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  • FIRST POST
    • Boiler Query
    • By Boiler Query 3rd Mar 18, 10:58 AM
    • 2Posts
    • 0Thanks
    Boiler Query
    Combi-boiler Vs System Boiler
    • #1
    • 3rd Mar 18, 10:58 AM
    Combi-boiler Vs System Boiler 3rd Mar 18 at 10:58 AM
    Hello all,
    I am in the process of changing our current combi-boiler. I was wondering if you could help/advise on the following;

    I generally understand the pros and cons of both systems in terms of size, initial out lay cost and room requirement, but for say a typical family of 4 in a three bed semi with all things being equal i.e. if they use the same amount of hot water and heating in a typical year, in terms of the running costs of gas and heating up the water and other losses, would it be cheaper to go with an A rated combi-boiler or A rated system boiler or is there no significant difference.
Page 1
    • Owain Moneysaver
    • By Owain Moneysaver 3rd Mar 18, 11:11 AM
    • 9,325 Posts
    • 11,194 Thanks
    Owain Moneysaver
    • #2
    • 3rd Mar 18, 11:11 AM
    • #2
    • 3rd Mar 18, 11:11 AM
    I suspect slightly cheaper with the system boiler as it won't be firing up every time someone runs a hot tap for 10 seconds.

    Also with a system boiler it's easier to have a backup for hot water in the form of an immersion heater in case the boiler breaks down.
    A kind word lasts a minute, a skelped erse is sair for a day.
    • grumpycrab
    • By grumpycrab 3rd Mar 18, 11:36 AM
    • 4,063 Posts
    • 1,954 Thanks
    grumpycrab
    • #3
    • 3rd Mar 18, 11:36 AM
    • #3
    • 3rd Mar 18, 11:36 AM
    I'm not an expert but in general I think combi boilers are a rip-off. Why? Installation cost versus reliability and life expectancy is really poor. If you can afford one get a system boiler.
    If you put your general location in your Profile, somebody here may be able to come and help you.
    • Anthorn
    • By Anthorn 3rd Mar 18, 1:31 PM
    • 4,294 Posts
    • 1,196 Thanks
    Anthorn
    • #4
    • 3rd Mar 18, 1:31 PM
    • #4
    • 3rd Mar 18, 1:31 PM
    I mostly live alone and I have a Combi boiler which provides hot water on demand as I need it and I don't store hot water in a tank. That's cheaper for me. But if I was a family of 4 I would have a hot water tank and store the hot water because there is less chance of the hot water cooling and having to be topped up with hot water.

    The one disadvantage in my case is in the consumption of water: The hot water has to be run in order for the combo boiler to heat it. That could be wasted water which is relevant for metered water.
    • CashStrapped
    • By CashStrapped 3rd Mar 18, 2:42 PM
    • 1,292 Posts
    • 725 Thanks
    CashStrapped
    • #5
    • 3rd Mar 18, 2:42 PM
    • #5
    • 3rd Mar 18, 2:42 PM
    Combis are actually often at their most wasteful for shot burst of hot tap water.

    When you use a hot tap, the combi boiler has to cycle up. Evacuate the combustion chamber, which uses a wodge of gas, start up the burner, heat the water and get it to the tap. This all happens in less than 10 seconds normally.

    However, in most quick hand wash instances, the hot tap fails to get the hot water to it before you have finished. So by the time it has done this, you have finished......

    -----

    I also typed out the differences between boiler types for another thread recently. I just thought I'd copy it in again, although I see you say you understand the differences.

    http://forums.moneysavingexpert.com/showthread.php?t=5802348

    -----

    Remember, all modern boilers are condensing boilers. Sot hey are all very efficient in themselves. A system boiler with a tank maybe have slight efficiency losses as you are storing water. However, modern pressurized tanks are very well insulated and have minimal losses. They are not like the copper tanks of old with a loose jacket.

    But, using a combi boiler inefficiently, or specifying one when unsuitable for the property can introduce greater inefficiencies.

    Then it comes down to personal preference. I have a traditional system. Personally, every property I have lived in with a combi does not have the consistancy in performance of a traditional system. Water running cold in the shower, flow suddenly reducing. Taking an age to fill a bath. Waiting for 5mins to get hot water from the tap.....

    ----

    Personally, I would take a system boiler over a combi for a house of your size any day. Especially if there is more than one bathroom

    You just have to ensure that the tank is sized for the number of people or bathrooms. I would look at a 200litre + tank in your case.
    ----

    Your installer should also check your incoming mains pressure to ensure a combi or system boiler is suitable,
    Last edited by CashStrapped; 03-03-2018 at 2:56 PM.
    • Boiler Query
    • By Boiler Query 3rd Mar 18, 5:22 PM
    • 2 Posts
    • 0 Thanks
    Boiler Query
    • #6
    • 3rd Mar 18, 5:22 PM
    • #6
    • 3rd Mar 18, 5:22 PM
    Thank you, I am in agreement with your reasoning and feel long term the system boiler although more expensive to install will prove a better more reliable choice with fewer things to go wrong like diverter valves and clogged up plate to plate heat exchangers
  • archived user
    • #7
    • 3rd Mar 18, 6:48 PM
    • #7
    • 3rd Mar 18, 6:48 PM
    Changing from a combination to a system boiler is not as simple as taking one out and putting a new one in. You need to take into account the need to site a hot water cylinder (vented/unvented?). CH systems also require motorised valves to control the flow around the cylinder and around the CH system. - 'S' and 'Y" plan being the most common. Then there are the additional plumbing costs.

    You also need an installer that will accurately measure your hot water and CH heat requirements. For example, a 18kW modulating boiler might be more than enough to heat your home but not big enough to give you constant hot water (or fast hot water re-heating). Are you happy with hot water priority or do you want CH and hot water heating at the same time?

    My advice: take your time and do your research using people who know about CH systems.
    Last edited by Hengus; 03-03-2018 at 7:05 PM. Reason: Typo
    • matelodave
    • By matelodave 4th Mar 18, 9:52 AM
    • 4,391 Posts
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    matelodave
    • #8
    • 4th Mar 18, 9:52 AM
    • #8
    • 4th Mar 18, 9:52 AM
    Why are you changing your boiler? is the first question I would ask.

    Is it defunct, not operating satisfactorily - not heating the place sufficiently, insufficient hot water or are you moving it somewhere else.

    As others have said, if you decide for a system boiler you'll need space for the hot water tank and associated plumbing which will be more complex. You'll be storing hot water which will have it's own heat losses. You can run out of hot water if the tank isn't large enough and the heat recovery rate isn't fast enough.

    A combi, has the disadvantage that it fires up every time you call for hot water so it's worthwhile learning to only use hot water when you need a decent amount - not just rinsing hands or stuff. You've got continuous hot wat if that's what you want, although the flow rates can be a bit low. They are also more complex, so there's more to go wrong and so can be more expensive tp repair.

    I've had both types and on balance prefer a combi, but that's just my opinion.
    Last edited by matelodave; 04-03-2018 at 10:03 AM.
    Never under estimate the power of stupid people in large numbers
    • Mr.Generous
    • By Mr.Generous 4th Mar 18, 10:14 AM
    • 2,354 Posts
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    Mr.Generous
    • #9
    • 4th Mar 18, 10:14 AM
    • #9
    • 4th Mar 18, 10:14 AM
    The other downsides of combi's nobody has mentioned include that when they fail, unless you have an electric shower, you have no hot water. With a cylinder you can at least use the immersion heater, expensive as it is, to get some hot water.

    From all the properties I have owned, managed and renovated I would say those with Combi's have far more go wrong. And when it does it tends to be expensive and complex. Expansion vessels - hate them. Sited in really difficult to change locations, and when they fail the PRV usually needs changing too.

    I personally think an airing cupboard is a useful place for keeping bedding and towels too.

    Undoubtedly for most small households a combi is cheaper to run gas consumption wise. A 300 repair bill every couple of years probably wipes that out.
    • boliston
    • By boliston 4th Mar 18, 10:27 AM
    • 2,863 Posts
    • 2,435 Thanks
    boliston
    I suspect slightly cheaper with the system boiler as it won't be firing up every time someone runs a hot tap for 10 seconds.

    Also with a system boiler it's easier to have a backup for hot water in the form of an immersion heater in case the boiler breaks down.
    Originally posted by Owain Moneysaver
    If I only need a small amount of hot water there is often enough hot water in the system to run the hot tap for up to about 2 minutes without firing up the boiler - the trick is to only turn the tap half on rather than fully on as this will avoid triggering the bolier flame.
    • cornfred
    • By cornfred 4th Mar 18, 1:32 PM
    • 3 Posts
    • 0 Thanks
    cornfred
    cornfred
    Definitely not a combi boiler. Water tank with immersion, for emergency hot water, is cheapest way for water. I've got this and the water is only on for 2 hours 7am and 2 hours 7pm.
    • dcweather
    • By dcweather 16th Apr 19, 9:13 PM
    • 19 Posts
    • 2 Thanks
    dcweather
    I'm trying to decide as well. I have an old and spectacularly good Potterton Netaheat conventional boiler that is possibly 30 years old serving 8 radiators in all, one which is rarely on in a spare room. All controlled with a self installed Tado/Alexa smart thermostat.. Only ever had a couple of relatively cheap electrical board problems and a couple of pumps. But it must fail soon and the availability of parts and finding somebody that understands it is getting harder. As we are approaching 70 I don't want to be left needing a new boiler in the middle of winter so am thinking of pre-empting a change. As there are just two of us I had naturally assumed a combi. We come from an era when we don't have showers every 5 mins like our grandchildren but have moved on a bit from Friday night once a week bath night! Baths are out of the question but we have a decent pumped walk in shower. We only need the hot water on for an hour, twice a day at the moment and it's always hot (which is great for a bit of washing up if not using the dishwasher) even with a cylinder with a jacket.
    It will be nice to get rid of the raised storage tank in the loft. If I went for a system boiler would the new pressure tank go in the airing cupboard instead of the existing copper hot water tank? My main factor in choosing though is disruption to the decor in terms of pipework changes. Not sure existing pipes are that accessible. What do you think I should go for?
    • macman
    • By macman 17th Apr 19, 12:30 AM
    • 43,238 Posts
    • 18,406 Thanks
    macman
    Definitely not a combi boiler. Water tank with immersion, for emergency hot water, is cheapest way for water. I've got this and the water is only on for 2 hours 7am and 2 hours 7pm.
    Originally posted by cornfred
    I assume you mean that the immersion heater is on 4 hours a day, not just the water?
    Given the hours quoted, you are not on E7 metering, so your hot water will be costing approx 300% more than heating it with a gas boiler, as will your space heating, unless you have another fuel for that?
    No free lunch, and no free laptop
    • macman
    • By macman 17th Apr 19, 12:37 AM
    • 43,238 Posts
    • 18,406 Thanks
    macman
    If you go for a combi, then your power shower will have to be changed, as you can only run that from a tank.
    Since you already have a tank, then the disruption caused by new or obsolete pipework is going to be similar whether you have a combi or a system boiler.
    No free lunch, and no free laptop
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