Calculation for Time taken to heat a room ?

[mitchk]

Hi Guys,

Can somebody help me to work out how long it would take to heat a room please.

We have snagged the radiators being too small in our new house and have just had the report back from the NHBC telling us that the radiators are adequatly sized for the rooms. If I take 1 room as an example could someone show me the workings out ?

Bedroom L 2.22m W 1.71m H 2.4m

The report states that the rooms Heat Loss is 377w and the Corrected Radiator Output is 500w

I'm assuming that this means that the room is heated by 123w / Hr

can anybody help please - think we are looking at a 4c increase

 

[kbdiy]

Not that I am particularly expert in this but am sure you will need to know the various heat loss values of all the construction elements of each room to calculate the heat requirement.

Who is the developer?

 

[mitchk]

hi mate - heat loss has already been calculated mate at 377w

in the report developer however did gave the consultant the U value of .28 for the external walls which seemed quite good and low.

The house is Full Cavity Insulated apparently

I'm just trying to find out that at 123w, how long should it take to heat the room ?

 

[Onetap]

The report states that the rooms Heat Loss is 377w and the Corrected Radiator Output is 500w

I'm assuming that this means that the room is heated by 123w / Hr

can anybody help please - think we are looking at a 4c increase

You've misunderstood this, I think.

The heat loss is the amount of heat escaping through the structure and by ventilation when the room is at a constant setpoint temperature (21 degC say) and the outside is at a constant design condition temperature (-4 degC or so). So long as you put in the same amount of heat via the rad, lights, occupants, etc., , the room will stay at the design 21 degC.

It's common to add 10% or so, to heat the room from cold, but since the outside design conditions are usually a small number of hours per year, it's not of much relevant.

You'd need lots more information to calculate the time taken to heat up the room (mass of structural components, their specific heat capacity, etc.). I think I used to know how to do it, but have never used it and forgotten. Unless you wait until it's -4 degC outside, and time it, there's no quick answer and that answer is not the one you want.

How have you "snagged" the rads as too small? If they just look small, you'd expect smaller rads for a house insulated to modern standards than for older buildings.

 

[cumfy]

A 4m2 room will need a very small radiator.

If it's a bedroom you won't really want it above 17-18 C anyway.

If you had very small rads in large non-bedrooms, I would understand.

I presume the 377W figure is based on a 20C equilibrium.

If you have a 500W that would be 22-23C under the same equilibrium conditions.

I guess you will know in January, but I think the worst case scenario is you'd have to wear a jumper.

 

[D_Hailsham]

We have snagged the radiators being too small in our new house and have just had the report back from the NHBC telling us that the radiators are adequately sized for the rooms.

Do you mean that this is a brand new house and you are the first owners? If so, the heat loss has to meet the standards set out in Building Regulations when the design was approved by the local Building Control Dept.

But you will know that, otherwise you would not be able to say with any degree of certainty that the radiators are too small.

If I take 1 room as an example could someone show me the workings out?

Not without knowing the exact construction of the walls, ceiling, floor and window, as well as the design temperatures.

Bedroom L 2.22m W 1.71m H 2.4m

The report states that the rooms Heat Loss is 377w and the Corrected Radiator Output is 500w

That sounds about right. If you have moved from an old house with poor insulation single glazed windows and draughty rooms, you will be surprised how much smaller the radiators need to be in a new-build house.

I'm assuming that this means that the room is heated by 123w / Hr

Where did you get that idea from? The heat required depends on three factors: heat loss from the room, design outside temperature (normally -1C) and required room temperature (normally 21C).

 

[cumfy]

Where did you get that idea from?

123=500-377

 

[sooey]

You haven't answered the question, why do you think the rads are too small? Are the rooms taking a long time to warm up? Are they cold?
What temperature setting is the boiler on?

 

[Weedrip]

The rooms losses are 377w [per/hr]

the rad is 500w output [per/hr]

The rad is ok!

To help understand the heat loss to the room is calculated to achieve the desired temp internally at the design min temp externally so the room will loose 377w per hr. by your calculations.

hope that helps.

 

[Agile]

The OP is not telling us what boiler flow temp he is using! Thats a major factor in the answer.

Practically, it would be hoped that a room will reach within 1 C of the target within about 30 minutes.

The heat output of a rad is higher as it warms the room up!

Tony

 

[Weedrip]

The heat output of a rad is higher as it warms the room up

Interesting thought!

At its simplest, the heat output of a radiator is a function of the mean temp of the water flowing through the rad and the temp difference between it and the room air temp.

Given that we would assume a constant mean temp of the flow within the rad in any calculation, the only real variable is the rise in air temp of the room. which in turn reduces the temp differential, which in turn lowers the effective output of the rad!

on the other hand.....perhaps I live in different reality.

 

[Agile]

Thats quite right!

With a room temp of 10 C the diff is 60 C

This reduces to 50C when its up to 20 C.

Its a useful feature!

Tony

 

[ALEC1]

all heat exchangers give off more heat the bigger the temperature differential between the two mediums, in this case air and water, through steel.