Unvented/ secondary circulation

[Bamber gaspipe]

Working at this job involving unvented cylinder,I`m not fitting it because I`m unqualified,it`s a big job & I am getting involved in the other aspects of the plumbing side. It`s a big Restaurant with Toilets, Kitchens, Blah,blah.Anyway, it`s an indirect cylinder with a system boiler. The thing is until I attend the course (soon now I`m getting the work) I can`t find anything on t`internet regarding pipe layout especially the secondary return. As I understand it this is just a hot water return straight to the cylinder. I thought this was returned from the furthest Hot water appliance back but I`ve been asked to put a drop in every area where there are hot taps etc.

Surely I don`t have to return every Hot outlet back? If anyone could point me in the right direction t`internet wise on gaining some much needed advice I would be grateful.

 

[Slugbabydotcom]

Surely I don`t have to return every Hot outlet back?

I haven't fitted an SC system but have worked on some so heres most of what I know.

The idea of secondary circulation is to run a loop of DHW pipework from the cylinder to within a reasonable distance of every hot water outlet and back to the cylinder. By keeping each drop from this circuit as short as possible you are reducing the time it takes for hot water to appear at each outlet with the added benefit of saving water.
Without any readily available information I would say that it is reasonable to aim for running this circuit to within 3m of each outlet especially for outlets with a high usage expectancy.
This circuit will increase the amount of water heated under pressure so it may be necessary to increase the size of your pressure vessel.
A brass pump will be required to circulate this water and manufacturers recommend fitting a check valve.
Running this pump on a timer saves running costs of the pump when the premises are not in use.
Needless to say that as the 'SC' is always hot it will need the thickest available insulation to reduce heat loss
There is always a reference to 'SC' in the MI's but like you say that doesnt tell you much about the secondary circuit itself apart from where it connects to the cylinder.
I did have a quick google about and couldnt find much but I did see that elson [email protected] may be able to offer some advice as may the manufacturers of the UV cyl in question

 

[Bamber gaspipe]

Thanks for that Slugs, I understand all about unvented except for the secondary circulation. When you say a loop back to the cylinder i.e a secondary return should this not be from the furthest hot tap away from the cylinder then back?
Pardon me for being thick but i don`t see the point of piping all hot outlets
back to the cylinder, can`t get me head around this. I`m sure the penny will drop when I actually work with the guy piping the cylinder

I should have said that I can see the point but it`s seems a helluva lotta pipework for the task it does. It`s like piping up the Hot supply twice

 

[Slugbabydotcom]

Think of a ring main where no outlet is more than say 3m from this circuit and every outlet is connected to the ring main.
As with a primary circuit There is a flow going out from the top of the cylinder and a return from the tee to the last [not necessarily furthest] outlet going back to the cylinder as a secondary return.
Heres a quick sketch

I found this which should go some way to explaining why SC is sometimes necessary

 

[Onetap]

Working at this job involving unvented cylinder,I`m not fitting it because I`m unqualified,it`s a big job & I am getting involved in the other aspects of the plumbing side. It`s a big Restaurant with Toilets, Kitchens, Blah,blah.Anyway, it`s an indirect cylinder with a system boiler............

IMHO, if this is in the UK (assuming your location isn't USA), I think the relevant legislation is the HSE L8 document, 'Control of Legionella ...'.
This requires hot water at >50 degC at the hot outlet within 60 seconds of opening the tap. So, back of fag packet, flow rate from tap, volume per m of pipe, distance back to the HWS main circuit, gives an absolute maximum length for an uncirculated HWS supply branch.

The relevant question is who has decided on taking the circulation up to all the outlets and whether they know what they're on about.

Where you can anticipate problems is with spray taps which are designed to minimize HWS wastage & so have a low flow rate; you may have to take the return close to the outlet. Also there will be a disabled toilet which should have a TMV and this would also benefit from getting the return as close as practical. The water should be stored at 60 degC (which is too hot to wash hands) so many premises decide to fit TMVs on all outlets; legislation may make this obligatory in a few years.

If it turns out as a F&R circuit, you need lockshield balancing valves (suitable for very low flow rates) on the return connections at the fittings to balance it, or you won't get any flow to the furthest/least-favoured fittings. If the pipes are properly insulated the required flow rate should be the merest trickle. Q is total heat loss from pipes through insulation, dT is (60 -50) deg.

 

[Softus]

If it turns out as a F&R circuit, you need lockshield balancing valves (suitable for very low flow rates) on the return connections at the fittings to balance it, or you won't get any flow to the furthest/least-favoured fittings.

If it's been installed correctly, the circulating pump is the last thing on the return before it connects to the cylinder.

Onetap, I don't understand your suggestion to use balancing valves - why do you think this is necessary?

If the pipes are properly insulated the required flow rate should be the merest trickle. Q is total heat loss from pipes through insulation, dT is (60 -50) deg.

Were you intending to post an equation here?

 

[Onetap]

If it turns out as a F&R circuit, you need lockshield balancing valves (suitable for very low flow rates) on the return connections at the fittings to balance it, or you won't get any flow to the furthest/least-favoured fittings.

If it's been installed correctly, the circulating pump is the last thing on the return before it connects to the cylinder.

Yes, so the least favoured/ index circuit would probably be the first branch off the flow pipe.

I don't understand your suggestion to use balancing valves - why do you think this is necessary?

If I may rephrase your question, say the index/least favoured circuit has a resistance of 2,000 Pa at the design flow rate and the most-favoured (nearest the pump) circuit turns out to have a resistance of 100Pa at the design flow rate.

Why do you think water would flow along a circuit with a 2,000 Pa resistance when it can flow along a circuit with a 100Pa resistance?

In reality, it doesn't; the least favoured circuit has negligible flow rate. The most favoured has a huge flow rate, many times in excess of the design. The water tears along the main flow pipe and only flows through those circuits nearest the pumps. You need the balancing valves to add resistance to the circuits nearest the pumps.

It really doesn't work unless you do have some balancing devices. You can use ball/service valves, but the settings are lost the first time they're closed.

In the real world, I worked on a 10+ storey building with a 70 years old plumbing system & big HWS circulating pumps & storage calorifiers/cylinders in the basement. Stop-cocks and gate valves on the HWS F&R pipes, but no lock-shield valves & no balancing. Hot (50 degC+) water at all taps within 60 seconds was needed.

What was happening was that taps on the first floor (which being close to the pumps should be getting lots of flow) NEVER got above tepid (30 degC+). I waited for 20 minutes with one tap running and it still didn't get hot. On other floors, some were OK, some weren't, it depended on who had been running hot water off. In the basement there was a history of HWS pipe failures caused by erosion due to the excessive water flow rates.

If the pipes are properly insulated the required flow rate should be the merest trickle. Q is total heat loss from pipes through insulation, dT is (60 -50) deg.

Were you intending to post an equation here?

No. I'm certain that we all know the relevant equation.
Please don't disillusion me.

 

[Slugbabydotcom]

What was happening was that taps on the first floor (which being close to the pumps should be getting lots of flow) NEVER got above tepid (30 degC+). I waited for 20 minutes with one tap running and it still didn't get hot. On other floors, some were OK, some weren't, it depended on who had been running hot water off.

That could be why UVC manufacturers recommend a check valve near the pump on the return.
How big is this place? Can you get away with just a single circuit?

 

[Softus]

I don't understand your suggestion to use balancing valves - why do you think this is necessary?

If I may rephrase your question, say the index/least favoured circuit has a resistance of 2,000 Pa at the design flow rate and the most-favoured (nearest the pump) circuit turns out to have a resistance of 100Pa at the design flow rate.

Why do you think water would flow along a circuit with a 2,000 Pa resistance when it can flow along a circuit with a 100Pa resistance?

That doesn't rephrase my question, just asks a different one without answering me.

I worked on a 10+ storey building with a 70 years old plumbing system & big HWS circulating pumps & storage calorifiers/cylinders in the basement.

You haven't said whether there were multiple parallel secondary circuits, or just one. I suspect multiple, in which case it's not as per the diagram posted above.

If the pipes are properly insulated the required flow rate should be the merest trickle. Q is total heat loss from pipes through insulation, dT is (60 -50) deg.

Were you intending to post an equation here?

No. I'm certain that we all know the relevant equation.
Please don't disillusion me.

If you want to create an illusion for yourself, then I wouldn't dream of destroying it. However, your definition of the word "we" appears to exclude any lay-plumbers who might be reading your post.

Bottom line: in the circuit in the diagram above, there is a benefit to using balancing valves, but many systems would work in many scenarios without them. In any case, this had wandered from the point of the OP, which was to gain an understanding of a basic SC; something he appeared to lack, viz:

...seems a helluva lotta pipework for the task it does.

 

[Onetap]

I don't understand your suggestion to use balancing valves - why do you think this is necessary?

If I may rephrase your question, say the index/least favoured circuit has a resistance of 2,000 Pa at the design flow rate and the most-favoured (nearest the pump) circuit turns out to have a resistance of 100Pa at the design flow rate.

Why do you think water would flow along a circuit with a 2,000 Pa resistance when it can flow along a circuit with a 100Pa resistance?

That doesn't rephrase my question, just asks a different one without answering me. ]

It answered your question; you seem to be a little hard of understanding.
Balancing valves are necessary because the circulation system won't work without them, for the reasons detailed above.

I worked on a 10+ storey building with a 70 years old plumbing system & big HWS circulating pumps & storage calorifiers/cylinders in the basement.

You haven't said whether there were multiple parallel secondary circuits, or just one. I suspect multiple, in which case it's not as per the diagram posted above.

Well, what do you think Softus?
A 10 storey building piped with one continuous HWS circulation circuit as per your diagram above?
Do I really need to explain this further?

It illustrates why balancing valves are usually necessary; excess flow through near-by circuits, no flow through remote circuits.

If you want to create an illusion for yourself, then I wouldn't dream of destroying it. However, your definition of the word "we" appears to exclude any lay-plumbers who might be reading your post.

The OP is not a layman, and does not require further explanation.
Most DIYers who have got as far as installing their own HWS circulation systems will be familiar with the simple maths involved.

Bottom line: in the circuit in the diagram above, there is a benefit to using balancing valves, but many systems would work in many scenarios without them. In any case, this had wandered from the point of the OP, which was to gain an understanding of a basic SC; something he appeared to lack,

No. In the circuit in the diagram you've shown above there is no necessity for balancing valves. In most real installations the lengths of the branches feeding the fitting would be excessive to deliver hot water in a reasonable time and F&R branches and balancing valves would be required. It doesn't work without balancing. Given multiple circuits, most of the water flows along the path of least resistance.

Your response shows disgraceful bad manners. You ask a question about a subject that you clearly know very little about and when someone with the relevant knowledge and experience takes the time to provide you with an answer, you respond with your sneering, derisive comments and your stupid little 'rolling eyes' emoticon.

Are you Corgi registered, by any chance?
Do you describe yourself as an 'Engineer'?

 

[Onetap]

What was happening was that taps on the first floor (which being close to the pumps should be getting lots of flow) NEVER got above tepid (30 degC+). I waited for 20 minutes with one tap running and it still didn't get hot. On other floors, some were OK, some weren't, it depended on who had been running hot water off.

That could be why UVC manufacturers recommend a check valve near the pump on the return.
How big is this place? Can you get away with just a single circuit?

No. The HWS circulation pump will circulate water along the flow and return pipes by applying a very small pressure difference, about 10 kPa, at a guess, for domestic systems.

When a hot tap is opened, the outlet pressure (usually about 2.5 bar = 250kPa) is pushing water to the tap down both the flow and return pipes. This will obviously overwhelm the pump and send water in reverse up the return pipe. The water from the return connection will be cooler. The NRV is intended to prevent this reverse flow. It has nothing to do with balancing.

 

[Softus]

It answered your question; you seem to be a little hard of understanding.

Perhaps you should practise explaining to people who don't understand, instead of aiming redundant comments at those you think do.

Well, what do you think Softus?

Well, to be perfectly frank I think you're being deliberately obtuse.

A 10 storey building piped with one continuous HWS circulation circuit as per your diagram above?
Do I really need to explain this further?

It's entirely up to you. If you think there might be some curious DIYers reading this, who might not understand, then do.
Otherwise don't. Or do. Or don't. Up to you.

The OP is not a layman, and does not require further explanation.

I beg to differ. If anyone is hard of understanding it's the OP.

Most DIYers who have got as far as installing their own HWS circulation systems will be familiar with the simple maths involved.

You have a very unusual impression of plumbing DIYers, if you think that they're that clued up.

Bottom line: in the circuit in the diagram above, there is a benefit to using balancing valves, but many systems would work in many scenarios without them.

No. In the circuit in the diagram you've shown above there is no necessity for balancing valves. In most real installations the lengths of the branches feeding the fitting would be excessive to deliver hot water in a reasonable time and F&R branches and balancing valves would be required.

Bunch of nonsense.

Your response shows disgraceful bad manners. You ask a question about a subject that you clearly know very little about and when someone with the relevant knowledge and experience takes the time to provide you with an answer, you respond with your sneering, derisive comments and your stupid little 'rolling eyes' emoticon.

Are you Corgi registered, by any chance?

This topic has nothing to do with gas, but the answer to your question is that I never have been, never intend to be, and have never pretended to be. And it's "CORGI", BTW.

Do you describe yourself as an 'Engineer'?

That all depends on what you mean by those inverted commas and that capital letter.

 

[Balenza]

Onetap

Are systems like the one in the first schematic on page 2 shown in the PDF below described as "Basic" self balancing ?? ie no flow regulators needed assuming no sub branches.

Are the ones without the electric actuater just basically like TRV,s on a radiator system except they are sensing flow temp instead of air temp.??

http://heating.danfoss.com/PCMPDF/vd57x102.pdf

I dont know. I am only asking.
I have installed a few domestic systems thats all.

 

[Onetap]

Onetap

Are systems like the one in the first schematic on page 2 shown in the PDF below described as "Basic" self balancing ?? ie no flow regulators needed assuming no sub branches.

That's interesting, thanks for posting that.

The MTCV valves in that diagram are automatic balancing valves. As you have said, they are " basically like TRV,s on a radiator system except they are sensing flow temp instead of air temp".

They shut down when the water is hot, restricting the flow and increase the flow when the water in the return branch is too cold. The section entitled "2. DYNAMIC METHOD - Thermal balancing" at the bottom right of page 4 refers to the use of these valves. I don't think the layout of the document is very clear.

The traditional method, using fixed balancing valves is what I was trying to describe above. It is described in the adjacent section "1. STATIC METHOD - Hydraulic balancing" and in the formulae above that. You would calculate the required flow rates and pressure losses in each branch, then select regulating valves to add a resistance where required.

Danfoss sell control devices (very good ones). Their paper is comparing their automatic balancing valves with fixed balancing valves. You really do need some balancing devices on most HWS circulation systems or you'll run into problems with tepid water and legionella.

Note the first formula used;

m =Q/ ( c x dt )

as we all know.

I've never used the Danfoss valves, but they seem like a good idea. I wonder how much they cost? I'd guess a lot.

 

[Onetap]

Most DIYers who have got as far as installing their own HWS circulation systems will be familiar with the simple maths involved.

You have a very unusual impression of plumbing DIYers, if you think that they're that clued up.

The DIYers often don't know but, more importantly here, they know that they do not know. They will research the topic, get a book or search the internet to find out.

Many "Proffessional Heating Engineers" don't know but think that they do know. They get the hump if anyone has the effrontery to suggest that they don't know, as you have. They don't see any need for study to fill the gaps in their knowledge, because they don't think there are gaps.

The OP asked a question about something outside his experience.
Now he knows more about it.

No. In the circuit in the diagram you've shown above there is no necessity for balancing valves. In most real installations the lengths of the branches feeding the fitting would be excessive to deliver hot water in a reasonable time and F&R branches and balancing valves would be required.

Bunch of nonsense.

Is it? Which bits are nonsense, Softus?
You didn't correct the poster who'd made a mistake about the function of the check valve.

Do you describe yourself as an 'Engineer'?

That all depends on what you mean by those inverted commas and that capital letter.

You do call yourself an "Engineer", then.

You're not a professional, you're not an engineer, you're not competent and never will be.
Deal with it.