Combi or regular boiler ????

paulsully

Hi, i was wondering if someone could give me some advice. i'm in a bit of a mess trying to figure which sort of boiler to get.

Our old back boiler has finally been condemned by the plumber, it was over 40 yrs old and getting very dodgy. The problem is that we cant decide whether to get it replaced with another heat only boiler or whether to fit a combi.

We live in a high pressure very hard water area and have a small 2 bed house with a water tank and cylinder etc, we dont use much hot water really except for our 'Power Shower' which uses a 1.5bar pump coming from the cylinder, this gives us a good shower using an 8 inch wide rain shower head. If we have a combi fitted we need the shower to be as powerful because a powerful shower is what we like, how fast taps run isn't really important. We're worried a combi wont be able to do what we need.
I've looked into the flowrate of combi's, wocester and vailance do nice ones which do 14 LPM but is it enough for a nice powerful shower with a wide rainhead? and what happens to the pressure when someone flushes the loo or runs another hot tap? and what's a good powerful shower flow rate?

Does a combi need a dedicated cold feed from the mains? or can it take its cold supply from somewhere else in the house without flow rate problems?

I really dont know whas best for us, there are con's and cons to both, a combi risks getting clogged with limescale, has no immersion for backup, and has a limited hot flow rate, but on the other hand is more efficient, means you can get rid of the water tank and lots of pipework.

Any suggestions anyone? :j

macman

A combi is not 'more efficient'. Why would it be? You can't run your existing pumped shower off a combi, it needs to run off the tank supplies.
A combi will always suffer a reduced flow if another hot tap is opened.
The first thing to check is that your mains flow and pressure is even suitable for a combi, and that can only be established following a site survey by your preferred RGI.
A combi must be fed from the rising main.

Cardew

As indicated above, if you require a proper power shower then you can rule out a combi; they simply cannot supply enough hot water.

paulsully

I thought a combi was more efficient because it supplies water on demand where a regular system stores heat in the tank which slowly loses the heat,

Our plumber says our water pressure is plenty high enough for a combi, and i was looking at different combi's and the different flow rates they have.
Isn't 14LPM enough for a power shower? i've seen combi's rated from 9.8LPM to over 16 but just cant seem to find what flow rate is required for a decent shower.

Our shower pump is only 1.5 bar, we just need a combi to equal that.

Or should be just get a regular boiler to replace our back boiler?

Lokolo

When I bought my flat I needed a new boiler, I replaced it with a combi.

You are correct about the loss of heat with regular system stores compared to combi, it doesn't mean the boiler itself is more efficient just the whole process is.

I get good water pressure (if I didn't I wouldn't have got a combi) and its perfectly fine unless someone else runs another tap, which they don't as its only 2 of us and we know not to.

If you have a 2 bed house then you might have yourself, partner, 2 kids or something and it might be more difficult to make sure no-one is turning other taps on and off.

Have you thought about asking neighbours whether they have combis and seeing how they find it? They would have the same pressure.

Cardew

A modern Hot water tank, at 65C, in 24 hours will lose heat taking around 2kWh to replace - they are tested to a British Standard. So around £30 a year(with gas) if kept at 65C all the time; in practice the loss will be less than £30 a year.


However that heat is not wasted for much of the year as it 'escapes' and warms the fabric of the house - and many HW tanks are in an airing cupboard.


If you get the biggest combi on the market, you might just about get an adequate supply of hot water - but you certainly don't need a huge combi for a 'small two bed house'. In fact with modern condensing boilers you shouldn't have them too large as they won't get in condensing mode.

AndysDad

Just had quotes for Boilers and the quote for the combi was over £1000 more expensive,so that's another thing to consider.

paulsully

I didn't realize that using a higher powered combi would cause a problem?

penrhyn

Combos also need a 22 mm gas pipe, 15 mm won't supply sufficient. So bear in mind a new run of Gas pipe may be needed.

Cardew

paulsully wrote: »

I didn't realize that using a higher powered combi would cause a problem?

Not a problem as such, just less efficient. See:


http://www.homebuilding.co.uk/advice/key-choices/heating/boilers

Getting the Right Size


It was once common practice to oversize boilers by around 30%, but nowadays this is considered wasteful, making it important to specify the right size for your home. This is decided by means of a heat loss calculation and will be affected by the house’s size, the materials used and the level of insulation and airtightness, as well as your hot water requirements, i.e. how many bathrooms are there? A heating engineer can work this out for you, but you can also use a free online calculator — try the one at sedbuk.com. - See more at: http://www.homebuilding.co.uk/advice/key-choices/heating/boilers#sthash.DwGCfSBB.dpuf

macman

paulsully wrote: »

I thought a combi was more efficient because it supplies water on demand where a regular system stores heat in the tank which slowly loses the heat,

Our plumber says our water pressure is plenty high enough for a combi, and i was looking at different combi's and the different flow rates they have.
Isn't 14LPM enough for a power shower? i've seen combi's rated from 9.8LPM to over 16 but just cant seem to find what flow rate is required for a decent shower.

Our shower pump is only 1.5 bar, we just need a combi to equal that.

Or should be just get a regular boiler to replace our back boiler?

You cannot use a pumped shower with a combi because you are not allowed to pump direct from the mains supply-you can only use such a shower with a stored hot and cold water system. If your RGI told you otherwise then it's time you got a new one (RGI, not shower).
A 'plumber' is not qualified to tell you if a combi is suitable, this should be done by an RGI conducting a proper pressure and flow test-has he done this?