Balancing a one pipe system - possible? Or should all Lockshields be fully open.

[JonnyH12]

I have poor flow through some of my radiators in a one pipe system. The actual pipe is blisteringly hot, and it's almost as hot on the return valve too. Some rads pump out heat, but most suffer from poor flow and the water isn't even getting to the lockshield return.

A Plumber came out last year and said all of the lockshields should be fully open, and now im doubting that advice.

 

[stem]

Yes generally he's correct. The pump circulates the water around a pipework loop, the radiators have an internal resistance higher than the difference between their flow and return on the pipe below them, so to get a flow through each radiator it relies on the natural circulation of hot water rising cooler water falling to get to circulate inside them. Closing the lockshield increases the resistance and makes circulation through the radiator even slower.

With a two pipe system the pump pressure is exerted directly on the radiator flow and forces it through.

The major problem with a one pipe is that the radiators don't get as much water circulating through them as with a two pipe, so are cooler and those further down the loop are even cooler than those at the start. To try and improve this the pump speed is often increased which blasts the water around the loop and creates a higher return temperature. Lowering the pump speed can help here, the one pipe loop is generally a lower resistance than a two pipe system as it is simply a loop of pipe, so the pump has less work to do. But the radiators at the end of the loop will get cooler if the return is cooler.

PS I have a one pipe system with 7 radiators on. All lockshields are fully open and the pump is on the lowest setting. Works brilliantly. When I bought the house 25 years ago I always intended to upgrade it but never needed to.

 

[JonnyH12]

Yes generally he's correct. The pump circulates the water around a pipework loop, the radiators have an internal resistance higher than the difference between their flow and return on the pipe below them, so to get a flow through each radiator it relies on the natural circulation of hot water rising cooler water falling to get to circulate inside them. Closing the lockshield increases the resistance and makes circulation through the radiator even slower.

With a two pipe system the pump pressure is exerted directly on the radiator flow and forces it through.

The major problem with a one pipe is that the radiators don't get as much water circulating through them as with a two pipe, so are cooler and those further down the loop are even cooler than those at the start. To try and improve this the pump speed is often increased which blasts the water around the loop and creates a higher return temperature. Lowering the pump speed can help here, the one pipe loop is generally a lower resistance than a two pipe system as it is simply a loop of pipe, so the pump has less work to do. But the radiators at the end of the loop will get cooler if the return is cooler.

PS I have a one pipe system with 7 radiators on. All lockshields are fully open and the pump is on the lowest setting. Works brilliantly. When I bought the house 25 years ago I always intended to upgrade it but never needed to.

Your post gives me some hope I can get tjjs working better.

What pump have you got? Mine is a grunfos pump, and I'm not sure if I should be using the constant pressure setting, which is designed for underfloor heating, or the standard constant speed

 

[stem]

I had the same dilemma, and even started a post with that very question. See here.

My old pump was on speed 1, now I'm using Constant Pressure 1. I can't say that I can detect any difference in performance, but based on the linked thread, I understand that it will same me some electricity.

Generally with one pipe systems, if they are properly designed, the radiators at the end of the loop are often oversized slightly to take into account that they will be cooler than those at the origin.

 

[JonnyH12]

Do you have just one pump?

I have three. One to the right of the low loss header, flowing out of the boiler.

One on the return central heating pipe, and one on the return hot water cylinder pipe.

I have 20 rads in total spread across two floors, so I might need cp2 instead of one. I just don't know if I need that on both pumps or just one.

 

[stem]

Ah that's a bit different, I don't have a LLH so I'm guessing that you have something like this (except with the pumps in the return instead of the flow).....

.....Which is a different beast. I'm not sure about the pumps here as have a very limited knowledge of this type of system. AFAIK, the boiler pump circulates water through the LLH and back to the boiler, so what isn't pulled off from the LLH by the HW and CH pumps goes straight back to the boiler return. So, if there wasn't much water being pumped away then much of the flow would go straight from the boiler through the LLH and directly back to the boiler.

If I had to guess I would think that reducing the CH pump would mean reducing the LLH pump too. But that's just my guess and could be completely wrong here. So I think you probably need a LLH pro. to advise here.

20 radiators is a lot, but it's more the do with the run of the pipe loop. for example you could have a 30 metre pipe run with 2 radiators above it, or the same length of pipe with 5 radiators above it. The pump would have the same duty to perform. But you are probably right in your case CPII would probably be better for you.