combi v system

[rhinosaw]

had a chap round today to price for a replacement boiler going from vented to system boiler 12kw.
He said I would save loads by going for a combi, not initial cost but running it.
thing is I like the idea of a back up for hot water and my other half has OCD so turns the hot tap on every few minutes.
with the old vented system that we have the boiler goes on for just over 2 hours a day ,( at different times) and that gives all hot water requirements that we need.
so would we save loads on running costs? thanks

 

[vulcancontinental]

Depends on insulation levels on the cylinder and the pipework between the boiler and cylinder. Combi is less efficient than system but when external pipework taken into account is more. About 2% either way.

 

[rhinosaw]

Thank you, as I thought, not going to save loads of money per year. Could about 8 quid actually

 

[bernardgreen]

so would we save loads on running costs?

Possibly, but with a combi the boiler has to fire up every time a hot tap is opened. The fire up process has to heat the mass of the heat exchanger before any heat gets to the now flowing water. When the tap is closed the heat in the heat exchanger disipates from the boiler.

The cost effectiveness ( or absolute efficiency ) of combi boilers supplying hot water varies with the way the hot water is drawn. One long draw or many short draws of the same total volume of hot water produced. Many short draws tend to be lower efficiency than one long draw, hence depending on how you use hot water a cylinder ( one long burn to heat it ) may be more cost effective than taking hot water direct from the combi.

 

[rhinosaw]

Thanks Bernard, I still wonder why installers tend to pressure me into a combi boiler when it must be easier to install a system boiler from a vented system

 

[rhinosaw]

And the hot water cylinder is staying vented

 

[bernardgreen]

why installers tend to pressure me into a combi boiler

Many installers probably know very little ( if anything ) about the alternatives.

 

[vulcancontinental]

Many of Bernard's points apply in reverse also, draws offs from cylinders will bring the boiler on if the temperature drops a few degrees.

The further the cylinder away from the sink the greater a short draw off will be as the combi is often next to the sink and the cylinder is further away so more cold to run off first even for a short draw off. Additionally the further the source of heat, the boiler, from the hot water section the greater the heat losses between hence the reduced efficiency.

The advantage of a cylinder is the immersion heater backup, an electric shower would be the same with the combi but not as effective.

As for vented cylinder that would be my preference over unvented, I want to wash not blast dirt off me and the maintenance is far, far more expensive with unvented.

I have now a combi because of the house layout but would prefer a system boiler and a vented cylinder arrangement that served me well in the 35 years I spent in the house now occupied by my son.

Difference in efficiency? Forget that it's negligible. Mass of combi? Forget that, buy one that is low mass; you can buy a high mass system boiler also so that doesn't wash particularly.

 

[rhinosaw]

Thanks Vulcan , interesting points,
The cylinder stat on my vented cylinder has never worked, we just turn the boiler on about 3 times a day to heat a tank of water which works fine. If I get a new cylinder stat when I get a new boiler will the boiler keep firing up when temp of water drops?

 

[bernardgreen]

draws offs from cylinders will bring the boiler on if the temperature drops a few degrees.

That can be prevented by having a timer that determines when demand from the tank thermostat is allowed to fire up the boiler.

 

[rhinosaw]

Thanks Bernard, I have a timer/programmer, so it's just a case of using that to allow the thermostat to turn the boiler on/off

 

[vulcancontinental]

Personally, if you have a older cylinder a new one will be insulated far better and also insulate any hot pipework in the airing cupboard, I was amazed when told just how much is lost through the pipework.

 

[bernardgreen]

Another consideration is the need to maybe up-grade the gas pipe from meter to boiler.

Often this requires a 28 mm diameter pipe to be installed.

Given the rules about gas pipes many installers take the easy option and run it externally. It can look ugly, is a temptation for metal theft and in a few building may contravene covenants or restrictions. ( 15 metres of newly installed 28mm pipe had to be removed from the exterior of a listed building )

 

[rhinosaw]

Another reason for not having a combi, I know I may also have to upgrade my existing gas pipe for a 12kw system boiler but perhaps less likely.
Thanks Bernard

 

[vulcancontinental]

A guide to annual losses from a cylinder and primary pipework. From a field trial 10 years ago. Cylinders are available with better insulation now. Lag any primary pipework you can. A kw from natural gas is around 5p.