What`s the point?

[charliechaplinspants]

If boiler sizes are calculated by Hot water requirements then why do fitters measure each room then size the boiler for the total amount of kilowatts required for each radiator/ room in the house?

 

[Terrywookfit]

They are only sized for hot water if it is a combi.

If you have hot water storage, the boiler is sized on heating requirements.

 

[D_Hailsham]

If boiler sizes are calculated by Hot water requirements then why do fitters measure each room then size the boiler for the total amount of kilowatts required for the house?

That is only true for combi boilers as the boiler output is determined by the hot water flow rate and the temperature rise (usually assumed to be 35C).

So if you want to deliver 15 litres/minute of hot water with a temperature rise of 35C your boiler will have to produce nearly 37kW.

Now the house may only require 15kW to heat it up in a cold winter. The installers will measure up each room to find out what size radiator is required for the heat loss from the room.

If you have a "standard" boiler with a hot water cylinder you workout the heat loss from the house and then add 2 to3kW for the hotwater cylinder.

The "whole house" method is recommended for sizing the boiler (see EST online boiler calculator). But this is not suitable for combi boilers and you still need to determine the radiator sizes.

 

[charliechaplinspants]

So the bigger the combi, the better hot water flow rate? what`s the point of measuring the rooms at all then, wouldn`t it make more sense just to whack in the biggest combi on the market for a good flow rate, I mean it`s obviously gonna take care of the heating side also.


Is that right or am I barking up the wrong tree?

 

[glasgowgas]

Is that right or am I barking up the wrong tree?

WOOF WOOF WOOF WOOF!

 

[mrgas]

One needs to measure the rooms to get the correct size radiator for the room. Unless a house is massive, the smallest combi will be oversized for the space heating requirements.

 

[D_Hailsham]

So the bigger the combi, the better hot water flow rate?

No: it's the other way round. The higher the flow rate (from the cold mains) the bigger the boiler you can have. If your flow rate is only 10 litres per second then you do not need such a big boiler. You also have to take into account the water pressure and the number of taps/baths/showers which could be running at the same time.

what's the point of measuring the rooms at all then?

To get the correct size radiator for each room.

 

[charliechaplinspants]

Ok, so measuring up the room gives you the right Kilowatt for each room regardless of boiler size then. I read somewhere that in order for a condensing combi to be of use the rads need to be oversized if a larger kilowatt of boiler is installed in order for the return temperature to be correct ( is it 55 degrees )? But oversizing the rads would negate the need to measure the room wouldn`t it?
I`m asking because I don`t know.

 

[fatplumber]

Modern boilers effectively range rate themselves so you don't have to wory about oversizing the rads.

 

[charliechaplinspants]

Doesn`t that just mean that the gas valve modulates to demand? How is the return temperature achieved in order that the boiler condenses properly & not just at startup?

 

[mrgas]

pcb controling heat input and pump speed

 

[D_Hailsham]

Ok, so measuring up the room gives you the right Kilowatt for each room regardless of boiler size then.

Correct. But you still need to make sure your boiler can deliver the total kW required by the rads. That's no problem with a combi, as the HW requirement is usually much greater than the heating, but a regular boiler has to deliver the rad output plus 2kW for heating the hot water.

I read somewhere that in order for a condensing combi to be of use the rads need to be oversized if a larger kilowatt of boiler is installed in order for the return temperature to be correct ( is it 55 degrees )? But oversizing the rads would negate the need to measure the room wouldn`t it?

Let's start at the beginning!

The traditional, non condensing boiler was designed to work with a flow temperature about 75-80C and a return temperature about 10-11C lower. Radiators are tested on this basis - they actually use a flow of 75C and return of 65C with the room temperature set to 20C.

A condensing boiler only condenses if the return temperature is below approx 55C. If you have a flow temperature of 75C, that means a temp difference between flow and return of 20C.

If you run a radiator with flow of 75C, return of 55C and a room temperature of 20C, you will find that the output of the radiator is about 15% lower than the mfr states. You therefore have to install a rad which is about 15-20% larger than the calculator says if you want to make full use of the boiler's condensing abilities and heat the room to the required temperature.

So you:

1. Size the rads (without adding anything for condensing).

2. Size the boiler using the Whole House method, or hot water requirement for a combi.

Compare the Rad Total kW with the Whole House heating kW to make sure there is no serious discrepancy. Then select the appropriate boiler.

3. Increase the rad sizes by 15-20%, which will allow the boiler to run with the return temperature below 55C, i.e in condensing mode, more often.

 

[bengasman]

If boiler sizes are calculated by Hot water requirements then why do fitters measure each room then size the boiler for the total amount of kilowatts required for each radiator/ room in the house?

Because they have always done it that way. Also makes them look like hardworking experts.

 

[charliechaplinspants]

Thanks for your time D.Hailsham, gotta go now but have lots more questions, talk to you again I hope.

 

[doitall]

Anyone mention the cost difference between a 1000 x 600 Dp than a 2000 x 600 Dp.