Heating coil pipe for indirect dhw cylinder - 22 or 28mm?

[doitall]

Diyitall wrote

By the same token you should be looking at 35mm from the boiler until the split.

35mm is grossly oversized.
Guessing again are we ?.

What for a 35kw load I don't think so balski

 

[Agile]

Unless you house a football team you are unlikely to have to use the full reheat capability very often.

For half the year you have to share boiler output with the CH. Without that you have full pump and boiler power for the HW.

I would use a 28 mm motor valve and a 22 mm pipe run with two 22 mm gate valves for isolation and balancing if it were ever required.

The cylinder heat absorption is only at the maximum when the stored water is at about 10°C. It rapidly reduces as the temperature differential falls so the 22 mm pipe run will have little effect for most of the time.

Tony

 

[DIYAddict]

By the same token you should be looking at 35mm from the boiler until the split.

When you say the first rads branch off close to the boiler, you are aware you can only tee off the common return once.

The boiler itself has 28mm primaries so 35mm is not an option.

I wasn't aware you could only tee off the common return once - the central heating part of the system was already there when we moved in. What is the consequence of more than one tee?

 

[Agile]

The boiler itself has 28mm primaries so 35mm is not an option.

You can use any flow and return pipes you choose! I saw a boiler today with 22 mm connections which had 35 mm pipes to the manifolds.

Just why do you think you cannot use 35 mm ???

Tony

 

[doitall]

By the same token you should be looking at 35mm from the boiler until the split.

When you say the first rads branch off close to the boiler, you are aware you can only tee off the common return once.

The boiler itself has 28mm primaries so 35mm is not an option.

I wasn't aware you could only tee off the common return once - the central heating part of the system was already there when we moved in. What is the consequence of more than one tee?

If you are going for 28 x 28 then 35mm should be considered.

If you tee off the common return more than once you get reverse circulation.

You will need to take the cylinder return back to the boiler so its the last tee before the boiler.

 

[DIYAddict]

The boiler itself has 28mm primaries so 35mm is not an option.

You can use any flow and return pipes you choose! I saw a boiler today with 22 mm connections which had 35 mm pipes to the manifolds.

Just why do you think you cannot use 35 mm ???

Tony

Agile and Doitall, looks like I still have a lot to learn about plumbing! I'd assumed that 35mm would offer no benefit if the flow was constricted at the boiler by 28mm connections, in the same way that 30A cable would be no advantage if connected to the end of 5A cable.

I must admit that the 28mm boiler connections had confused me as I thought 35kW required 35mm too.

The CH/HW split will be approx. 1.5m from the boiler, just after the pump and balancing valve ( they're already plumbed in years ago - not by me). It's a fair amount of work and expense to change - do you still think it's worthwhile?

 

[DIYAddict]

Unless you house a football team you are unlikely to have to use the full reheat capability very often.

For half the year you have to share boiler output with the CH. Without that you have full pump and boiler power for the HW.

I would use a 28 mm motor valve and a 22 mm pipe run with two 22 mm gate valves for isolation and balancing if it were ever required.

The cylinder heat absorption is only at the maximum when the stored water is at about 10°C. It rapidly reduces as the temperature differential falls so the 22 mm pipe run will have little effect for most of the time.

Tony

Good point. Unfortunately I have now bought 30m of 28m pipe and fittings now so I might as well stick to that. I take comfort from the fact that you professionals are divided about this - it means it wasn't an entirely stupid question

 

[DIYAddict]

If you tee off the common return more than once you get reverse circulation.

You will need to take the cylinder return back to the boiler so its the last tee before the boiler.

Ah. I wonder if that explains why some of the radiators suck in air even though the f&e tank is plumbed in correctly for positive pressure throughout. I must admit I'm not quite sure of the difference between teeing off the common return and a radiator's return pipe joining the common return where it's supposed to.

I've definitely been intending to make the cylinder return the last tee before the boiler.

 

[Agile]

Agile and Doitall, looks like I still have a lot to learn about plumbing! I'd assumed that 35mm would offer no benefit if the flow was constricted at the boiler by 28mm connections, in the same way that 30A cable would be no advantage if connected to the end of 5A cable.
?

Unfortunately you also misunderstand electric cables.

Pipes and cables operate in exactly the same way.

Their ability to pass a liquid or current flow depends on their RESISTANCE

ONE meter of 5A cable has the same resistance as SIX metres of 30A cable !!!

Boilers have smaller internal pipes because that keeps manufacturing costs down and its only short lengths. Outside the boiler the distance may be 10m so to keep the flow resistance down its necessary to use larger diameter tube.

Simple really!

Tony

 

[DIYAddict]

Thanks Agile.

So, if the constricted bore is unimportant if not too constricted and/or not too long, why do shower pump manufacturers insist on full bore valves instead of isolator valves? Playing it safe I suppose...

Cheers, Paul

 

[doitall]

Thanks Agile.

So, if the constricted bore is unimportant if not too constricted and/or not too long, why do shower pump manufacturers insist on full bore valves instead of isolator valves? Playing it safe I suppose...

Cheers, Paul

Because you are considerably increasing the normal flow rate of the pipe, and if you starve the pump it will not last very long.

By the same token, the gravity cold feed to the cylinder should be increased to match the output of the pump and any other drawoffs.

 

[Agile]

Because there is so little head of water on the INLET to a shower pump its absolutely essential that the pipe is not restricted at all otherwise the inlet water flow will virtually cease. Only full bore valves should be used. Nevertheless cowboys still use cheap valves often.

On the other hand the outlet is at a high pressure and restrictions are of little importance.

Tony

 

[DIYAddict]

By the same token, the gravity cold feed to the cylinder should be increased to match the output of the pump and any other drawoffs.

I'm using a 28mm cold feed and the tank is in the loft above. The ceilings are quite high (4m) so I'm hoping that will give enough flow. The shower pump will be sited on the floor at the base of the cylinder.

Again this was on the (mistaken) assumption that the size of the cylinder boss dictated the plumbing bore. Do you think 28mm will be enough? It's not too late to change to 35mm.

I'm siting the cylinder cold feed above the other cold feeds on the tank to prevent scalding should the tank run dry.

 

[doitall]

Depend on the output of the pump and what its being used for. just a shower then 28mm is fine

 

[lurcher]

Balenza
You will of course have a bypass on the CH circuit in the form of a lockshielded radiator in the room where the room stat is located. I was talking about the HW circuit.
W Plan not quicker than S Plan? Are you sure you know what a W Plan is?