Combi or Conventional boiler

[Saracen]

Kumagoro,

If I'm right, you are saying combi never runs out of hot water?

Well, yes and no.

A combi basically heats water on demand and (at least, usually) there is no storage tank. Since there's no storage tank, you can't empty it if too many people have baths or showers in quick succession, so can never run out.

So that's good then?

Yes, and no.

Since there's no storage tank, there's no stored hot water. So if the boiler packs up, you have neither heating nor hot water. With a system that has a tank, however, you have :-

1) A tank full of hot water
2) Usually, an electric immersion heater element.

So ..... while it's expensive to run, if the boiler packs up, you can at least heat a tank of water electrically with the immersion heater until you get the boiler fixed.

And as for the "heating on demand" thing with the combi, well, you can kinda do that with the non-combi too.

I have two circuits on the boiler, one providing hot water for the radiators, and the other providing it for the hot water tank. Both are controlled by a timer which specifies when the boiler can fire, but both ALSO have thermostats. The heating has the standard room thermostat (well, it's a radio link type not the older wired one), so even oif the timer is saying the boiler can fire the heating, it still only does so if the thermostat says that the room temp is below the trigger temp.

The other thermostat (again, radio link) is on the storage tank. So ..... I can set the hot water circuit to be permanently on, but it still only fires to heat water if the tank temperature drops and the thermostat cuts the boiler in. And that happens when hot water gets used and therefore the tank temp drops.

Clearly, it will also happen if the tank temp drops though normal heat loss, but with a properly insulated tank, my experience is that I use it before that becomes an issue.

[Saracen]

Kumagoro,

If I'm right, you are saying combi never runs out of hot water?

Well, yes and no.

A combi basically heats water on demand and (at least, usually) there is no storage tank. Since there's no storage tank, you can't empty it if too many people have baths or showers in quick succession, so can never run out.

So that's good then?

Yes, and no.

Since there's no storage tank, there's no stored hot water. So if the boiler packs up, you have neither heating nor hot water. With a system that has a tank, however, you have :-

1) A tank full of hot water
2) Usually, an electric immersion heater element.

So ..... while it's expensive to run, if the boiler packs up, you can at least heat a tank of water electrically with the immersion heater until you get the boiler fixed.

And as for the "heating on demand" thing with the combi, well, you can kinda do that with the non-combi too.

I have two circuits on the boiler, one providing hot water for the radiators, and the other providing it for the hot water tank. Both are controlled by a timer which specifies when the boiler can fire, but both ALSO have thermostats. The heating has the standard room thermostat (well, it's a radio link type not the older wired one), so even if the timer is saying the boiler can fire the heating, it still only does so if the thermostat says that the room temp is below the trigger temp.

The other thermostat (again, radio link) is on the storage tank. So ..... I can set the hot water circuit to be permanently on, but it still only fires to heat water if the tank temperature drops and the thermostat cuts the boiler in. And that happens when hot water gets used and therefore the tank temp drops.

Clearly, it will also happen if the tank temp drops though normal heat loss, but with a properly insulated tank, my experience is that I use it before that becomes an issue.

[eden188]

Crikey! Saracen. Still not asleep ?!?!

Thanks for the detail BTW!

[eden188]

Sorry to bring this subject up again. I have just received a quote from the plumber for a combi boiler - Vaillant 38kw combi or the Solaris 35kw. Anyone heard or have experience of these boilers please?

Thanks!

[rox0r]

We had a Vaillant ecoTEC 837 condensing boiler fitted last year to replace a severely old and war-torn back-boiler, whole job was about 3.5k but since my partner's dad did the job at cost we got the boiler, all new radiators and complete re-plumb throughout for that which is a ruddy steal for a big three bed semi.

The Vaillant is a cracking piece of kit and because ours has the pre-burner thing it never takes more than 4-5 seconds to get hot water to the tap. You can add some nifty modules to it in the control panel including a facility to monitor and control it from t'internet, so you can switch the heating on before you go home from work on a cold evening for instance.

[eden188]

thanks for the info.

Never knew boilers could be so hi tec nowadays .

[Steve]

/In rocks the boiler geek/

Well well well... time for my tuppence-worth, although I haven't worked in the boiler industry for 4 years now.

Pro-combi
Combis take up less space (no water tank).
Heating your water on demand is a lot more efficient than trying to store it, no matter how good your insulation is.

Pro-'conventional'
Better suited to serving larger families where hot water might be drawn from more than one place at once (combis will only provide decent hot water up to a certain flow rate)
Erm... I think that's about it.

As for reliability, a good combi is just as reliable as a good 'normal' boiler, but if it breaks you're potentially in a worse situation. That said, you'd be surprised how many hot water storage tanks don't actually have an immersion heater... or do have one... that's broken. If you have an electric shower, then I'd say the choice is irrelevant.

Modern, energy-efficient condensing boilers need servicing every year or they will break. Strictly speaking all boilers need a yearly service, but condensing boilers will run into trouble firing sooner if they're poorly maintained; and a good service guy will spot any other potential issues before they arise.

So, for flats/small houses I recommend combi boilers. For bigger properties, a full system is probably wiser.

[Rob_B]

I thought you weren't allowed to use conventional boilers anymore unless you got specific approval to install it (due to energy efficieny regs?)

~ £2k is about right for a fitted combi with no extras. To get a rough idea ask British Gas to come and give you a quote, then knock 1/3 off that price & you have what you should be paying for the work

[Steve]

It's quite easy to argue that you need a conventional boiler due to the placement of the existing boiler and the fact that you may need to rework other bits of the system etc.

But by and large people are better off getting condensing boilers anyway.

[eden188]

But by and large people are better off getting condensing boilers anyway.

Condensing boiler, I presume that's a combi boiler?

[Steve]

Sorry, a condensing boiler is an energy efficient boiler, named as such because the flue gasses have so much heat removed from them that the vapour in them condenses.

[eden188]

Thanks Steve, sorry to sound dumb, so is that a conventional or combi? I really don't have a clue with these . I suppose I can google this

[Steve]

Condensing boilers can be either combi or system boilers. Same for "conventional" where conventional means "not condensing" rather than "not combi"... just in case I wasn't causing enough confusion

[Cog]

I've heard that combi boilers are very efficient, but cant supply any more than one or two taps with hot water at the same time

[BUFF]

I've heard that combi boilers are very efficient, but cant supply any more than one or two taps with hot water at the same time

depends upon the combi - e.g. a Worcester Highflow 440 will provide 20 litres a minute at 35C temp rise which is way more than 1 or 2 taps. 12-15 litres a minute @35C temp rise would be a more normal figure though.
All depends upon your needs but for the majority of households a combi is fine.

[VictorE]

@ eden188 Just a heads up if you do decide to have it fitted ask the plumber what pipe system he's going to use eg copper or plastic i think for the price he's quoting sounds like it may be a flexible plumbing system try this link http://www.hepworthplumbing.co.uk/po...e-pipework.asp just to give you an idea what it looks like.

Its ok if he is going to use this system just make sure he intends to use a good one there are some cheaper polybutylene systems out there so ask questions nicely of course before you hand over any pennys.

furthermore its always a good idea to get atleast 2 or 3 other quotes and when they call ask them questions about flexi pipe and combi systems you will then have some idea of what you want and expect. hope this helps.