Single pipe system - poor flow through rads

[MarkSmith]

Hi,

We have a single-pipe heating system. Several rads are new and have new valves (not TRV, just normal ones), all of which are completely open. All rads have bypasses under them.

Pipe leaving the boiler is very hot. Pipe returning to boiler is almost as hot. The feed pipe to every rad is very hot. The return pipe from each rad is cold. However the rads themselves are only warm-ish at the top, cold at the bottom. House doesn't get warm, even with heating full on.

I don't think it's a balancing issue: upstairs is as cold as downstairs.

What's going on, and what can I do to improve the situation?

Converting to a two-pipe system is unfortunately out of the question: oak and tiled floors which cannot be lifted.

My (inexpert!) diagnosis is that it does appear to be basically working, but the flow through the rads is poor. Being a single-pipe system it only has convection to move water through the rads, and this doesn't seem to be powerful enough to move much water. Most of the water is going through the bypasses, back to the boiler, almost as hot as it left.

I have one idea: in a few places I can get to the bypasses (two are above the floor and one is below a bit of floor I can lift). I could put isolator valves inline with the bypasses, which I could partially close to force the water through the rads instead. No idea how conventional this is - professional plumbers will probably be turning in their graves (if they're dead and still reading this) - although it does feel like it's probably work.

Advice much appreciated!

Thanks!
-Mark

 

[ALEC1]

you need high lift valves for single pipe systems.....

 

[ServiceNumpty]

Some new rads are not designed for single pipe systems, low water content I believe. I know the rettigs aren't suitable for 1 pipers. Try balancing.

 

[simond]

There is no fundamental reason why a one pipe shouldn't work OK.

Your post suggests that the system doesn't work; but sitting here it seems likely that the one pipe was put in at least 35 years ago, and I doubt the occupants of your house have tolerated no heating over the intervening years.

So the question is, what have you changed?
One pipe systems work better with a top and bottom opposite entry. As Alec says, full bore valves, not the pinhole ones used for two pipe must be used if you are going to get adequate circulation. The radiator branches (tees) off the one pipe must not be close together.

I suspect these are are the root causes of the problem, which is likely to be of your own making......

 

[swbjackson]

What speed is the pump set to? I've found in the past that if you set the pump speed too high the radiators don't heat up very well and a lot of installers seem to whack the pump on maximum no matter what.

 

[Norcon]

The radiator branches (tees) off the one pipe

Is that how they were piped in the ole days? Did they work?
If it was 3" in diameter then your options are indeed rather limited.

Danfoss show the by-pass pipe branched off the one pipe.
Its a big difference.
http://heating.danfoss.com/PCMPDF/VDC6V102_1PipeSolution.pdf
http://heating.danfoss.com/PCMPDF/VDCHA202_Restrictors_teamcen.pdf

ChrisHutt has posts in the archives relating to this method afaik. He certainly has ones relating to the former method being wrong as I've been researching his comments on the subject. (some might call that googling)
Two 28mm T's for example have a 3m equivalent pipe resistance.

 

[MarkSmith]

Thanks for the replies all.

you need high lift valves for single pipe systems.....

Could you point me towards a high lift valve please? I have searched screwfix etc. and been unable to find any.

What speed is the pump set to? I've found in the past that if you set the pump speed too high the radiators don't heat up very well and a lot of installers seem to whack the pump on maximum no matter what.

Was set to high. I've set it to medium and this seems possibly to have made an improvement - thanks! Will take a few days to be really sure but this does look hopeful.

http://heating.danfoss.com/PCMPDF/VDCHA202_Restrictors_teamcen.pdf

Looks interesting - their bypass restrictors are exactly what I was suggesting. Not such a bad idea perhaps.

Try balancing.

I'm unsure what to balance. There are 7 rads: the first is inline with everything (directly after the boiler output) and is properly hot, and then it splits into one single-pipe system for upstairs (2 more rads) and one single-pipe for downstairs (4 rads). All 6 rads on the one-pipe systems are inadequately hot.

[...] it seems likely that the one pipe was put in at least 35 years ago, and I doubt the occupants of your house have tolerated no heating over the intervening years.
[...]
I suspect these are are the root causes of the problem, which is likely to be of your own making......

Certainly it has been a problem since we moved in 18 months ago. The preceeding 33.5 years, I don't know. There have clearly been changes in the relatively recent past - new boiler and probably the addition of the bathroom radiator (the one which is in-line with the rest, and the only one which works properly) - but when it stopped working effectively I don't know.

Thanks all,
Mark

 

[Norcon]

Could you point me towards a high lift valve please?

Danfoss Ra-g.
http://tinyurl.com/cxdd7gj

 

[ServiceNumpty]

Turn off the 1st rad after the boiler, and see what happens to the rest of the rads.

Mike

 

[Agile]

You dont buy specialist professional components at Screwfix!

My advice would be to do whatever is necessary to access the bypass pipes under EVERY rad and to fit adjusting valves.

Then the system will work very well if you have high flow radiator valves.

There could be a good argument for fitting non-valve connectors to each rad and full bore valves in each supply pipe. Both on a cost basis and performance basis.

Tony

 

[simond]

If I understood the OP correctly, the first rad has been inserted in series with the one pipe. If this is the case and it is turned off, flow will cease....

There is no magic getting these things to work, you don't need to put restrictions in the pipe. You need full bore valves, ideally top and bottom entry on the rads.

We've got Vaillant, Viessmann and Worcester boilers working successfully on one pipe domestic systems. In commercial, one pipe are even more common. For instancem we have an ecoTEC 630 that has been working on a 4 zone one pipe for the last six years.

I wouldn't go making significant alterations as it will probably be pointless. Pump speed can be low because you have no significant resistance in the circuit.

 

[D_Hailsham]

One pipe systems work better with a top and bottom opposite entry.

That explains why Danfoss show TBSE in their diagrams!

http://heating.danfoss.com/PCMPDF/VDCHA202_Restrictors_teamcen.pdf

 

[DeltaT2]

Dear oh dear!!! What a load of drivel!!! For starters all the Danfoss details are showing one pipe drop systems, completely the wrong design from what the OP has!!

A single pipe system should have the main Flow pipe cross under the emitter & a short pipes(F&R) rise up to the emitter. The main will be a circulation, but the radiator circulation may work on convection only.

The biggest consideration is 'point of least resistance'. If you have a small rad it'll heat better than a large rad full of cr.p.

 

[Nige F]

Thanks for the replies all.

. I've set it to medium and this seems possibly to have made an improvement - thanks! Will take a few days to be really sure but this does look hopeful.

. There are 7 rads: the first is inline with everything (directly after the boiler output) and is properly hot, and then it splits into one single-pipe system for upstairs (2 more rads) and one single-pipe for downstairs (4 rads). All 6 rads on the one-pipe systems are inadequately hot.

. There have clearly been changes in the relatively recent past - new boiler and probably the addition of the bathroom radiator (the one which is in-line with the rest, and the only one which works properly) - but when it stopped working effectively I don't know.

Thanks all,
Mark

Turn pump to min . - it`ll still have more head than the SMC commander that was fitted on the original system - Assuming the replacement boiler doesn`t have a min. flow rate requirement . If the rads are original they should have large bore valves . If you must balance , turn off all the upstairs and see how the downstairs performs on it`s own - that heated water loves to rise in pipes . What does "in line with the rest" mean re; the bathroom rad Good Luck

 

[MarkSmith]

Turn off the 1st rad after the boiler, and see what happens to the rest of the rads.

They all stop.

Could you point me towards a high lift valve please?

Danfoss Ra-g.
http://tinyurl.com/cxdd7gj[/QUOTE]Thanks. Wow - expensive. More expensive than the rads themselves!

There could be a good argument for fitting non-valve connectors to each rad and full bore valves in each supply pipe. Both on a cost basis and performance basis.

Could you point me towards a non-valve connector please?

If you must balance , turn off all the upstairs and see how the downstairs performs on it`s own - that heated water loves to rise in pipes . What does "in line with the rest" mean re; the bathroom rad Good Luck

If I turn upstairs rads completely off (not the in-line bathroom one, see above, but the other two), downstairs rads still do not get adequately hot. (I presume this is because both these upstairs rads have bypasses (inaccessible under oak floor sadly) and most of the water goes through these, back to the boiler, without bothering to go downstairs through the much longer loop.

In-line with the rest means exactly as simond put it:

If I understood the OP correctly, the first rad has been inserted in series with the one pipe. If this is the case and it is turned off, flow will cease....

The split into two single-pipe loops happens after this rad.

Small update: pump speed to medium made it worse. (The reason I thought it had helped was that the water going back upstairs was much colder - so I assumed it had more effectively transferred into the rads. However, fact is the house was much colder after two days on that setting, so I think the water just lost more heat on account of taking longer to move around.)

I'm going to try and get high-lift valves. I'm very very reluctant to pay £400 for them, though, because I still don't understand how it will encourage the water to go through the longer, more resistive downstairs loop rather than just around the upstairs loop and back in. Seems like some sort of restriction upstairs is needed, to force the water downstairs.

Thanks again all!