7mm or 15mm? copper or polyplastic?

[Nanothermite]

how does the type of piping to radiators affect how much heat is delivered to the radiatoirs?

how does the following piping compare:
7mm copper
15mm copper
7mm polyplastic
15mm polyplastic

I have radiators that are not getting hot enough, I am wondering if the piping to the radiators need changing.

if a radiator is giving out its maximum btu/kw rating, what temperature is the radiator likely to be? shouldn't the radiator be piping hot, ie too hot to touch? my rads are hardly even warm, yet their btu rating is pretty much what the calculatin say they should be for the room.

 

[ericmark]

clearly the less water in the pipe the quicker it reaches the radiator however the reduced flow rate will mean it takes longer to heat up.
My system uses 8mm and furthest radiator from system switch on to feeling warm is around 10 minutes. I have mixture of 22mm, 15mm, 10mm, and 8mm. The pipes reduce in size as it radiates out. The main delay is old boiler which takes quite a while to heat the cast iron tube (big lump of iron) in the fire tube boiler.

But the pipe size will not really effect how hot a radiator gets. A part which need not be touched for normal operation Metallic is limited to 80 degrees C. regulation 423.1 of BS7671:2008. So unless no exposed pipes and using a Myson Radiator that your limit. A part intended to be touched but not hand-held reduces to 70 degrees C. So the maximum temperature out of the boiler is somewhere around the 70 degrees. (That's if electrical controlled as temperatures are from electrical regulations). We expect around 15 degrees fall across the radiator plus pipe work losses so the radiator should be around the 60 degree mark. So question one how hot is the radiator? In other words are you expecting too much?

Using oil and of course metal pipes with insulation on them you could start at 200 degrees allowing smaller radiators although would have to be like those used in a car so just hot air to can't touch metal but never seen in domestic. May arrive in the future as it would allow off peak power storage but not as yet.

So down to earth again and lets consider standard system.
1) Boiler (bad name as it does not boil) heats water to approx 70 degrees.
2) Pump gives water a head to move it.
3) Radiators allow a restrictive flow designed to give 15 degree drop.
4) Any radiators in warm rooms will auto switch off.
5) As radiators switch off by-pass valve opens.
6) As warm water starts to return to boiler flame is reduced.
7) As hot water returns to boiler flame goes out.

What are the problems? Well major one is any radiator not set correct will allow hot water to return before room is hot. If this is allowed to happen boiler will shut down. Second and related problem is if one radiator is wide open there is no head to push water through others in the same system. However the thermostatic valve will to some extent address the problem of badly set lock shield valves. And you will get the following.

Easy radiator to get water through heats first. Hot water returning from that radiator will reduce output or shut down boiler. Once that room warms up the thermostatic valve will close and next easiest room will start heating. As long as there is no electric thermostat it will one by one heat the rooms as each room cools the valve on that radiator will open and re-heat that room. So although from cold it will take ages it will in the end heat whole house and once hot will hold it that way.

However if there is an electric thermostat as well in one of the rooms once it reaches that room then it will go no further.

Now most of us have central heating fitted in the summer. So on testing all seems to work. Only as winter comes is the problem evident and even then likely only after it's been switched off for a day.

So then comes the setting of lock shield valves. OK it says 15 degrees easy without thermostatic valves but with thermostatic valves installed as each radiator switches off the head will increase so more water flows so set a radiator at 15 degrees do the rest come back to first and it's no longer at 15 degrees. So the idea of two K type thermocouples connected to my comparison meter may seem easy but just does not work in practice. So start with cold system. Open furthest radiator full open (we don't want boiler to switch off) the rest turn completely off. Then one by one open valve slowly until warmth is felt in pipe at that point stop. Then close last one completely and switch off boiler let it cool. Once reasonable cool switch one again and open last radiator until warmth is felt.

Ok now we have some heat in all radiators not enough but some. Next is slowly to tweak each one. Very little at a time open cool ones a little more. (At this point all thermostatic valves wide open) Over the next few days one may have to open or close a valve 1/4 of a turn. You want a difference on every radiator between incoming and outgoing temperature.

Once this is all set the boiler will not shut down as quickly so hot water will be feed to each radiator for longer. Once all working A1 consider how long it took and then realise why the guys who fitted it did not get it right. It takes a long time. OK for home owner watch TV and in adverts tweak the radiators a little. But commissioning guy just does not have that time.

So do you think I have guessed right? Was it your lock shield valves needing setting?

 

[Nanothermite]

thanks, I'm guessing the problem is not the pipes then.

"So question one how hot is the radiator? In other words are you expecting too much?"

I would estimate the temperature by touch of all 10 rads around 15-25 celcius after hours of burning, i have no instrument to measure it but i have a tropical fish tank which warms to ~20C and the rads are around the temp of the fishtank water.

bolier is a wood burner with output 8.7kw with max of 13kw,(I have directed all heating to the central heating, none to hot water), kept the stove burning at constant 400-500C (stove pipe temp).
system is S-plan with 250L hot water cylinder.
8 rads are 1.2 kw each, 1 rad 1.1kw, 1 rad 0.7kw, so total is 11.4kw rads.
2 of the rads are heat leaks, these are a bit warmer than the others, but the heat leaks are not too hot to touch.
I have turned off 3 or 4 rads manually (not the heat leak ones) which doesn't seem to raise temp of the the others.
electronic room stat (remote connection, not hard wired) is set way up at 35C.
the temperature of the 28mm return pipe to the stove is much warmer than the temperature of the rads which doesn't make sense to me.
had a look in the expansion tank in the loft and it looks pretty gunky, like stirred up pond water. all pipes and rads are new installed by a plumber so I wasn't expecting that.

had it recently installed, plumber doesn't seem to think there is a problem. the weather is mild, but CH doesn't heat the rooms, I dread to think what is going to happen if we get a harsh winter.
I did not see the plumber balance them, but given the temperatures of the rads were never very warm I don't see how he could have balanced the system.

edit - the spec of the stove says it will power hot water and 10 rads, and the 10 rads are roughly in line with my btu calculations for the rooms.

 

[durhamplumber]

Here is an answer - I hope it is what you were looking for.

Flow (Laminar) of Liquid through Pipe:

The appropriate equation for laminar flow (i.e., not turbulent) of a
liquid through a straight length (L) of pipe or tubing is:

FR = (Pi (R^4) (P - Po))/(8 N L)

where FR is the volumetric flow rate of the liquid (e.g., gal/sec),
Pi = 3.14159..., R is the radius of the pipe or tube, Po is the fluid
pressure at one end of L, P is the fluid pressure at the other end of
L, N is the fluid's viscosity, and L is the length of the pipe or
tube. The temperature dependency here is in N because the fluid's
viscosity depends on its temperature. A significantly different
equation must be used for gases.

 

[durhamplumber]

in short,all else being equal,if you double the radius of the pipe,you increase the flow by 16 times!