Theoretical questions regarding low loss headers (or close-coupled tees)

[Ric2013]

T'other day, the subject of hydraulic separation came up again on this forum and I decided it would be interesting to look at the subject again. Thinking about the kinds of design shown on this page:

Why all the fuss about hydraulic separators? Closely spaced tees have worked just fine for me for 30 years... and it’s a cheaper way to go!

Just wondered if those with experience would comment on my understanding of the matter as I'm making a few guesses here.

Most boilers spend most of their lives running at part load, particularly if the building has been insulated to a higher standard since the system was first installed:

1. Within a domestic heating setting, a hydraulic separator would help the boiler to modulate more accurately (if it is modern enough to do so) and so reduce stress on the heat exchanger and improve efficiency by helping it run in condensing mode.

2. On a less modern boiler with no modulation, a hydraulic separator would reduce the length of boiler cycles, thus keeping flow temperatures to emitters more consistent and ensure boiler flow is always at a good rate without the need for a dedicated bypass. But this might actually increase gas consumption and wear to boiler due to increased cycling?

3. If the system is open-vented, the hydraulic separator could be given the cold feed and vent pipes and then it could also be used to integrate an uncontrolled heat source such as a solid-fuel appliance with a gas system, thus saving on the space and expense of an accumulator/thermal store/ buffer tank/whatever you want to call it. (Obviously the boiler would have to flow UP to the header, not DOWN as in the diagram).

4. All the diagrams I see of systems with hydraulic separators show the pumps on the return rather than the flow. I'm not sure why this has to be.

Any thoughts?

 

[sammathias]

What diagrams you been looking at mate? Pumps generally always on the flow?

 

[Ric2013]

What diagrams you been looking at mate? Pumps generally always on the flow?

The link in my post. The majority of diagrams I get online seem to show pump on return. Perhaps it's an American thing?

 

[ShaunCorbs]

its more of balancing flow rates and making sure your not putting too much flow through the hex

might not be the case depends how its sized etc

you could install a gravity heat source on a llh and then pump it from there

lower temp, pump will last longer

 

[Ch4 plumbing]

Try this site.

https://www.google.co.uk/url?sa=t&s...FjABegQIBhAB&usg=AOvVaw3KljFgVxW3JXzjdMFMNy-B

 

[sammathias]

Oh really, not sure. Can't say I've ever fitted one on the return..not that I've done alot. Separators are good to eliminate thermal shock on the hex giving the system a more balanced flow rate. Increasing efficiency due to condensing more. Bigger pumps are a prime example with irregular flow/return temperatures.

 

[Ric2013]

lower temp, pump will last longer

But would you get the same disadvantage you'd have running a 'normal' sytem with pump on return, namely you'd be running at a negative pressure - air ingress etc?

 

[ShaunCorbs]

But would you get the same disadvantage you'd have running a 'normal' sytem with pump on return, namely you'd be running at a negative pressure - air ingress etc?

no as you would keep the boiler pump on the flow and then the other ones / circuits ones on return

unless you sealed it

 

[fixitflav]

But would you get the same disadvantage you'd have running a 'normal' system with pump on return, namely you'd be running at a negative pressure - air ingress etc?

No, provided the cold feed (or the expansion vessel on a pressurised system) is close to the pump suction. That's the point of lowest pressure, so everywhere else is higher.
I prefer pump on flow, piped boiler - open vent - cold feed - pump, as any released gas will be from the boiler, but open vent - cold feed - pump - boiler is OK. Once the system has settled down there shouldn't be much gas. Then the pump runs cooler and might last longer.

 

[Ric2013]

No, provided the cold feed (or the expansion vessel on a pressurised system) is close to the pump suction. That's the point of lowest pressure, so everywhere else is higher.

Website down for maintenance, so here's the Google cache:

Or, as it should be if open-vented:

Vent at A, cold feed at B. Distance A - B must be less than 150.

Yes, I suppose if the tees or the LLH is itself the neutral point, then the primary circuit will be at positive pressure because the pump is sucking on the cold feed. But the pump on the return of the secondary circuit is surely creating a vacuum all the way from B to the pump outlet?

 

[chris watkins]

I would seal any system given the chance now days, it just doesn't make any sense to leave them open under normal circumstances.

The Neutral point is where the expansion vessel connects to the circuit, so it wants to be close to the circulator inlet that way as much of the system is under positive pressure.

I think the return is preferred for the circulator as they work best pushing (into the high resistance of the heatX) rather than sucking from it.

 

[Ric2013]

Yes, makes sense what you are saying, but as I intend to specialise in solid fuel eventually, I need to understand where I would put my venting if I need it.

In a normal system, we have cold feed (or Ex Vessel) then pump pushing into the boiler via the heating load circuit, so your suggestion of putting the pump on return would have the exact same effect in this context, but it's taken me a while to see it this way. Your comment about pushing into the high resistance of a modern boiler has helped me grasp this concept, thank you.

The CIBSE article Ch4 has linked to states that pumps should suck on the LLH which would have the pressurisation on the bottom of it, thus it would, as you say, have virtually the whole system under positive pressure.

 

[chris watkins]

Not great on solid fuel Ric (looking forward to you teaching me as you learn) but my understanding is that as the F&V are so key to a safe in these systems they should only go on the Primary pipework or if using a buffer /thermal store to overcome the limitation of not having an automatic fuel source both (F&V) should connect directly to this.
Buffer is then the neutral point of secondary side.

 

[fixitflav]

In a normal system, we have cold feed (or Ex Vessel) then pump pushing into the boiler via the heating load circuit, so your suggestion of putting the pump on return would have the exact same effect in this context

Not quite sure what you're saying there. Pushing into the boiler via the heating load circuit is the same as having the pump on the boiler flow.
Pumped return gives higher pressure at the boiler inlet than pumped flow, which might be an advantage (though I can't see how), but it's still pushed into the boiler, just that with pumped flow it's been round the heating circuit first.

 

[fixitflav]

But the pump on the return of the secondary circuit is surely creating a vacuum all the way from B to the pump outlet?

I'll have to have a think about that! It depends on the location of the cold feed and vent, which aren't shown. Does the air separator act as an open vent? ie just a small vessel with a gas outlet pipe. Where is the cold feed? Its position is important, as we've discussed.

 

[Ric2013]

I'll have to have a think about that! It depends on the location of the cold feed and vent, which aren't shown. Does the air separator act as an open vent? ie just a small vessel with a gas outlet pipe. Where is the cold feed? Its position is important, as we've discussed.

The cold feed would be at point B as I have added to one of the diagrams in the above post. If the system is open-vented, the cold feed and vent logically have to be somewhere on the LLH or the primary circuit, but the system need not necessarily be open-vented, in which case...

 

[fixitflav]

Looking at the links, it might be an advantage on a commercial installation with several zones, with a separate pump to each. It seems way OTT for a domestic set-up. Why would you want to install an additional pump? Air release and filtration can be done other ways.

1. Within a domestic heating setting, a hydraulic separator would help the boiler to modulate more accurately (if it is modern enough to do so) and so reduce stress on the heat exchanger and improve efficiency by helping it run in condensing mode.

I don’t see why. If the rads can’t dissipate the heat, the boiler will modulate till the heat produced = heat dissipated. Why would a hydraulic separator make a difference? It doesn’t store heat or water. And to work in condensing mode the rad area has to be generous so (in most weather conditions) the house is warm with lowish water temperature (flow and return). Again, I don’t see how a hydraulic separator helps.

2. On a less modern boiler with no modulation, a hydraulic separator would reduce the length of boiler cycles, thus keeping flow temperatures to emitters more consistent and ensure boiler flow is always at a good rate without the need for a dedicated bypass. But this might actually increase gas consumption and wear to boiler due to increased cycling?

Perhaps I'm being slow, but I don’t see how a hydraulic separator makes a difference one way or the other.

 

[Ric2013]

Not quite sure what you're saying there. Pushing into the boiler via the heating load circuit is the same as having the pump on the boiler flow.
Pumped return gives higher pressure at the boiler inlet than pumped flow, which might be an advantage (though I can't see how), but it's still pushed into the boiler, just that with pumped flow it's been round the heating circuit first.

What I meant was this. That if you take a standard open-vented heating installation then you would put the cold feed just before the pump inlet and your vent would be just before the cold feed. So from vent and feed (the neutral point), you would have the pump sucking on the cold feed and pushing through the resistance of the radiators and the restrictive boiler. The pump will necessarily run on the warmer water at the temperature it leaves the boiler.

But, on a standard system, the pump would not be directly into the boiler return and running on the cooler return water unless the radiators are between the suction side of the pump and the boiler. In which case, the radiators would be in the negative pressure zone.

However, with a LLH, if the LLH itself is the neutral point, the pump on the primary circuit is sucking directly on the cold feed and pushing water the cooler return water through the boiler back to the LLH. Because the radiators are hydraulically separated from the primary circuit, they are not put into negative pressure by having the primary pump directly on the boiler return.

 

[Ric2013]

Perhaps I'm being slow, but I don’t see how a hydraulic separator makes a difference one way or the other.

As I was trying to explain why it did make a difference, I realised it probably doesn't make a difference!

 

[Ric2013]

Looking at the links, it might be an advantage on a commercial installation with several zones, with a separate pump to each. It seems way OTT for a domestic set-up. Why would you want to install an additional pump? Air release and filtration can be done other ways.

I'm not convinced about air release and filtration either, but I do think it might be possible way of integrating solid fuel back boilers or solar with a gas heating system without having to use a thermal store. I expect the extra pump would increase electricity consumption though.

 

[sammathias]

It seems way OTT for a domestic set-up. Why would you want to install an additional pump?

Large domestic system which require an additional pump..or a domestic property with with UFH, The LLH steals temperature from the return before it gets back to the boiler so the return to the boiler tends to stay at condensing range 50-55 degrees increasing efficiency. Expensive tho!

Even Some of the higher output vaillants will tend not to work correctly without a LLH.

 

[fixitflav]

The cold feed would be at point B as I have added to one of the diagrams in the above post. If the system is open-vented, the cold feed and vent logically have to be somewhere on the LLH or the primary circuit, but the system need not necessarily be open-vented, in which case...

The cold feed at point B gives the reference head (neutral point) for both circuits. Looks bad to me. Assuming the secondary circuit is pumped clockwise (as shown by the arrow on the pump), the minimum head is between heating load and pump suction. If say the header tank static is 2m, and the pump head 5m, head at pump suction is minus 3m.
Did you mean ".....vacuum all the way from B to the pump inlet?"
The head between pump outlet and B is the header tank static (2m in my example) plus a tad of pipe friction.

 

[chris watkins]

Hydraulic separation using a LLH (expensive) or close coupled tees (cheap) allows modern boilers to receive the minimum flow rate required by MI's, their control systems can then make adjustments to the output (burner modulation) & anti-cycling routines based on the heat being used against a typical 20degC difference between the F&R. In conventual designed UK systems the flow rate changes as zone valves, TRV's & automatic by-passes open & close, they can't make sense of what the system actually requires.
This is more apparent the larger boilers & systems as the burner can not go low enough & turns off as heat is not being carried way. Then they fire back up after a sort while (cycling) this is very wasteful so many have a program which stops it, limiting the stop/starts per hour, trouble is areas of the building then go cold or HW cylinders don't heat up in.

 

[Ric2013]

Did you mean "...vacuum all the way from B to the pump inlet?"

Err, yes. That was a good typo, wasn't it. I meant inlet.