Combi-boiler Vs System Boiler

Boiler_Query

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.

Owain_Moneysaver

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.

grumpycrab

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.

Anthorn

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

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......

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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.

https://forums.moneysavingexpert.com/discussion/5802348

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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.....

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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.
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Your installer should also check your incoming mains pressure to ensure a combi or system boiler is suitable,

Boiler_Query

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

System

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.

matelodave

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.

Mr.Generous

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

Owain_Moneysaver wrote: »

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.

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

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.