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I havebeen trying to discover the precise definition of a “one-pipe system” forcentral heating radiators. I know what atwo-pipe system is.

For aone-pipe system, there “appear” to be two possibilities:

Possibility1: The pipe carrying water from the pump enters the first radiator at the handvalve. The water flows through theradiator and exits at the lock shield valve. The water then flows on to the next radiator in sequence, and so on. In this configuration, there is only a singlepath for the water to flow through all the radiators in the system.

Possibility2: The pipe carrying water from the pump reaches the first radiator. The pipe then divides into two just beforethe hand valve. Some water flows throughthe hand valve into the radiator but some flows along the other pipe therebybypassing the radiator. The water exitingthe radiator at the lockshield valve enters a pipe that joins the bypass pipe,so that there is then a single pipe that carries the water onto the nextradiator in sequence, and so on.

Which ofthese two possibilities is a “one-pipe system”. Or are they both possibilities?

Eachpossibility will have different attributes. For example, you could not have any TRVs in a Possibility 1 system because,when the first TRV closes, it would stop all the water from flowing. But you could have TRVs in a Possibility 2system. And, if the pipes carrying waterbetween the radiators are hidden underneath the floorboards, it might be relativelyeasy to detect whether you have a Possibility 1 system. But it might not be so easy to distinguish betweena Possibility 2 system and a two-pipe system.
 
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Why do you want to know ? are you having a problem with your system ? how old is system ? what boiler do you have ? what are you hoping to do ?
 
P1 And P2 Are both 1 pipe systems, as the radiator takes it's flow and return from the same pipe.
 
A proper one pipe system will have a single pipe running beneath each radiator and both radiator valves will connect to the same pipe. Historically a one pipe system would consist of a fairly large bore pipe slowly rising to a high point from the boiler and then dropping quite rapidly to the return connection. The water circulation would have been by gravity. Individual radiators would have been teed off from this single pipe.

The advent of affordable circulators meant that it was no longer necessary to use gravity as the means of propulsion and the circulation pipework could get smaller and central heating systems became more affordable. The circulator only pushed the water around the system at a slow rate as the heat rose from the single pipe via gravity.

It's easy to tell the difference between the different types of system. With the hot water off and any bypasses closed shut one radiator valve. If this stops the flow you have an incorrectly piped one pipe system. If it doesn't shut the rest of the radiators down. If this stops the flow you have a two pipe system, if it doesn't you probably have a one pipe system.
 
foralone there seem two attributes one pipesystem super diet pills when pump go synchronised there were 7 gerbils unusual positionsthen WERE FULL and relative difficult times not when we were oncewhen?
 
If that was a question on the chase quick fire round I would have to say one pipe through all the rads connecting to both flow and return.

CORRECT.

Who was the first man on the moon?
David Hasslehoff.
 
Why do you want to know ? are you having a problem with your system ? how old is system ? what boiler do you have ? what are you hoping to do ?

First of all, my thanks to all who have replied.

The stimulus for this query was that British Gas recently installed a new Grundfos pump in my central heating system. The British Gas engineer was insistent that the pump speed be set at a particular value but, when I looked at the Grundfos instructions later, what the engineer had insisted on was not the same as what Grundfos recommends. Furthermore, the speed setting that Grundfos recommends depends on whether the system is a one-pipe system or a two-pipe system.

Now, I had never encountered these terms before. I had always believed that, for all radiators in all systems, water enters from the flow pipe and exits to the return pipe - that is, a two-pipe system. So I started to investigate on the Internet what is a one-pipe system, and it was then that I started to get mixed messages about what is the precise configuration of a one-pipe system.

To answer your remaining questions. Our house was built in 1978 and I assume that the radiators and their pipework were installed at that time. However, in 2009, British Gas installed a new boiler (a Worcester Greenstar 24Ri gas fired condensing boiler), along with a new hot water cylinder, new zone valves, a new pump, a new bypass with a bypass valve, and new pipes to connect them all together. New controls and new electrics were also installed. But the original radiators and their pipework remained, although TRVs were installed on just the bedroom radiators.

What am I hoping to do? Well, I had always assumed that we have a two-pipe system but, in light of the revelation described above, I would now like to be able to confirm which type of system we do have so that I know precisely which speed setting Grundfos recommends for our system.
 
is your space bar broken

Sorry about the missing space characters. I generally prefer to compose posts offline in Word first of all and then copy and paste the text into the forum. This usually works without any problems but, on this particular forum, some space characters go missing for some reason. Any advice you might have to avoid this happing would be appreciated.
 
Sorry about the missing space characters. I generally prefer to compose posts offline in Word first of all and then copy and paste the text into the forum. This usually works without any problems but, on this particular forum, some space characters go missing for some reason. Any advice you might have to avoid this happing would be appreciated.

If you are going to prepare the post in Word, and then paste it, its better to hit the "Go Advanced" button. You will then get an enhanced edit window control panel, with additional icons. The 4th in the top row is a specific "paste from Word" which makes allowance for some of the odd formatting habits that Word has.
 
A proper one pipe system will have a single pipe running beneath each radiator ... if it doesn't you probably have a one pipe system.

A very informative reply. Thank you.

I don’t believe that we have an “incorrectly piped one pipe system” because we have TRVs on the radiators in our bedrooms.

One problem with doing as you suggest is that our system has a bypass with a bypass valve, but I don’t think there is any means of closing the bypass.

Another perceived problem is that, if I were to close all the radiators and the bypass, might not that cause some damage (to the pump? to the boiler?) if the pump can’t circulate the water?

And a third problem is how would I know whether the flow has stopped?
 
If you are going to prepare the post in Word, and then paste it, its better to hit the "Go Advanced" button. You will then get an enhanced edit window control panel, with additional icons. The 4th in the top row is a specific "paste from Word" which makes allowance for some of the odd formatting habits that Word has.

I copy text from a Word document to my clipboard. Then, having positioned the cursor in the input area where I want the text to be pasted, I click the “Paste from Word” button. But nothing happens.

The best method I have found so far is to use the ordinary paste function and then click the “Preview Post” button. I can then see where the space characters are missing and I can insert them manually.

I don’t know what the problem is; it doesn’t happen on any other forum that I use.
 
At 1978 era, domestic heating systems would normally all be 2 pipe systems, unless done by an idiot.
1 pipe systems are usually inferior & were done in the 60s.
 
As Mike said earlier, a one pipe system is very basic.

One pipe runs from the FLOW to the RETURN of your boiler.

Both ends of your radiators are tapped off this pipe. There should be no valves on this pipe pther than those at the boiler.

Here endeth the lesson.
 
The vast majority of one pipe systems I've encountered have had the valves on opposite corners of the rads too. The flow would usually be top left and the return bottom right. Having said that, just last night I saw a microbore OPS, sludged up like mad but still working a treat. On that particular system, there was a manifold with a van load of microbore coming off it, and a balancing valve to one end. Literally a pair 3/4" pipes from the boiler, a gate valve and a load of branches off. Looked a right mess! All different sizes too, from 6mm to 10mm. I'd post a picture but haven't a clue how to.
 
to cut to the chase
one pipe fitted in the pioneering days of heating design
they quickly discovered it was crap
rip it out install 2 pipe,it works
 
At 1978 era, domestic heating systems would normally all be 2 pipe systems, unless done by an idiot.

Thank you for your very reassuring post. I suspected that was the case. It probably accounts for the fact that, in 2008/2009, when I knew that my boiler needed replacing and I was reading up about central heating systems, all the diagrams of radiator circuits that I saw were of two-pipe systems. However, they weren’t called by that name so I didn’t realise that one-pipe systems existed.
 
At 1978 era, domestic heating systems would normally all be 2 pipe systems, unless done by an idiot.
1 pipe systems are usually inferior & were done in the 60s.

This^ I'd be amazed to find you had a 1 pipe system in the late 70's.
 
Maybe if it was a DIY job?

Almost certainly, the installation of our radiators and their pipework was not a DIY job. We live on an estate where all the houses were built and fitted with central heating systems in 1978. Besides, the ground floor of our house is made out of concrete, not floor boards, and the pipes for the ground floor radiators are set in this concrete. And the pipes for the upstairs radiators are hidden under tongue and groove chipboard floor boards.
 
A very basic rule of thumb - whatever setting the old pump was on, put the new pump on assuming it got everything hot.
 
One pipe.

Boiler flow and return in series , rads are mutually exclusively in parallel to that series pipe.

Two pipe std

Boiler flow and return exclusive and rads in parallel between two exclusive
 
This^ I'd be amazed to find you had a 1 pipe system in the late 70's.

i don't know, i've seen two new ones in my 5 years....

however one of them was plumbed in by a numpty who plumbed it in valve to valve ......

soon as one trv turned off the entire system stopped......
 
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