Combi or Standard Boiler?

Sponsored Links
Joined
26 Jun 2010
Messages
11,412
Reaction score
4,676
Location
Bedfordshire
Country
United Kingdom
Isn't common sense enough?
I hope so.

Common sense would seem to suggest a low efficiency, non-modulating, high water content, high mass boiler moving a lot of hot water very slowly over metres of distance through large bore normally uninsulated pipework to a hot water store while losing heat all the time before being drawn off at point of use would compare unfavourably with a high efficiency low mass, modulating, low water content primary and secondary heat exchangers separated by less than 400mm of pipework with primary water moved at high velocity and used only for short periods on demand.

I was intrigued at your opinion that it could be otherwise regardless of how a gravity system is operated
 
Joined
30 Dec 2018
Messages
12,260
Reaction score
1,726
Country
United Kingdom
Thanks, yes I am finding everyone wants to do a combi with seems at odds with online advice.

Why would this be?

If we stuck with a standard boiler, would there be any point/benefit in replacing the tank? We have a 30+ year old beast with no kind of insulation like they have now.

There might an advantage, it depends how long hot water is stored for/ how quickly use make use of it, once hot. The heat lost from the cylinder, is not entirely lost - it warms the airing cupboard and in turn the house itself.

A combi is often a one box solution, which means they are more complex, potentially more to go wrong in the box, which will need a gas engineer to fix. Parts external on a gravity system are an easy DIY fix. They also work under greater pressure than gravity systems, so you see lots of posts in the forum about combi boilers loosing pressure.

Every time you turn an hot tap on, the combi has to fire up, so lots more wear on the boiler and there can be a delay before the water becomes hot at the tap, more that with a gravity system - hence gas and water wasted, whilst you wait, though some combi store a small amount of hot water. The boilers Kw output, in order to heat the water, may need to be double that of a gravity system, which can be sized to provide enough space heating - another source of poor efficiency.

A potential deal breaker for the installation of a combi boiler, is that in order to meet the greater demand for gas, compared to a gravity boiler, is that a combi will probably need a new larger gas supply pipe, run all the way from the meter to the boiler. Depending on the house structure and where the boiler is installed, that might involve an ugly pipe being run outside around the house walls, as there might be no other options.

Combi's excel, where a house or flat is occupied unpredictably, when you might suddenly get home and need instant hot water.

Visitors to here, are astounded by how quickly a bath can be filled, because full flow can be had from both hot and cold taps at the same time.
 
Last edited:
Sponsored Links
Joined
30 Dec 2018
Messages
12,260
Reaction score
1,726
Country
United Kingdom
low water content primary and secondary heat exchangers separated by less than 400mm of pipework with primary water moved at high velocity and used only for short periods on demand.

The difference is what exactly? On a gravity system there is some small heat-loss while ever the boiler is trying to heat the cylinder, then it is done. With a combi, there is more wastage heating the boiler and water in the boiler, but every time there is a demand for hot water. Seems six or two three to me.
 
Joined
26 Jun 2010
Messages
11,412
Reaction score
4,676
Location
Bedfordshire
Country
United Kingdom
The difference is what exactly? On a gravity system there is some small heat-loss while ever the boiler is trying to heat the cylinder, then it is done. With a combi, there is more wastage heating the boiler and water in the boiler, but every time there is a demand for hot water. Seems six or two three to me.

I see where we are at cross purposes.

There are no gravity boiler out there to be bought and haven't been for many years. I think you're confusing what we call a system or heat only boiler with a gravity boiler.

A heat only boiler is what you refer to but I'm 'almost' positive you cannot buy a gravity boiler and if you did you would be in breach of building regs to install it.

A system boiler is basically a boiler containing all the components of a combi except the hoy water section and diverter valve. It's pressurized and has pumped primaries not gravity primaries.

A heat only boiler is the same as a combi less the hot water section, pump and expansion vessel.

They all have electronics and are far more complicated than a gravity boiler.

The BRE did a comparison test comparing the two systems, a combi and a boiler with hot water cylinder for efficiency, the boiler was slightly more efficient than the combination boiler but taking into account the losses between the boiler and cylinder and the cylinder itself the differences were negligible.

Any heat into the house which is above that required, DHW emissions in the summer is a reduction in efficiency, in winter it may be useful but it's misleading to call it an improvement.

If I had a suitable property I would indeed have a system or heat only boiler (not gravity) with vented hot water cylinder, a CWS cistern in the loft and gravity to the DHW and DCW taps. Not all properties lend themselves to this, mine does not, so I have a combi, and increasingly housing is less suitable as properties for the general public are getting smaller.
 
Joined
30 Dec 2018
Messages
12,260
Reaction score
1,726
Country
United Kingdom
A heat only boiler is what you refer to but I'm 'almost' positive you cannot buy a gravity boiler and if you did you would be in breach of building regs to install it.

A system boiler is basically a boiler containing all the components of a combi except the hoy water section and diverter valve. It's pressurized and has pumped primaries not gravity primaries.

A heat only boiler is the same as a combi less the hot water section, pump and expansion vessel.

They all have electronics and are far more complicated than a gravity boiler.

Yes, my apologies, I got the word 'gravity' into my head, but not withstanding - I standby what I said..
 
Joined
20 Mar 2018
Messages
1,571
Reaction score
186
Location
Stoke-on-Trent
Country
United Kingdom
Yes, my apologies, I got the word 'gravity' into my head, but not withstanding - I standby what I said..
By gravity I assumed you meant a system with a F/E tank on the boiler circuit, as opposed to an expansion vessel. Not an old-fashioned system with gravity HW, pumped CH. As we all know, there's pros and cons with F/E tank and expansion vessel.
 
Joined
10 Mar 2007
Messages
10,688
Reaction score
2,268
Location
Poole, Dorset
Country
United Kingdom
o with a possible £500 a year saving
No chance of that from a new boiler.
Boiler alone might save you ~10%
New controls, insulating the cylinder and so on will save a lot more for a lot less - but you can fit those to the existing system, new boiler not required.

Unless the existing boiler is broken and can't be repaired, keep it.
If and when a new boiler is required, get the equivalent of what you have. A combi is not the answer.

If you are desperate to spend new boiler money now, spend it on insulation for the building.
 
Joined
30 Dec 2018
Messages
12,260
Reaction score
1,726
Country
United Kingdom
By gravity I assumed you meant a system with a F/E tank on the boiler circuit, as opposed to an expansion vessel. Not an old-fashioned system with gravity HW, pumped CH. As we all know, there's pros and cons with F/E tank and expansion vessel.

I did mean a system with an F&E tank in the loft and a second larger tank for the hot water cylinder.
 
Joined
8 Dec 2007
Messages
14,581
Reaction score
5,916
Location
Cheshire
Country
United Kingdom
Your installer will want for various reasons to persuade you to convert to a combi. A gravity system already installed, has many benefits and would cost you a lot more than a combi to install, if you were starting from scratch. If what you have is in good condition - keep it, just replace the boiler.

Despite modern boilers being able to modulate, a gravity system (depending on how you use it) has potential to be run more efficiently than a combi. As above, you may also have the benefit of an immersion heater with a gravity system, should the boiler fail.
Or we could have a fire pit in the centre of the house, remove all doors and windows then get rid of the first floor and remove a few roof tiles to allow the smoke to escape.
Boil water in a cauldron over the fire for efficiency!
I agree Bernard. The old ways are the best.
 
Joined
27 Oct 2009
Messages
25,239
Reaction score
9,832
Country
United Kingdom
We are having new gas mains in the village, people with gas combi boilers may have no means of heating water ( other than a kettle ) while their gas service is not available. Those with a hot water cylinder probably have the option of heating by electric immersion heater.

yeah not having hot water for the 1 day they are renewing the main in your lifetime should be the deciding factor
 
Joined
3 Nov 2006
Messages
27,017
Reaction score
2,919
Location
Bedfordshire
Country
United Kingdom
yeah not having hot water for the 1 day they are renewing the main in your lifetime should be the deciding factor
Not only loss of gas supply but fault in the boiler, it could be days before it gets repaired,

One assumes the renewal work goes to plan and loss of supply is less than a day.

A friend still has the small 2 ring electric hob that the gas people gave to her when there was an un-planned 4 day loss of gas supply. She did try to return it but was told it was non returnable.
 
Joined
27 Oct 2009
Messages
25,239
Reaction score
9,832
Country
United Kingdom
Not only loss of gas supply but fault in the boiler, it could be days before it gets repaired,
As with any boiler and its is not unusual for immersions not to work when they get used years after they are fitted .
Fan heaters/mini hobs are never collected it is not cost effective as they need pat testing again etc
 
Joined
30 Dec 2018
Messages
12,260
Reaction score
1,726
Country
United Kingdom
Or we could have a fire pit in the centre of the house, remove all doors and windows then get rid of the first floor and remove a few roof tiles to allow the smoke to escape.
Boil water in a cauldron over the fire for efficiency!
I agree Bernard. The old ways are the best.

If that's what floats your boat, go ahead.. Room sealed instant gas water heating boilers have been around for very many decades. The modern equivalent can do both HW and CH, it makes them an option and not compulsory.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Top