Two pipe heating systems

[John84]

Hi all,

Need some help on understanding how heating systems work.

Firstly

With two pipe systems I heard you are limited to the number of radiators due to the friction imposed by radiators, is that true, if so, isnt that a limitation with single pipe systems aswell?

Secondly

I thought there is actually water circulating in radiators, flow where hot water comes into radiator and return where water returns back out. If there is no water circulation flow (in) return (out) then wouldnt the flow pipe be under severe pressure in the sense that water can only be pumped into the radiator?

please advise - any suggestions appreciated,

many thanks

 

[justlead1]

Hi. I dont quite work like that. Think of an inner and outer brake cable on a bike. The water being the inner and outer the pipe. The water is not pumped but circulated. (as with inner cable, move one end and other end responds) One pipe systems rely on frictional resistance on the pipe between the rad connections to push / pull water through the rad. Two pipe system has a dedicated flow and return which is not guided by pipe friction in the same way as one pipe system. Although lock shield valves on rad are used to control volume / energy as circulated water will take the easiest route, which can result in poor circulation to the periphery of the system, the lock shield valve allow the system to be balanced. Offering each rad and correct proportion of circulated water. The bigger the system the greater the piper sizes, in order to carry the loading the rads require. Good Luck

 

[Bernie2]

Two pipe system:

Hot water flows from boiler down one pipe and returns by another to be reheated then goes out again. Its really a circle. There is of course friction all along the route. But proper pipe sizing usually compensates for this.

In short the bigger the pipe the less resistance. Simply because less of the water touches the walls in a big pipe than it does in a small one. And as its the walls that cause the friction then the bigger pipe has less friction loss.

So if you wanted a radiator 10 meters away from a boiler then 15mm may do.

But if you wanted it 20 meters away then 22mm would be better.

The things is, if you use fittings on the pipe you have to add on their equivalent length, so if you used a pile of elbows for instance they may add another 10 meters on to the length of pipe, so instead of pumping the water 10 meters, if you add the fittings it may be more like 20 meters in which case you would perhaps use 22mm pipe instead of 15mm.

Its not that easy of course, but that is perhaps a rough working idea.

If you have 15mm pipe at 2 bar pressure and change it to 22mm the pressure stays the same.

If you reduce a 22mm to 15mm the pressure drops slightly but the velocity increases.

If you scale up a 15mm to a 22mm the pressure rises and the velocity drops.

Have a look at Bernoulli's Principle animation its interesting, to Plumbers anyway!

 

[Plouasne]

Bernie, you forgot to say its interesting only to plumbers who are interested in advancing their knowledge of the craft of plumbing

 

[John84]

Thanks for your replies chaps,

My thought is that because two pipe systems have more pipework, there becomes more resistance hence correctly sized pumped are vital is that correct?

With regards to water circulation in modern radiators, isnt there a constant flow of of water through the radiator from flow to return or does the water flow into the rad first, then via condensation it drops to the return leg?

I initially thought that as the boiler heats up the water it flows to the rad and then through to rad to return?

please advise your previous points are invaluable

 

[Bernie2]

1. Yes pump and pipe sizes must be good.

2. In the old rads and some modern ones, the hot flow goes in the top and cools as it drops through the rad. Today however they rely on what Plumbers call balancing. And that is each rad has a valve at each end. One you can adjust and one you can't unless you know how. The one you can't, called a lock shield, basically because when set they don't want you to touch it, is the interesting one and the one you do the balancing on.

To do it properly requires two pipe thermometers, what you do is put one on each pipe going into the rad. You then adjust the lock shield until you have a temperature difference between the two.

What that tells you is the temperature drop over the rad. You go to each rad and do the same. Then to the boiler and check the temperature drop there.

There is a bit more too it than that of course, but if you Google something like "balancing central heating" it will probably tell you how to do it in more detail.

So what you are really doing is slowing the water going through each rad. As you say in theory if all the valves where fully open and there was no resistance in the system it would pump out one side of the boiler and return on the other. But system resistance and valves adjust things to slow the water and give it chance to release its heat.

In the old days they did the same thing without a pump. The pump was regarded as an assistant to natural circulation. Today however, lots of systems would probably not circulate without a pump.

 

[John84]

Thanks bernie thats sound advise.

Just one area of clarification - are radiators filled with water then or is there an air gap?

 

[Hugh]

Bernie you have read alot of books! cheers for intresting reading!

 

[Bernie2]

No rads are filled with water. Air can help corrode metal in the system.

If you read Migos and the other Administrators as well as Plouasne's and others mail's, you will find they know a lot more than me. Its all part of Plumbing.

The short courses only give you a start off. Don't worry though, just do what you can do and read as you understand.

Being honest when learning Plumbing, I often did things I didn't know much about, but learned more later.

Somehow it sometimes seems easier to learn that way.

Like getting lost in a forest, soon makes you realize how useful a compass and navigation skills would be. So when your cistern looks out of level, you soon realize its probably better to set out levels before you install than try to do it afterward.

Its all part of learning. I always feel sorry for the new lads nowdays as everybody wants you working full belt from day one with no mistakes allowed.

In my working day you had more time to make mistakes and put them right. In other words we had a longer flatter learning curve.

 

[Plumbstar Tom]

Have a look at Bernoulli's Principle animation its interesting, to Plumbers anyway!

I have googled this as I haven't seen it before but it refuses to load, it may be my computer

The best way to explain it is what happens when you squeeze the end of a hose pipe

 

[doitmyself]

With two pipe systems I heard you are limited to the number of radiators due to the friction imposed by radiators.

The friction in a radiator is negligible. It's only the friction in the pipework whcih has to be taken into account. This determines which size pump is required. Unfortunately there are only two sized commonly available, so a compromise has to be made when selecting the most suitable pump.

I thought there is actually water circulating in radiators, flow where hot water comes into radiator and return where water returns back out.

Obviously there is water in a radiator - if there wasn't the radiator would not get hot!

If there is no water circulation flow (in) return (out) then wouldnt the flow pipe be under severe pressure in the sense that water can only be pumped into the radiator?

I think you mis-understand how a two pipe system works. It's like a ladder. One side is the flow, the other is the return and the rungs are the radiators. You can shut off a radiator by closing the valves (equivalent to removing a rung) and the water will still continue flowing round the circuit.

The flow of water through a radiator is mainly due to the fact that there is a difference in pressure either side of the radiator.

 

[traineedrip]

I think you mis-understand how a two pipe system works. It's like a ladder. One side is the flow, the other is the return and the rungs are the radiators. You can shut off a radiator by closing the valves (equivalent to removing a rung) and the water will still continue flowing round the circuit.

The flow of water through a radiator is mainly due to the fact that there is a difference in pressure either side of the radiator.

Just to add if you stopped the flow to every radiator there is still a bypass fitted somewhere in the system, allowing flow to enter the return pipe and circulate.

 

[bobpape]

Something fundamental that ought to be mentioned,
Is when one fills a system with uninhibited water (no fernox) the oxygen in the water
reacts with the bare steel inside the radiators and produces ferrous oxide and in doing
so produces hydrogen gas, this gas behaves in the same way as air in the system
except that unlike air it is inflamable, you can establish whether or not you are dealing
with gas or air by attempting to ignite the produce when bleeding the system,

When a system is filled with uninhibited water the oxygen in the water remains active
for about three months and then becomes inert no longer creating rust and gas.

as the oxygen has depleted