calculating btu value for copper underfloor heating

[pappasmurf]

can someone pls help......i have just installed underfloor heating in our newly built kitchen using 22mm copper pipe. The subfloor was membraned and properly insulated using 100mm jablite and concreted and the pipe work was covered with denzel tape and then the floor screeded and tiled. The under floor heating in this area works just fine. i am this summer thinking of carrying on this underfloor heating link into the adjacent through lounge. my question is can my boiler handle all this pipework?..... as i have no way of calculating the btu value of just pipes covering my floors!!...... i have a worcester bosch 40cdi condensing boiler supplying the houses needs. my lounge is approximately 27ft x 16ft.
the kitchen was piped only in the usable walking areas and not underneath the units!.... pls help. i dont wonna install any plastic poly tube. many thanx in advance.

 

[Onetap]

pls help. i dont wonna install any plastic poly tube. many thanx in advance.

Why not? It is far superior to copper for this application. Copper hasn't been much used since about, approximately shortly after PEX became available.

 

[bengasman]

As your under floor pipe is pumped by the manifold pump, it has no influence on your boiler pump.

 

[DP]

You cannot have joints on plumbing that is not accessible, so copper on under-slab application is questionable

 

[pappasmurf]

with all due respect to the above replies.... i have already chosen copper over pex piping because i will be installing diverter valves in case god forbid the pipework underneath should ever leak i can then manually divert the supply of heat back to the radiaters in the room and since the rest of the house is made up of 22mm pipework i dont want to restrict the flow of heating by using smaller diameter piping . And hopefully my joints will be more then secure "without" any elbows being inserted; instead bending all the pipework to provide for a more efficient system. i have done exactly this in the kitchen and for the last 2 years everything has been fine. oh and the boiler is a condensing "combi" so does not have any external pumps. i just need to know how to calculate the btu value i.e every 1m of 22mm of pipe i will use "x" amount of btu..... someone pls helpppppppp.

 

[bengasman]

Well, every under floor heating system needs a separate pump, manifold, and thermostat.
BTU calculations can not be done for a bunch of pipes in a floor. Besides that, even if you could, what would it matter? The pipes are there whatever the value. What would you do with the value?

 

[pappasmurf]

the reason i need to know how to calculate the btu value is if it is even viable that my boiler can handle this area. and i cant have a pump in a combi system anyways! and yes there will be a room thermostat...so pls stop trying to point out the obvious i know that if it is under the floor it is under the floor!..... if u cant help then pls dont reply. thank u!

 

[bengasman]

Not a roomstat, a manifold thermostat which controls the temperature of the water in the underlfoor section separate from what goes round in the rads.

The pump I mentioned pumps the water in the underfloor section, again totally independent from the pump in your combi that pumps the rads.

This is why you also need a manifold.
Without this, the problem you will have is opposite to the one you expect, although the result will be the same; your rads will not get warm enough and the temperature drop over your rads will be too low.

The result of what you have done will be a warm floor and a cold house.

 

[DP]

Typical response from a 'handyman' who wants advice but does not want to listen to what does not agree with his/ her plans.

SIMPLE FACTS. 1) Boiler pump is needed to send water to the load, be it radiators, HW cylinder or COPPER (not suitable material) under floor heating.


2) Radiators will take flow water at temperature of 85 degrees C, even 95 when boiler shuts down safely.

3) UF heating loop is around max 50 degrees C

How do you plan to distribute water between the underfloor loop and radiators at different temperatures?

 

[bengasman]

Too late dp, we have been found out. All we want to do is baffle the poor punter with science so we can sell him all sorts of stuff he doesn't need.
He is not stupid, he knows you can just stick a bit of pipe under the floor as long as it is not that inferior plastic stuff, but proper copper.

 

[LongTooth]

pappasmurf,

the heat output of a pipe, like any other radiator, depends on the insulation around it and the temperature difference between the pipe and the surrounding materials. When you buy a radiator, the quoted BTU is an approximation based on assumptions of water temperature, the air temperature of the room, and the position of the radiator. As we all know, putting a towel over a radiator increases the insulation around it and so reduces its power output.

You can calculate the BTU (or at least watts) of any radiator, including copper pipe, if you know the input and output temperatures of the water and the volume of water flowing per minute. Calculating the volume per minute is not trivial, but if you can disconnect the circuit, you can get an approximation by filling a bucket!

If V = Litres per minute of flow through radiator or pipe; dT = Temperature difference between outlet and inlet of radiator or pipe; Then power output (P) in kilowatts (kW) is given by:

[code:1]P = 0.07 x V x dT[/code:1]

Hope that helps.

 

[doitall]

Excellent long in the tooth, but you need to factor in the pipe size

 

[LongTooth]

For a given volume, pipe size will affect the flow rate in metres/second (and other things like the head loss etc), but for the heat calc all you need is volume of water per unit time and the temperature change.

In other words, if you joined a 22mm copper pipe to a 10mm plastic pipe (so they had the same volume per minute flowing through them), then if the copper part had a 10C temperature loss across its length and the plastic part had another 10C across its length, then both pipes are radiating the same amount of energy/second (Watts).

Of course, the 10mm plastic pipe would need to be MUCH longer than the copper bit to achieve this result, but the point is that the same volume of water per minute and the same temperature drop = same heat output, irrespective of pipe diameter or material.

 

[Terrywookfit]

The surface area of the pipe will determine its heat loss.

 

[LongTooth]

The surface area of the pipe will determine its heat loss

Yes, many factors determine the heat loss. What you are saying is that a larger surface area will emit more heat (at the same temp), and so if one wants more heat output, one might use a larger pipe or radiator. This is correct.

However, what I am saying is that the amount of heat loss is related only to the volume of water per second and the temperature difference. I am not saying that 1 metre of 15 mm pipe radiates the same heat as 1 metre of 28mm pipe.

This is simply a physical property of the water. 1 litre of water needs about 4200 Joules to raise its temperature by 1 Celsius. If you have 1L per second of water flowing through a pipe and the temperature drop between the ends is 1 Celsius, then you have 4200 Joules per second being lost = 4.2kW, irrespective of the pipe size or material.

A 28mm pipe that had 1C temperature drop would be shorter than a 15mm pipe with 1C temp drop in the same conditions (e.g. free air). The 28mm pipe will have a lower water velocity at 1L per second than the 15mm pipe will have at 1L/sec, but these factors do not affect the total heat radiated for a given volume per second and temperature drop.