Rule of Thumb for Combi ?

[WKayani]

So still in the middle...Combi or Conventional.

Are Combis really that bad ? I had a gas company round who said that 3 our of every 4 install they do is a Combi installation. Either hes busy doing lots of flats or people have been swayed.

I'm not too keen on having big water tanks in the loft. On a side, pressure into the house is good but could be better. The visiting engineer said that pressure would be increased immensely by a Combi..is he doing his typical salesman pitch ?

 

[kevplumb]

pressure of the hot water will decrease with a combi well actually flowrate but i dont want to get into that
so yes he is talking b*ll*cks

 

[oilman]

Pressure is not the significant factor. If everyone turns their taps off, the pressure will go up. If you open a cold tap you will get the best water supply. If the water comes through a restriction (your boiler) the flow rate will be lower. If you now have other taps turned on, this restricted supply will have to be shared around.

There are benefits of combies, but most people live quite happily with a hot water tank and don't suffer from diseases. The combi was invented by boiler companies, and is useful for small dwellings where the occupants are out most of the time. The makers then had a sales job done on it, (rather like Canadian timber companies sold all their rubbish wood by selling it as "knotty pine"). They told porkies though, saying it was more economical, how? A car is better than a horse, but not if you want to go across fields.

Arm yourself with some questions.

What is my water pressure ? (So what?)
What is the flow rate? This should be done with all mains cold taps full on, and measure all the flow rates and add them up.
What is the flow rate through the boiler?
For what temperature rise? (Otherwise you get a cold bath in the winter).
What is the temperature of the cold taps in the winter?
How long will it take to run a bath? (This is amount of water in bath divided by boiler flow rate, and don't forget the temperature).

You can have the answers to some of the questions before the man with audible genitalia arrives.

Then buy a conventional boiler and allocate some space for water tanks. Stop relegating water supplies to a low position on your priorities list. It is THE important service in the house, and you are worried about the space a tank takes up? You will not die from a poor service, but you will be seriously pised off.

 

[PEDANTICVINDICTIVEMAN]

Get a combi.

 

[AdamW]

Never tried to run a bath from a combi, but let's do a little calculation:

A typical bath is about 200 litres (my one is, and it is quite small).
Assume incoming water temperature in winter is 5 degrees
Assume you want bath water at 40 degrees (slightly more than body temp, I guess this would be a moderately hot bath, but not a really hot one)
SHC of water is 4.2 joules per gramme per degree celsius
Combi power is 35kW

So, you want to heat 200 litres of water by 35 degrees. Which means, you need:

200,000 x 35 x 4.2 = 29,400,000 Joules of energy.

Boiler can supply 35,000 Joules per second,

29,400,000 / 35,000 = 840 seconds = 14 minutes to run a bath

Now that assumes a 100% efficient boiler, and a 100% efficient bath. So in reality I would imagine it would take AT LEAST 20 minutes to run a bath, taking into account the fact that the water will cool down before you get in it. Does this compare with the experiences of combi-owners here?

 

[croydoncorgi]

Unless of course you consider a 'storage combi' such as the Worcester Bosch Hi Flow 400 which has quite a large capacity heatstore...... Given that you already have a floor-standing boiler, that might also be the simplest to install.

(But what do I know?...)

 

[masona]

Now that assumes a 100% efficient boiler, and a 100% efficient bath. So in reality I would imagine it would take AT LEAST 20 minutes to run a bath, taking into account the fact that the water will cool down before you get in it. Does this compare with the experiences of combi-owners here?

My mum's combi take approximately 20-25 mins to run a bath Got so fed up with it, so now bought a egg timer sitting on the lounge table telling her when the bath is ready

 

[PEDANTICVINDICTIVEMAN]

What you supplying your hot tap with, 8mm or what, my Isar combi doesn't take anywhere as near as long to fill a large bath as what your talking about, and it's bloody hot, this isn't particularly a brill combi so god knows what you lot are fitting.

 

[oilman]

It's ok for CONCISEHELPFULMAN, he can fix his own boiler with trade priced parts. That's compensated for by being able to pee on old folks and rip out tanks,

 

[kevplumb]

wwoooaaahhh down boy steady tiger

 

[PEDANTICVINDICTIVEMAN]

Nice one, like it

 

[Noraslad]

Tell you what don't know about WKayani but youv'e all put me off ever changing to a combi

 

[WKayani]

Ditto....put off a combi myself now!!!

Will look at both options...think the combi will come in cheaper though. Having an old boiler has really taken its toll this winter....takes over an hour to heat the water and approx the same to heat the radiators !!!

p.s.Whats a megaflo ? The engineer mentioned the best thing since slice bread is a megaflo but only designed for houses with 2+ baths.

 

[kevplumb]

unvented cylinder ask oilman

with a family it could well be a better option than a combi

what about a fully pumped system with a high recovery cylinder

 

[AdamW]

What you supplying your hot tap with, 8mm or what, my Isar combi doesn't take anywhere as near as long to fill a large bath as what your talking about, and it's b****y hot, this isn't particularly a brill combi so god knows what you lot are fitting.

Ye cannae disobey the laws of physics! The important factor here is the heat output of the boiler, the above calculation assumes everything else is perfect.

Just had a look at the Baxi website, to get an idea of some of the combis, they do up to 45kW. Also, they give a maximum HW flowrate of 16l/min at 35 degrees temperature rise for that most powerful boiler. That would still take over 12 minutes to fill a bath. Not as bad as 20 minutes, but still not as good as a pair of gravity-fed taps with nice big fat pipes.

Do you like to bathe in 3 inches of water, per chance?