Silly question about parallel CH pipe runs :-(

I suspect this is a silly question, but I'm going to go ahead and ask anyway.

In a normal parallel pipe central heating installation, from what I understand the last radiator has the supply into one valve and the return from the other valve. For all other radiators the supply and return pipes are tee'd off, similar to the following:



My question about this setup is that if the last radiator on the circuit is turned off, doesn't that mean water will "sit" in the supply and return pipes between the previous radiator and the last radiator and couldn't this allow silt to form in those pipes, especially if they are plastic with inserts! For example, if the last radiator was a dual fuel towel rail and so one of the valves was closed off to the CH for the Summer months when the electric element was used to heat it.

Wouldn't it be better if the supply pipe to the last radiator "became" the return pipe? This way, just like any other radiator in the system, if the last radiator is turned off then water will bypass it and return to the boiler, avoiding having (potentially) silty water sitting in the pipes.

I know this is a silly question, but it would be useful to know why the supply pipe doesn't become (feed into) the return pipe at the last radiator.

1. The water sits in both the flow and return pipes all the time.
2. When the heating system is on, it is heated and flows.

The flow to each radiator is quite slow to balance the system, most valves are only partially open. If you short the flow to the return it will create a low resistance bypass and radiators will not heat.

oldbuffer said:

1. The water sits in both the flow and return pipes all the time.
2. When the heating system is on, it is heated and flows.

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I guess it does, especially during the warmer Summer months.

dal5band said:

The flow to each radiator is quite slow to balance the system, most valves are only partially open. If you short the flow to the return it will create a low resistance bypass and radiators will not heat.

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Yes, I thought that might be reason. I guess water will take the line of least resistance, which will be to just flow back to the boiler if the supply and return were connected at the end of the run.

So, normally, when the flow hits the last radiator there will be some resistance in the flow and this will ensure the water also flows into each of the radiators, through the partially opened valves.

Look at the FAQ's for the post on 'Balancing', this explains a bit more about it.

What you have called a pressure releif valve in your drawing is a by pass not a PRV

Where is your "silt" coming from. If a product of corrosion then you need to address that problem since a properly installed, flushed and treated system will not form silt.

The "Silt" to which you refer is "Magnetite" ie Rust ...dissolved steel !!

I would be checking that Barrier pipe has been used.

ianmcd said:

What you have called a pressure releif valve in your drawing is a by pass not a PRV

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I agree. I "borrowed" that picture, so it isn't my own. My boiler has a built in by pass, so this isn't something that is included in my CH piping.

Jackrae said:

Where is your "silt" coming from. If a product of corrosion then you need to address that problem since a properly installed, flushed and treated system will not form silt.

Click to expand...

Terrywookfit said:

The "Silt" to which you refer is "Magnetite" ie Rust ...dissolved steel !!

I would be checking that Barrier pipe has been used.

Click to expand...

I had an issue with the loft CH last year when the 2 radiators and towel rail stopped warming up. Changing their TRV's and lockshield valves didn't solve the problem and in the end I had to disconnect it from the rest of the CH and spent some time flushing it through. The original piping for our 22 year old house is all copper. When the loft extension was done, around 17 years ago, they extended the CH pipe run into the loft then used plastic barrier pipe (the correct type - I checked) for the loft. Because the loft bedroom is quite warm and rarely used, the radiators had been turned off for a long period of time. In addition, the method used to connect the piping to the radiators is a star shape, rather than the usual parallel piping. When I flushed it through, I realised there was a LOT of magnetite that had hardened at joints, where inserts were present and this was blocking the system. I was able to flush them all through and it has been working well since. I also cleaned out the whole system using fernox cleaner and added inhibitor. I plan to re-route the pipes to improve the flow in the loft and also change the loft feed to 22mm, sourced from nearer the boiler. This will be done as part of extending the system to introduce an additional UFH bathroom zone and LLH + pump. As a result the flow into the loft will be much better.

The last radiator I was referring to in my post was actually a towel rail in the bathroom, from where the loft feed used to originate, but I have now terminated this, hence it is the last radiator on the circuit (just like it was before the loft extension). This was the reason for my question.

Currently, I have two tails sticking out of the wall waiting for the towel rail to be installed after the bathroom has been tiled. These pipes never get warm as there's no flow through them. If there's no flow through them, they "potentially" could silt up over time (I would assume), which is no different to the towel rail being turned off during the Summer months when the electric element will be used.

I guess I'm being cautious as a result of the silting issue I've already experienced, but I appreciate this was a combination of bad flow and plastic piping being used. This is certainly an example where I believe copper piping is better than plastic due to the lack of inserts that narrow the bore of the pipe.