Measuring the U-Value of a Room

[Bon]

There's the properties of the methane contained in Fido's f@rts to contend with too!!

 

[kirkgas]

send me an email address (get mine from my profile) and i will send you a tutorial which we use to teach plumbing students how to do this

 

[kirkgas]

There's not really any such thing as a U-value for a room. A U-value is the amount of heat lost through one square metre of a given component. A room would have at least 5 components: the ceiling, the walls, the floor the window and the door, and all these would have different U-values. Combining the U-values together does not produce one overall U-value.

you are right, what people talk about as u value of a room is actually the heat loss of the room (worked out by calculating the different u values for every m2 of material in the room, taking into account the use of the room and the temp through the wall from the room as well)

 

[Bon]

There's not really any such thing as a U-value for a room. A U-value is the amount of heat lost through one square metre of a given component. A room would have at least 5 components: the ceiling, the walls, the floor the window and the door, and all these would have different U-values. Combining the U-values together does not produce one overall U-value.

you are right, what people talk about as u value of a room is actually the heat loss of the room (worked out by calculating the different u values for every m2 of material in the room, taking into account the use of the room and the temp through the wall from the room as well)

Not strictly true.

You take the area of each type of fabric and that has a corresponding u-value which is used as a constant in the equation

Heat Loss (W) = Area (m2) x U-value (W/m2.K) x [Room Design Temp - External Design Temp] (K)

Totalling up for external walls, internal walls, windows, doors, floor and roof only gives you your fabric losses. You then need to take into account your infiltration losses and factor in any ventilation that may be in place.

That totalled up gives your your heat loss (I always add 10% as a margin too).

The actual U-value is given (or found in tables etc...) and isn't worked out for every heat loss. Either that or you can work it out as it's the inverse of the material's R-value!

 

[sandyn111]

Despite all the 'helpful' comments, this is an extremely sensible question and very relevant. It is possible to calculate what could be called the U value of a room. The U values you get from tables for walls and floors are only a guide, but depending on the fabric of the house, they can be way off. By using a dual channel temperature logger and a variable power source, you can get a figure for the heat loss of a room. It just so happens that I have been working on a system to do these measurements. I have a dual channel temperature data logger based on a Velleman 8055 USB interface card and I use the NTC resistors from a temperature acticvated switch....all available from Maplin. I have written the software which reads the 'temperature' and saves it to a file. and draws a graph of the temperature over time. The logging time can be 8 hours if required. I still have to devise a way of having a variable heat source, controlled by the PC, but that should be reasonably easy to do.
....measure inside tempreature, outside temperature, you know the total area of the room, and the power input, so you can calculate the heat loss and the equivalent of a 'U' value for the room.

 

[jeds]

Your device might tell you the heat loss from a room over a given period but that isn't the same as a U-value.

 

[Bon]

That gizmo would only really be helpful though when trying to evaluate whether a design for an existing system is adequate.

To do the design in the first place (where U-values are actually used) your gadget isn't very useful, especially as you'd need the room fully built in order for it to work, so you wouldn't be able to get your first fix pipework in, not to mention the time delay you'd need between the rooms being built and the heating being installed.

It may be beneficial in a domestic situation, but not in the commercial sector.

 

[sandyn111]

The gizmo is for measuring the thermal performance of a room I have insulated. I can calculate an equivalent of a U value. The U value is W/m2K. it is valid in a 3D structure. You get an measure of the REAL thermal transfer from inside to outside.

I can measure the inside temp, the outside temp. I can evaluate the amount of power to keep the room at a constant temperature when the outside temperature is lower. At that point, the power into the room equals the heat flowing out . I know the area of the room, including four walls, floor and ceiling, so I can calculate the heat loss(w) per metre squared kelvin, which are the units of U, therefore, it must be a U value for the room.

 

[Bon]

And where do the heat losses due to infiltration factor, which are based on the volume & ventilation of the room as opposed to the surface area?

Your tool sounds very impressive (!), however as I pointed out earlier in the thread, u-values are used as constants in theoretical equations in order for the required heat input to be calculated therefore proving them isn't really required.

I absolutely agree that tools for measuring the thermal performance of a room post construction are very useful to ascertain if the system is performing to the design criteria, however the hang-up on u-values is a little unnecessary IMO. Like I said earlier, we have design guides for this.

 

[IJWS15]

To find actual U value . . .. .

Set weather to maintain constant temperature outside . . Very difficult

Set heating to maintain a constant temperature inside the room.

Close doors and windows, remove dog and cat.

Measure energy used by heating, deduct portion used heating rest of house - difficult.

Measure area of walls, floor, ceilings

Measure temperature difference inside to outside (assumes it is the same for all walls - means you have to demolish the rest of the house and put the room on stilts or the temperature difference would vary by wall and wall to floor ).

The energy used to maintain a constant temperature over a set period will allow you to calculate the U value of the room.

Now if yo think about the difficulties of real life measurement you will understand why the theory is used.

 

[Bon]

To find actual U value . . .. .

Set weather to maintain constant temperature outside . . Very difficult

Set heating to maintain a constant temperature inside the room.

Close doors and windows, remove dog and cat.

Measure energy used by heating, deduct portion used heating rest of house - difficult.

Measure area of walls, floor, ceilings

Measure temperature difference inside to outside (assumes it is the same for all walls - means you have to demolish the rest of the house and put the room on stilts or the temperature difference would vary by wall and wall to floor ).

The energy used to maintain a constant temperature over a set period will allow you to calculate the U value of the room.

Now if yo think about the difficulties of real life measurement you will understand why the theory is used.

It's not just me who thinks this is an absurd waste of time then! Phew

 

[sandyn111]

a complete waste of time??? I'm not asking you to do it and my time is my own to do whatever I want with it.
I'm sure there will be things you do that I think is a complete waste of time.

Why does someone climb a mountain????because it's there. Why do I do it....because I can.
Don't take life so seriously guys. Ha ha ha

 

[Bon]

Steady on mate, you can do whatever you like with your time.

It's just that I get paid to design heating systems and the like, and I therefore know that the device you have come up with will do diddly squat to assist a Mechanical Engineer in designing systems.

You run along though and carry on with your widget.

 

[sandyn111]

For me, this topic is exhausted. My answer was aimed at DIYWell's original question. Yes it is possible to get an indication of the U value for a room using a USB interface card, which costs £40 and a bit of software.

 

[freddiemercurystwin]

For me, this topic is exhausted.

And for everyone else that happened a year ago!