as I have a larger than normal bath I ended up with a 1200x450 cylinder. (assuming that I fill the bath with 2/3 hot and 1/3 cold and that only 2/3 of the cylinder is heated). It's actually more than I need, in practice there's enough for a bath plus a couple of loads of washing up. If there's going to be 2 baths in a row I hit the +hour button before running the first one.
     
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  14. Agile

    Agile

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    The advice above is based on UNVENTED cylinders which are directly fed from the mains water just as a combi is.

    The minimum mains dynamic flow rate is about 22 litres per minute @ 1.0 bar.

    The cost of the cylinder is little more for a larger cylinder so don't bother to fit a small one. The rule of thumb is 50 litres PLUS 50 litres PER person. So 300 litres for up to five people.

    Cylinders about £800 and installation about £500. But the mains water supply capability needs measuring before considering an unvented ( or a combi for that matter.

    An unvented will supply several outlets at a time unlike a combi which has a very limited flow rate.

    Tony Glazier
     
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  15. JohnD

    JohnD

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    A bath holds about 100 litres of water, so certainly no smaller. If you have two bathrooms in use at the same time, you can easily use 200 or more.

    A modern boiler can nominally heat a modern cylinder from cold to hot in 20 minutes (when I tested that, I found my boiler modulates down as the cylinder nears its target temperature, so it slowed down and took longer).

    It is said that a cylinder is cheaper for gas on hot water, because the boiler heats up, runs until the cylinder is fully hot, then turns off, rather than going on and off every time someone uses a hot tap. IME there is also less delay in hot water arriving at the tap, though there are some combi boilers which have a small internal cylinder.

    If you have a good water service pipe all the way to the mains under the pavement, you will probably get the best possible results from an unvented cylinder (Megaflo or similar). If not, it may even be worth your while digging a trench to lay a new water pipe in a larger size, especially if the house is old and the flow poor.

    Combis are relatively complicated, they have more to go wrong, and they do. If you have a cylinder you always have the option to heat it with an electric immersion heater when the boiler is out of action, though it will cost more and take longer. If you ever get solar panels, you can heat the cylinder free.
     
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  16. Dan Robinson

    Dan Robinson

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    A bath holds around 100 litres of MIXED water usually no hotter than 46 degrees. Not hot water at 60 degrees.

    Combination boilers are no more complicated than a normal boiler and airing cupboard.

    If you're giving advice John at least make it accurate.
     
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  17. St0rmer66

    St0rmer66

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    Agree with Dan, the more you're storing the less efficient you're being. The whole point of a high recovery cylinder is that you get a quicker heat up and can store less in the first place as a result. Within moments of a draw off the boiler should kick in and start heating it, it won't be able to keep up with how fast you're running it off but it will last slightly longer and will be back up to temperature sooner.

    Combi's are not inherently "destined" to go wrong as long as regular servicing is kept up. I have a preference for hot water cylinders myself, but my house had a combi in when I moved in and putting in a cylinder would be a lot of upheaval and creating space for it etc so I just put in a new, more powerful, combi.
     
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