Convert one pipe system to two pipe system.

[Gareth James]

Hello,

We have recently had our ageing Baxi back boiler replaced with a new combi boiler. When the Plumber ventured under the floor during fitting, he noticed that the pipe system was an old one pipe system. Since the new boiler has been fitted we are getting excellent heat downstairs but it takes an age for the radiators upstairs to heat up and when they do they are not as hot as the downstairs ones. He suggested that we change the downstairs part of the system to a double one and that should give us better heat up the stairs - he said that he could change the compete pipe system but that would involve much more work and effectively would mean pulling up the upstairs floorboards etc.

I am not entirely convinced just changing the downstairs will make much of a difference, can anyone advise??

Thanks,

Gareth

 

[Daveydub]

If you turn off the downstairs rads do the upstairs perform better?
Do the downstairs rooms get toasty?
You could in this case consider TRVs to downstairs rads (need to be suitable for 1 pipe system)

 

[Gareth James]

If you turn off the downstairs rads do the upstairs perform better?
Do the downstairs rooms get toasty?
You could in this case consider TRVs to downstairs rads (need to be suitable for 1 pipe system)

Yes to all of the above. We have TRVs in all of the radiators bar the bathroom one which doesn't really get hot to be honest! The system was also flushed after the new boiler was installed.

 

[stem]

What you report is slightly unusual. Normally the 'one pipe' is run as a loop from and to the boiler, with the radiators sitting on top. Like this. The radiators at the start of the flow are the hottest, whilst those at the return end of the loop are cooler because of the heat lost from the previous radiators. Usually the pipe heads upstairs first, and so the upstairs radiators tend to be the hottest. Not the other way around.

If you have a large property, yours may have two loops, one upstairs and one down, and as water takes the easiest path the downstairs maybe getting the majority of the flow. Where there are more than one loop, usually manual valves are inserted in the pipework to balance the flow, but you would need to confirm your actual pipe layout to be sure there are more than one loop.

You mention the hotter downstairs radiators as if this is new to you. How did it work before the boiler was replaced?

I bought my home 18 years ago and discovered it had a one pipe system. I had planned to upgrade it to a two pipe, but there were more important matters to address first. However, it worked so well during our first winter, I just replaced the boiler, the controls and added normal TRV's. It all works perfectly, never makes a sound, non of the rads have ever needed bleeding, and the bills are low, so I've never replaced it.

 

[Gareth James]

What you report is slightly unusual. Normally the 'one pipe' is run as a loop from and to the boiler, with the radiators sitting on top. Like this. The radiators at the start of the flow are the hottest, whilst those at the return end of the loop are cooler because of the heat lost from the previous radiators. Usually the pipe heads upstairs first, and so the upstairs radiators tend to be the hottest. Not the other way around.

If you have a large property, yours may have two loops, one upstairs and one down, and as water takes the easiest path the downstairs maybe getting the majority of the flow. Where there are more than one loop, usually manual valves are inserted in the pipework to balance the flow, but you would need to confirm your actual pipe layout to be sure there are more than one loop.

You mention the hotter downstairs radiators as if this is new to you. How did it work before the boiler was replaced?

I bought my home 18 years ago and discovered it had a one pipe system. I had planned to upgrade it to a two pipe, but there were more important matters to address first. However, it worked so well during our first winter, I just replaced the boiler, the controls and added normal TRV's. It all works perfectly, never makes a sound, non of the rads have ever needed bleeding, and the bills are low, so I've never replaced it.

Hi,

As far as I know the pipes downstairs feed from the boiler first, then it goes upstairs.... I could be completely wrong though as I am not a qualified plumber! The plumber just mentioned to me it was a one pipe system but didnt say what radiators feed first. Before we replaced the boiler it was a similar story regarding the radiators being colder upstairs but at the time we didnt know it was a single pipe system so I assumed it was due to the back boiler and ancient pump being rubbish and not the pipe system!

Thanks,

 

[Daveydub]

As far as I know the pipes downstairs feed from the boiler first, then it goes upstairs

From your description I would say that you are right.
This is a better arrangement for you than the more traditional arrangement, generally folks want downstairs warmer than bedrooms.
You could try a bit of balancing to improve things a bit but this is a feature of a one pipe system.
Once all your downstairs rooms are warm & trvs shut down then upstairs will get hotter so try to stop the wife turning them all up to max.
Focus more on room temperature than radiator temperature

 

[Daveydub]

This is a better arrangement for you than the more traditional arrangement

By which I meant, better than flow going up and returning via ground floor not better than a 2 pipe system!

 

[simond]

A one pipe system will have cooler radiators on the last rads in the circuit. However, as the rooms downstairs start to warm up the upstairs ones should start to work better.

Important things to look at with one pipe systems are;

- the flow pipe should ideally enter at the top of each radiator, and return at the bottom
- any TRVs should be 'one pipe' type; as a rule of thumb, if you could pick them up in a merchant they won't be one pipe, they're normally special order only.

A one pipe will normally work almost as well as a two pipe, so I expect your problems are one or both the above. I've seen plenty of one pipe systems ruined by expert plumbers fitting incompatible TRVs.

 

[Agile]

A very simple modification that is usually far cheaper is to convert to TWO one pipe systems, one upstairs and one downstairs.

But it always surprises me that so few ever do that. But that could be based on charging the customer more for more work and not suggesting a small change with a small charge!

Tony

 

[stem]

As far as I know the pipes downstairs feed from the boiler first, then it goes upstairs....

That would explain why the downstairs radiators are hotter, but it is an unusual layout. Most one pipe systems normally follow the 'traditional' natural circulation pattern in that hot water rises from the boiler, goes upstairs and then as it cools falls to the ground floor to return to the boiler. This was a legacy from the days before the radiator circuit was pumped and the flow occurred naturally by what is referred to as 'gravity' this layout continued when pumps were added as the natural circulation assisted the flow.

With a properly designed one pipe system, the radiators at the end of the circuit would normally be 'sized up' a bit to take into account that fact that they would have a lower operating temperature.

- the flow pipe should ideally enter at the top of each radiator, and return at the bottom

- any TRVs should be 'one pipe' type; as a rule of thumb, if you could pick them up in a merchant they won't be one pipe, they're normally special order only.

I have fitted Standard Terrier TRV's to all of the radiators on my one pipe system (except the hall where the room thermostat is) because when I fitted them, I hadn't realised it was a one pipe system. The flow and return also enter the radiators at the bottom. All lockshields are fully open, there are 8 radiators on the loop and they all get hot, only the last two seem to be slightly cooler.

A very simple modification

I would hardly call replacing and rerouting the entire pipe system to all of the radiators a 'simple modification'

 

[Agile]

I have to say that most pumped one pipe system that I have seen have fed the downstairs first.

That would have been the expectation as originally the objective was 21 C downstairs and only 16 C upstairs in bedrooms.

But the simple modification I described above is very simple and gives much better performance.

Tony

 

[Steelmasons]

I have to say that most pumped one pipe system that I have seen have fed the downstairs first.

Really?

That would have been the expectation as originally the objective was 21 C downstairs and only 16 C upstairs in bedrooms.

....how many one pipers have you actually seen Tony?

 

[simond]

I have fitted Standard Terrier TRV's to all of the radiators on my one pipe system (except the hall where the room thermostat is) because when I fitted them, I hadn't realised it was a one pipe system. The flow and return also enter the radiators at the bottom. All lockshields are fully open, there are 8 radiators on the loop and they all get hot, only the last two seem to be slightly cooler.

Are you advocating this route?

It's wrong.

I have seen plenty of combi boilers working with drastically undersized gas supply pipework, but I'm not going to recommend it.

 

[Daveydub]

I would hardly call replacing and rerouting the entire pipe system to all of the radiators a 'simple modification'

Stem. What Tony was suggesting was to convert to 2 x 1 pipe systems. Without a quick glance or extensive description of your pipwork not sure how major a job this would be.
If your pipe run is around the ground floor back to the boiler area before rising to upstairs, around upstairs, and drop back to boiler then all your alterations would be beside boiler and quite simple.
If your pipe run is to far end of house & then rise to upstairs, you would then need to bring a pair of pipes (1 ne flow & 1 new return) back from here to the boiler.
Hope that made sense.
Nobody could advise you to fit Terrier TRVs because they may well not work. You have fitted them and they work so that is great news.

 

[Agile]

To do my suggested modification all that is needed is for ONE pipe to be connected at "T" where the pipe goes upstairs. That's usually at the furthest distance possible from the boiler.

Then the return from the upstairs is connected to the flow at the boiler.

But that only works well in some cases where the installation is very simple. If the system is fully installed with swept tees and a top feed then two pipes will need to be brought back to the boiler so that the flow can be kept in the same direction.

Tony