What is the best boiler / size

[Copper]

Hello folks.

What is the best boiler / size

A little bit info on the house.…

The flow rate from the cold tap in the kitchen is just over 16 ltrs a minute.
Smallish 2 bed terraced house with a total of 6 radiators
1 bathroom c/w shower. There is never more than one tap on at a time.

I will be doing a clean swap with an old Vokera 24kw combi.
I was thinking of upgrading to a little more power 28kw combi, but is it worth it ?

I am in the North of Scotland and I could still feel the cold on some winter nights. Does the extra 2kw heat up the house more, I can see that you can get an extra 1.5 – 2 ltrs of hot water a minute, but does it put more oomph into the radiators ?

I’m not blessed with money, so would it be wise to spend that wee bit more on something like a 24 kw Worcester / Valliant / Veismann. Or should I go for a Baxi Duotec or Platinum 28kw with a 7 warranty (or 10 year with the Platinum)

In fact I’ve been looking at a fair bit of boilers over the weekend, what’s frustrating is everyone of them gets a good review and everyone of them can get a bad review ! So for that reason, warranty, price and size is what I’m looking.

This is my (long) short list…….. Any help would be appreciated


BAXI DUO TEC 24 or 28
BAXI PLATINUM 24 or 28
IDEAL LOGIC + 24
IDEAL VOGUE 26
GLOWORM Ultracom2 24
GLOWORM Flexicom 24
GLOWORM Betacom 24 / 28
POTTERTON GOLD 24 or 28
POTTERTON TITANIUM 24 or 28
VAILLANT ecoTEC PRO 24 or 28
VIESSMANN VITODENS 100W 26
WORCESTER BOSCH GREENSTAR JUNIOR 24
WORCESTER BOSCH GREENSTAR Si 25
WORCESTER BOSCH GREENSTAR Cdi 28

Many thanks.

 

[petertheplumber12]

Nowadays and this can be controversial so I will say it. I would now choose intergas, they have less parts to wear out and a 10 year guaranty on the boilers heat exchanger. But there are plenty of people with different opinions so just don't go for cheap get a band A rated boiler.

 

[JohnD]

I am in the North of Scotland and I could still feel the cold on some winter nights. Does the extra 2kw heat up the house more, I can see that you can get an extra 1.5 – 2 ltrs of hot water a minute, but does it put more oomph into the radiators ?

you'd need an unusually large house to need 24kW of heat.

Much more likely is that your radiators are not big enough. Add up the total length of all your radiators (treat a double radiator as one-and-a-half times its length for simplicity)

As a very rough rule of thumb, a modern, finned, single radiator, 600mm high x 1000mm long, can output about 1kW of heat under (improbable) ideal conditions.
see http://www.screwfix.com/p/kudox-pre...ingle-convector-radiator-white-600x1000/36126 for example, and a double of the same size nominally about 1750, see http://www.screwfix.com/p/kudox-pre...ouble-convector-radiator-white-600x1000/65888

Depending on size, age and quality of insulation, I reckon a typical bedroom needs about a 1500W radiator, but if you fit, say, a 2kW double, then you can heat the room faster from cold. If you have TRVs then a bigger radiator will not use more gas, once the room is warm it will automatically turn itself down.

I used to be away a lot on business, so most of my rads are twice the size a calculator might say, so it warms up fast from cold, and has plenty of spare capacity for unusually cold nights. It still uses less than the boiler is capable of delivering, which is fine by me.

 

[transam]

Some are alloy heat exchangers all the worc's for instance & the logic

some are full stainless steel veissmann & vogue

some are probably composite & stainless steel heat exc's that need to incorperate a fuseable link

and some ?? most?? contain more plastic than u would find at one of them tupperware parties in the 70's

the viessmann has probably got the most plastic & rubber hoses connecting the heat exchanger to the manifold , the worcs has probably got a plastic/rubber hose connecting the gas internally ??

 

[D_Hailsham]

The flow rate from the cold tap in the kitchen is just over 16 ltrs a minute.

But what's it like when both hot and cold are on? Don't forget that the 16l/min has to be shared.

Smallish 2 bed terraced house with a total of 6 radiators

Check your heating requirement using Whole House Boiler Size Calculator. (Set the Domestic Hot Allowance to 0kW as you have a combi boiler.)

I’m not blessed with money

You're obviously not a plumber.

 

[JohnD]

is that document out of date?

it says
Boilers should be sized to provide sufficient heat to provide hot water, and warm the dwelling to the required comfort temperatures when the temperature outside is below freezing. (Historically, this has been -1°C although more recently -3°C is being recommended).
This means that for most of the heating season, during the times of day when comfort temperatures are required, the boiler will produce more heat than can be used by the system.
The heat excess to requirements is lost via the flue through convection. The cooler air entering at the base of the boiler cools it so that it fires-up again, even if the system does not need any heat at that time. When the internal thermostat is up to temperature the burner is switched off, but the heat produced is going nowhere except out of the flue. The boiler is doing what is known as short-cycling.

what about modulating boilers, and others with a fanned flue? It doesn't mention that the room stat will stop the boiler firing when the room is up to temp.

 

[ALEC1]

.. incorrect deleted ..

Examples are vaillants 350 and 470 controller, these wind down the temperature radiator as room temperature approaches set point (indoor compensation) or run the temperatures just warm enough to maintain a stable room temperature (weather compensation)

you still have to size the boiler correctly though...

 

[D_Hailsham]

The description of the effect of boiler oversizing is a bit simplistic, I agree, but even with modulation many boilers are still oversized in the cooler months. It's often worse when a combi boiler is involved as they are usually sized based on just the hot water requirements.

There is a later version of the calculation but it's only available as a document and not as an online calculator.

I don't know what difference it makes to the size of boiler, but an internal temperature of 19.6C seems a bit on the low side.

All condensing boilers have to take compensation controls which match heat generated to heat required, varying the boiler temperature.

That may be true in Germany but not, as far as I am aware, in the UK.