pros and cons of a combi boiler

Discussion in 'Plumbing and Central Heating' started by bernardgreen, 29 Jul 2015.

  1. bernardgreen

    bernardgreen

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    My boiler fires up once a day to heat two tanks of hot water. (*) Maybe twice if a lot of hot water is used.

    A combi would be firing up every time I used any hot water.

    I would have thought that firing up, short burn and shutting down many times each day would cause more wear and tear on the boiler's components than one or two long burns per day.

    (*) The layout of this cottage makes having two tanks, one in the bathroom and the other in the kitchen less wasteful of water in long pipe runs.
     
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  3. Dan Robinson

    Dan Robinson

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    Standing losses.
     
  4. bernardgreen

    bernardgreen

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    If by that you mean the loss of heat from the tanks then I consider that to be slow release heating that takes the chill of the cottage on a cold day so is not wasted heat.
     
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  5. Dan Robinson

    Dan Robinson

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    That's great mid July.
     
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  6. bernardgreen

    bernardgreen

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    We have had cold days in July. :mrgreen:

    But seriously being a very old stone built cottage with no damp proof a small amount of back ground warmth is advantageous.

    This morning I had a long hot shower even though the boiler was last fired yesterday morning so the heat "loss" is very low. There is still plenty of hot water.
     
  7. Dan Robinson

    Dan Robinson

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    Are you asking the question in the context of your house and lifestyle? Or in general?
     
  8. Agile

    Agile

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    The reality is that a modern cylinder has a loss of about a couple of kWh a day and that costs about 10p when heated by gas.

    A combi has to heat itself up every time it provides hot water and the waste from that could easily exceed the standing loss from a cylinder.

    In your case there is also the heat loss from hot water pipes to be added into the equation as well as the loss of cold water while waiting for it to get hot at the tap.

    Tony
     
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  9. bernardgreen

    bernardgreen

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    In general. partly prompted by a friend who is about to get gas supplied and hence replace his aging oil fired boiler. He is between two heating "experts". One is promoting combi and the other is strongly advising heat only.
     
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  11. mogget

    mogget

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    I have a vented hot water system and like the immersion backup. Also should the mains water go off without notice (which it has done a few times, but not for the last few years) there is some stored water available which can be boiled in a kettle.

    Disadvantages are the low pressure and the space the tanks take up.

    I would have thought a combi with preheat turned on would lose just as much heat as a modern cylinder.
     
  12. plugwash

    plugwash

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    My experiance of having a combi in the flat I rent (installed just after I moved in due to the heat exchanger in the tank failing) is it takes ages to fill a bath. There are also issues with the cold tap starving the boiler and combined with a mixer tap on the bath and the fact that the output water temperature from the combin varies with the flow rate makes getting the temperature right practically impossible unless you run the hot and cold sequentially. Whether the problems are because the boiler isn't big enough, because of poor installation* or what I don't know. Preheat seems pointless, it results in the boiler firing up periodically and doesn't make any noticable difference to the time to get hot water out of a hot tap.

    * The combi installer used the existing pipework which combined with the layout of the plumbing in the property means that the hot water takes a much longer route than the cold water and much of that route is in 15mm pipe.
     
  13. Razor900

    Razor900

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    Or more sensibly a combi in the kitchen to supply heating and hot water to that tap and a cylinder in the bathroom would be a far better solution :whistle:
     
  14. bernardgreen

    bernardgreen

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    The kitchen hot water tank also supplies the washing machine and a hot tap in the "outside" toilet ( used by the person who rents one of my outbuildings ) hence if I had a combi then the boiler would be firing up several times a day. And the internals of the combi are more complicated ( ie more parts to go wrong ) than the internals of a simple heat only boiler.

    So for my cottage what I have is the most sensible arrangement
     
  15. Dan Robinson

    Dan Robinson

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    Rubbish.
     
  16. vulcancontinental

    vulcancontinental

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    You can't use your property as a basis for a question regarding a friends property alteration. Too many variables and unknowns.

    Cold main pressure, cold main flow rate, position of combi, heat only, hot water cylinder and relation to outlets, daily hot water useage for your friend, occupancy, what and how many appliances, what outlets, backup (electric immersion heater or shower if boilers break) has he to change the whole system or just the oil boiler?

    What's an alternate thinker?
     
  17. bernardgreen

    bernardgreen

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    :?:

    A modern heat only boiler has one heat exchanger, (*) a variable speed fan, a modulating gas valve and a few sensors to ensure the unit is burning gas in a safe and efficient way and that nothing is overheating. Pump over run control is also provided for the pump which can be fitted remote from the boiler.

    (*) the single heat exchange may of course be a two pass one with a condensing section in the flue gas and the main section in the combustion area.

    A combi has all those and quite a lot more all packed into it.

    For a start in the combi there are two heat exchangers and a divertor valve of some sort to determine whether the heat goes to hot water or to central heating loop.

    Then there is a flow switch to detect when hot water is required to fire up the boiler when a hot tap is turned on.

    For some reason the circulating pump is almost always built into the boiler, probably necessary to pump the circulating hot water from the gas to water heat exchanger through the water to water heat exchanger that is needed to heat the domestic hot water.

    Then there are pressure relief valves and expansion vessels and again these are often packed into the case of the combi boiler.
     
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