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

[DIYAddict]

I've just bought a BS1566 1500 x 450mm (210L) indirect vented cylinder to connect to the existing central heating system, which uses a pumped 35kw gas boiler. I'm intending to use two 2 port zone valves for independent control of CH and HW.

The cylinder will be positioned 15m from the boiler, and the guy who sold it to me said it will need 28mm pipe to feed the heating coil, otherwise the boiler will 'hunt'. The coil itself is 28mm.

However, reading my plumbing book seems to indicate that 22mm will be more than adequate for a pumped system, and using larger bore pipe would result in an imbalance between HW and CH circuits - there would be very little resistance in the HW circuit so the radiators wouldn't get hot when the cylinder was being heated up.

My book says that 22mm pipe can handle up to 13.5kW over a max length of 36m. So the question seems to be what is the heat load of this cylinder? Assuming a temperature rise of 56C over 1.5 hours, the standard equation gives 4.186 x 210 x 56/5400 = 9kW - fine for 22mm.

But I'm confused as BS1566 cylinders are meant to have a recovery period of 30 minutes. If I feed that into the equation I get 27kW, which is higher than even 28mm pipe is rated at!

So what pipe size should I use?

 

[DIYAddict]

Ok, ignoring the dodgy maths: I have a 120,000BTU boiler and a 210L vented indirect cylinder 15m away.

Should I use 28mm for the primary heating coil given the size of the boiler?

If so, I'll probably have to include a restrictor to balance the dhw with the CH. Could someone please explain to me whether doing this would then defeat the object of using 28mm in the first place?

Come on guys - someone must know! Kind thanks in advance...

 

[twgas]

think the boiler MIs will tell you to use 28mm flow and return, more about getting all that heat away from the boiler than heating cylinder in any set time

 

[DIYAddict]

Ah - that makes sense. Thanks Mick. And presumably, if the rads are fitted properly with correfct size pipe (I think they are), then the cylinder flow won't have to be too constricted to balance the CH and HW.

 

[heatingman]

Check with the MI's for unvented unit, some of them dont recommend a balance/restriction valve on coil return

 

[twgas]

indirect vented cylinder

Check with the MI's for unvented unit

a little confusion me thinks

may be a good idea to put a balancing valve in, can be left fully open if it causes a problem

 

[heatingman]

oops, saw 210L, (measure twice, cut it once. Read it twice & dont appear daft)

Sorry, ignore my rambling.

 

[lurcher]

Install it on a W-Plan and forget bypasses and throttles. 28mm from the boiler if you so wish. The recovery time on the cylinder will be quicker than any S-Plan.

 

[twgas]

W-plan, this is 21st century now

 

[doitall]

My bobs worth says the bigger the engine the faster the car

22mm is fine if the pumps big enough to shift the water.

One assumes you are taking 28mm somewhere handy and splitting 22mm for the heating and 22mm for the hot water.

 

[DIYAddict]

Thanks for all your help everyone. The boiler MIs don't say anything about the primary coil thickness, but I like the thinking that 120000 BTU's is a lot of heat to shift if it's all directed just at the dhw cylinder, so I'm going for 28mm.

doitall, The Central heating itself starts out at 28mm and thins out gradually, the first radiators branching out pretty near to the boiler, so I'm connecting my hot water quite early on in the 28mm run.

lurcher, I did think about the W-Plan but I had already acquired a 28mm two-port (and 22mm two-port for when I though I could use 22mm for the heating coil), and basically chose the the S-Plan for that (admittedly very silly) reason. I'm hoping that there is sufficient flexibility in the programmer to emulate a W-plan in need be, and eventually there's a likelihood of lots of showers and baths being taken first thing so I didn't want the central heating to stall completely.

I'll be fitting it over the next few weeks and I'll keep you posted on how it performs.

 

[lurcher]

Just because twgas and I posted the same conclusion within 7 mins does not mean that we are the same person nor even long-lost twins. We are merely inherently and irrevocably right and superior etc ramble. Atag and Viessmann boilers do internal W-Plans. It's the only way. Gleaming.

 

[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.

 

[Balenza]

Install it on a W-Plan and forget bypasses and throttles. 28mm from the boiler if you so wish. The recovery time on the cylinder will be quicker than any S-Plan.

You still need a by-pass for the CH circuit and no the heat recovery for W plan is not faster than S plan.

 

[Balenza]

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 ?.