Sounds like it will have to be a combi then, even if it is short cycling.
What type of hot water system to have?
- Thread starter Zebra579
- Start date
Sounds like it will have to be a combi then, even if it is short cycling.
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This radiator: http://www.screwfix.com/p/kudox-pre...le-panel-convector-radiator-700-x-800mm/54912
is indeed 5478BTU, or 1605 watts. Two of those would be 3.2kW.
If you have a woodburner already, that is likely to be in the 3 or 5kW range as well.
Oh I see... So my total radiator requirement, which is indeed, as you say, just under 5kW, is too small for most combi boilers to function properly!
But surely there must be many small flats which have similar heat requirements. Does nobody make a combi boiler for the smaller household?!
But surely there must be many small flats which have similar heat requirements. Does nobody make a combi boiler for the smaller household?!
No. A combination boiler is a compromise between two things.
It has to be powerful enough to heat hot water at a rate people find acceptable. This realistically means 25kW at least, usually more.
Unfortunately unless your house is very large, you won't need all of that capacity for space heating, and while boilers can operate over a range of power, there is no way it will work properly with a 5kW heating load.
Even if the house was large enough to use most of the available capacity, it's still a total bust because a large house will inevitably have more than one bathroom, which a combi boiler cannot supply at the same time.
Combi boilers are suited to a small number of mid sized properties which have low requirements for hot water.
Everywhere else, they are fitted because they are cheap for the installer to buy, quick to install and are sold to customers at a relatively high price.
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I am renovating a 490 year old cottage and needed a boiler that went down to about 5 kw but could also give more than 12 Kw when needed. It also had to be compact to fix in the only place where a flue could be fitted without compromising the thatched roof.
I found Johnson and Starley do a 16 Kw that modulates down to 4 Kw when necessary. It comes in two versions, system ( Combi ) and heat only.
The HE16
http://www.johnsonandstarley.co.uk/domestic-boilers/heatonly-he25h.asp
or HE16s
http://www.johnsonandstarley.co.uk/domestic-boilers/system-he25s.asp
If you can find space for a hot water cylinder then you can avoid the problems of maintainance of the more complex assembly of the combi and hot water temperature varying with flow rate.
I found Johnson and Starley do a 16 Kw that modulates down to 4 Kw when necessary. It comes in two versions, system ( Combi ) and heat only.
The HE16
http://www.johnsonandstarley.co.uk/domestic-boilers/heatonly-he25h.asp
or HE16s
http://www.johnsonandstarley.co.uk/domestic-boilers/system-he25s.asp
If you can find space for a hot water cylinder then you can avoid the problems of maintainance of the more complex assembly of the combi and hot water temperature varying with flow rate.
You are likely to find installers will be ( very ) biased to one or two makes of boiler and will be ( very ) reluctant to consider any other type or make. They also will try to make the installation as easy as possible for themselves and for them a combi is a nice minimum work project.
To get a good result do the research using manufacturer's data and impartial and un-biased advice to select the type of system and boiler and then find an installer prepared to install that make of boiler.
One down side of a combi with pressurised radiators is that a very slight weep leak can lead to the boiler shutting down until the rads are re-pressurised. On a non pressurised system the weep would probably self seal and if it didn't the header tank would replace lost water.
A neighbour has had to re-pressure several times since Christmas, no signs of where the weep is.
To get a good result do the research using manufacturer's data and impartial and un-biased advice to select the type of system and boiler and then find an installer prepared to install that make of boiler.
One down side of a combi with pressurised radiators is that a very slight weep leak can lead to the boiler shutting down until the rads are re-pressurised. On a non pressurised system the weep would probably self seal and if it didn't the header tank would replace lost water.
A neighbour has had to re-pressure several times since Christmas, no signs of where the weep is.
Combi and range rate the heating to minimum, vaillant and worcester both are able to do this.
So much less faffing about.
So much less faffing about.
Thanks Bernard for your views. Do you know of a good source for independent unbiased advice, apart from here of course?
I did look into this before I installed my (8kw) woodburner, but it seems from my research that for that system to work you need an accumulator tank, you don't just run the radiators straight off the woodburner. I don't have anywhere I could put the tank.
By the way, a system boiler and heat only boiler are NOT combi boilers. A system boiler is a heating unit with an integral expansion vessel and circ pump, removing the need for a F and E tank, because it uses a sealed system.
Do those example models provide instantaneous hot water from cold mains?
A system boiler still requires a hot water cylinder.
Bernard's opinion that all installers put in whatever suits them and is easiest isn't true. I install for the customer, I tell them which boilers can meet their needs, and generally they choose by a preferred make, price and warranty.
People choose combi boilers to save space, eliminating the need for cylinders and tanks. Many combis will fit into a kitchen cupboard. And all modern pre-mix boilers modulate down to match heat load, most boilers can remain lit at lower than 4KW load.
The make of boiler and system connected to it are irrelevant if badly installed. A well designed, carefully installed and commissioned system with a decent boiler warranty is the most any installer can offer. The repair service (if needed) behind the warranty is more important than it's length imo. And the expense and availability of parts when the warranty expires.
Do those example models provide instantaneous hot water from cold mains?
A system boiler still requires a hot water cylinder.
Bernard's opinion that all installers put in whatever suits them and is easiest isn't true. I install for the customer, I tell them which boilers can meet their needs, and generally they choose by a preferred make, price and warranty.
People choose combi boilers to save space, eliminating the need for cylinders and tanks. Many combis will fit into a kitchen cupboard. And all modern pre-mix boilers modulate down to match heat load, most boilers can remain lit at lower than 4KW load.
The make of boiler and system connected to it are irrelevant if badly installed. A well designed, carefully installed and commissioned system with a decent boiler warranty is the most any installer can offer. The repair service (if needed) behind the warranty is more important than it's length imo. And the expense and availability of parts when the warranty expires.
My opinion is based on the quotations and actual installs at several friends and neighbours over the past few years. One had several quotes for a new system where all but one were for combis and ripping out the existing tanks. It was the British Gas man who suggested a heat only using the existing perfectly good hot water system.
For a new system in a modern new build compact house that can be a valid reason but if there is a existing system of heat only boiler why do so many plumbers suggest ( often strongly ) to rip out the tanks and install a combi. And why when there is space for tanks do they still seem to prefer to install combis ?
I accept my mixing of system and combis was an error on my part.
Some customers don't want to fit a new heat only boiler to replace the old one. If they're improving their home, the current system doesn't always meet their needs for installing a mixer shower etc. Keeping the existing system can substantially drive up their costs, and money is always a constraint.
I was under the impression that space was another constraint for the OP.
Many new houses have unvented cylinders or thermal stores, not combis. Although I've come across a few of these systems with inadequate controls or not fitted in compliance with the manufacturers instructions.
Why on earth doesn't somebody make a boiler which does central heating in the same way as a combi, ie without the need for a cylinder, but without heating hot water also? Then you could size it appropriately for your central heating system, and not have this problem of cycling?
If your only concern is your central heating, and you don't require hot water to be heated by the boiler, then there is such a boiler. A system boiler or a heat only boiler. The worcester 12i for instance can run at 3.45 kW. You can use it for your heating only. I take it an electric water heater would then be used?
You need to decide exactly what you require, and then more accurate advice can be given.
You need to decide exactly what you require, and then more accurate advice can be given.
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