Replace conventional CH boiler with combi?

30 Jan 2011
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United Kingdom
What would you do in this scenario?

Typical 3-bed semi. Very old HW and CH system with conventional boiler (no idea which one it is but it sits on the floor and flue is through chimney!). Old rads, HW tank in airing cupbaord upstairs. Cold water and expansion tanks in loft. Electric shower in bathroom.

I need to do the place up anyway as it hasn't been decorated in decades.

My stumbling point is what would you do about the HW and CH system? Would you just stick a new conventional boiler to replace the current one and leave everything else in place?

Or would you rip all the HW and CH system out and replace it all with a combi-boiler based system?

The cheaper option is to replace the existing conventional boiler with a new conventional boiler, PowerFlush the system and replace the rads.

The more expensive option is to rip the whole out and replace.

Advantages of the cheaper option: less disruptive. Keeps backup options for HW (HW tank has an electric immersion heater). Disadvantages are that the tanks take up space and it is more costly to run.

Advantages of the replace with combi option are that it is a neater (aesthetically) solution, cheaper to run too. Disadvatnages are that there is no backup if the combi boiler fails.
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Most landlords would fit a combi as they see that as simpler and a lower maintenance option.

Its not more expensive to use a hot water cylinder and you can get a higher HW flow rate.

I would personally just get the system cleaned out and the boiler changed, which is what I did.
My old system was 30 years old and was replaced with a Vaillant EcoTec 415.
I had one radiator changed, as it was a bit small and was rusting. All the others were left in situ.

Having the immersion heater as a backup, and the ability to get hot water out of more than one tap at a time, without affecting the flow, is a great advantage.

Having stored water when you get a power cut, is also quite helpful, as well as a warm airing cupboard to store towels etc.

I can run a bath, and still get loads of hot water out of the kitchen tap as well.

Remember also, that you will end up with a pressurised system with a combi. Look through this forum at the number of people suffering losses of pressure on their systems!

Combis are a cheap option for new installs, as Agile says.

In my opinion, after reading what people have posted here, combis are far more trouble, and seem to keep the plumbers busy ;)

So flushing out the old system and replacing the old conventional boiler might be the best option then?
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Well, you will get lots of different opinions on this forum.

I did not see any advantage in ripping out a system which has worked reliably for 30 years.

My family has always had plenty of hot water, even the gravity fed shower was not affected by anyone turning the taps on, as its cold feed came from the cold water storage tank.

The main problem was the old boiler which was a Thorn Olympic 38/50B, which was getting noisy and was not very efficient.

The Vaillant EcoTec 415, which I now have, has a maximum output of 15Kw, which is more than enough for my house. Vaillant are a good boiler, and well worth spending the extra cost.

Our installer flushed the old system out and refilled it with a chemical cleaner, which was left in, with the old boiler on, for 24 hours.
Then it was all flushed out, with quite a lot of brown muck.
The header tank was also totally cleaned out as well.

The new boiler was fitted in the same place as the old one, in the kitchen. As it is much smaller, I can now put a large fridge underneath.

A Fernox Total Filter was also fitted, to capture any future particles.

The pump, controls, and 3 port valve were changed, as they were also 30 years old and getting a bit worn out !

The whole process caused very little disruption, and I still had hot water during that period, by using the immersion heater, as this was unaffected by the work.

Also no messing around with expansion vessels, pressure relief valves, and possible pressure leaks from old radiators. Far too many things to go wrong with combis !

Seemed a "no-brainer" as the Americans would say.
Make sure you get a good installer who knows what they are doing, and cleans the old system out properly.

Thanks for your advice, Keith.

My survey revealed that the hot water cylinder needs replacing with a modern foam-insulated one. I imagine this is a fairly straightforward job if I decide to keep the conventional heated system?
You could also consider an unvented cylinder, which could probably go in the loft.

Mains pressure hot water, immersion heater backup.
Its all depends on how its fitted but if its easy then we charge about £460 inc for a standard indirect cylinder.

If its complicated then we usually quote accordingly and hope that someone with less skill will quote lower because they dont forsee the problems.

^ It's in a bedroom airing cupboard at the moment. Does that make it complicated or easy?
It all depends on the size and layout.

Some plumbers do some amazing stupid things when they fit cylinders like pipe connections made up before its placed in position so that they are inaccessible afterwards.

In a bedroom if its too small then its often easier to make the plumbing easy by having a new cupboard constructed around it. Many DIYers can do the cupboard themselves.

Get what works for you, bearing in mind that you need a fairly good reason for a combi to justify the cost and trouble of all the plumbing changes. Ignore the opinions, pro and amateur alike, about which system is "better" and concentrate on the differences between them for you as the one that has to use it. One of the main differences may be what you wish to do with the shower.

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