New vertical radiators on old single pipe system

[quin]

Any ideas why 2 new vertical (tubular) rads replacing old horizontal flat panel rads on a single pipe system are not heating up? Remaining flat panels further in the system are heating up well and original flat panels used to heat well. All rads fitted with TRVs. Satisfied that there are no blockages or unvented air.

[hercules]

I am in the process of replacing my old flat panel radiators for a one pipe system.

From various material i have read some they say its important to have TRV's specific to one pipe systems.

The other could be the bends; 90degrees are too restrictive, better to have shallower angle for a smoother flow of water to run through.

[plumbbob]

Sorry to be negative here, but the single pipe design was abandoned decades ago partly because of this reason. Modern radiators are simply no longer designed to work purely on a gravity basis.

Modern rads are designed to give a high output whilst remaining compact. This is partly achieved by reducing the internal bores which in fact is an advantage in a modern system. Even if the water did circulate to any degree, it is unlikely the radiator would ever achieve its full output because of the reduced supply of hot water.

Which of course brings me on to the other main disadvantage of a single piped system, namely the further along the chain a radiator is, the cooler the input water is.

Given that there are substantial known problems with single pipe systems, I am surprised anyone would consider installing one when efficiency these days is paramount.

[quin]

That's interesting. I have introduced 3 x 90degree bends by taking the pipes back into the wall. I have also heard about 'low resistance' or 'high flow' TRVs. A helpful chap on the Honeywell tech support line is sending me details - they aren't on their website, so obviously not a common item.
I am actually getting some flow, but painfully slow. Think I'll try the TRV route first before digging into the floor and walls again. Thanks.

[quin]

Thanks Plumbob.
Fully understand why single pipe systems were abandoned. Trouble is I've got one! It has actually been working pretty well for the last 30 years and the issue has only emerged as I have swapped the rads. Digging up concrete floors and re-working the whole system isn't really justified.
Your point about the design of modern rads is interesting. Just one more thing to tip the balance beyond functional limits. I'll keep playing and see if I can get some half decent performance from them.

[plumbbob]

I appreciate if you already have a one pipe system, and it is working well, you need a pretty good reason to abandon it. My comment was actually aimed at Hercules who says he is actually installing one now.

The feed off the main loop must rise immediately. Gravity won't work in horizontal pipes and any bends should be swept (shallow curves) rather than sharp corners.

If absolutely nothing else works, you could consider fitting a restriction in the main loop between each side of the radiator so force the water through. Although of course this makes it's own problems.

It can be a good idea to connect the feed pipe to the top of the radiator and the return diagonally opposite to encourage a gravity flow.

[hercules]

Sorry, i didnt explain myself very well. I currently have a one pipe system (originals built wit the house) and i am in the process of replacing the original old flat panel radiators.

The system works okay but some radiators furthest away from the boiler heat up slower and in some cases have cold spots.

By replacing them with double panel, double convector rads i hope to get more heat in the house.

Plumbbob, how easy would it be to convert a one pipe system into a two pipe? Could a new pipe be place alongside the one pipe to act as the second run? I am trying to minimise the work of having to rip out the old pipe?

[plumbbob]

[quote="hercules"]The system works okay but some radiators furthest away from the boiler heat up slower and in some cases have cold spots.

By replacing them with double panel, double convector rads i hope to get more heat in the house.[/quote]

You have just described the single biggest disadvantage of the one pipe system. In actual fact, your existing radiators aren't even achieving their optimum output now simply because the gravity flow they rely on is insufficient. Indeed installing larger or different radiators alone is not going to improve the heat output to the room. By changing the design, as Quin has discovered, it may result in a reduction in output.

Changing to a standard two pipe system is by far the best solution and of course, the existing pipework can be re-used. How easy is it to do? Well, that depends on where the existing pipework is routed so it is difficult to be precise - it could be either way.

The existing pipes may be imperial making connections a chore. It also makes a difference if the existing pipework is showing signs of corrosion.
Some of a new system will not require the large bore 3/4"/22mm pipe. Occasionally there is not enough room to place a new pipe along side the old.

[mattc]

Hi - please help!....

We have just been charged a substantial sum of money by a plumber to replace 6 radiators on an old single pipe system. All radiators are piping hot and in perfect working order except 1 which is stone cold. The plumber has advised that this is because of the nature of the single pipe system and that the only option is to replace all the pipework in the house to create a more modern design.

I have taken up the floorboards to review his work and the issue as far as I can see (with my miniscule knowledge of such things) is just as described in this thread - the pipework connecting the radiator to the supply moves through 4 right angles whereas the original pipework just went through one right angle and straight up into the old radiator (which worked OK).

Does it sound like the plumber is correct and I must spend several more thousands of pounds updating the system, or does it sound like he should be able to adjust the pipework around the radiator to create a smoother design?

Thanks in anticipation of your assistance!....[/img]

[hercules]

Hi Matt,

I currently have a one pipe system and am in the process of getting my rads changed. One pipe systems are not very common these days and therefore most plumbers i have spoken to have not had much experience of them. Because of this they would rather replace the one pipe with a two pipe system because they are more familiar with it.

I got a radiator changed for my one pipe system and it was a simple case of the plumber cutting the pipes just above the floor boards, lifting off the old rad, then inserting new one to sit on the exising pipe (keeping in mind he repiped to the same format as the old). This worked fine. I am not a an expert of why your rad was stone cold but i would be puzzled why you would have to change the whole system? Like i previously mentioned maybe they are not quite sure about reconnecting the new rad into a one pipe system.

Last year i was quoted of changing my whole heating system ie, boiler, 7 rads, new pipework, inc labour ~£3000 (thro a friend), £5000 (anonymous). Hope that helps, and let us know how you get on.




[quote="mattc"]Hi - please help!....

We have just been charged a substantial sum of money by a plumber to replace 6 radiators on an old single pipe system. All radiators are piping hot and in perfect working order except 1 which is stone cold. The plumber has advised that this is because of the nature of the single pipe system and that the only option is to replace all the pipework in the house to create a more modern design.

I have taken up the floorboards to review his work and the issue as far as I can see (with my miniscule knowledge of such things) is just as described in this thread - the pipework connecting the radiator to the supply moves through 4 right angles whereas the original pipework just went through one right angle and straight up into the old radiator (which worked OK).

Does it sound like the plumber is correct and I must spend several more thousands of pounds updating the system, or does it sound like he should be able to adjust the pipework around the radiator to create a smoother design?

Thanks in anticipation of your assistance!....[/img][/quote]