Can someone explain what the reasoning is behind placing the loop supply and return tees closely together on the main boiler loop?

Thanks in advance.

John

Got it, thanks.

So please can you share with us what exactly it is required to do so.

I believe the term and desirable objective being discussed here is called hydraulic separation. The objective is to keep multiple circulators from interacting with each other. The degree of circulator interaction depends on the head of the piping path that they share in common. So less shared common pipe results in less head loss and achieves the desired hydraulic separation so the multiple circulators are unaffected by each other.

Hydraulic separation is not always necessary or desirable. Most of our work is with high efficiency condensing boilers and we rarely use hydraulic separation.

The question is too general to answer.

We always use hydraulic separation as there is never a negative to adhering to this best practice. Our HR designs specify the distribution plumbing guidelines, the specific components to be used (circulator pumps, manifolds, etc.), the manifold circuit balance valve Cv settings, the differential pressures of the circulators, and all the circuit flow rates to simplify installation setup and to ensure that the installed HR performance is consistent with the design. If the installation is accomplished without providing for hydraulic separation, the design parameters may become invalid. If the design does not include the specific HR heating system components and these design parameters, than the installed HR heating system performance is left to the competence of the installer and this can be a hit or miss affair.

See here. But I disagree with the operating cost analysis, at least theoretically. Decoupling boiler flow rate from loop flow rates should a) allow each pump to always operate at its single most efficient operating point-- only the exact flow needed in each loop. b) allow the pri/sec series configuration which gives some control over priorities - might be useful in some cases.

On the other hand, maybe a modulating boiler benefits from more flow as more loops call for heat. Not having an independent boiler loop pump does that.

Right, hydraulic separators are a great way to accomplish this. I notice all the illustrations are straight out of Siegenthaler's Modern Hydronic Heating without giving credit.

Posted By sailawayrb on 17 May 2014 07:22 PM
Right, hydraulic separators are a great way to accomplish this. I notice all the illustrations are straight out of Siegenthaler's Modern Hydronic Heating without giving credit.

See the center picture on page 1.

Wow Dana, even with you pointing this out, I still struggled before seeing that John was the presenter. Thanks, I feel much better about this :-)

One of the benefits of reading AND experience it the ability to avoid saying "never" or "always".

The main thing that one can predict when hydraulically separating any boiler from a hydronic distribution loop, is that you will raise the chance of the heating system shutdown due to a failed pump by 100% since primary/secondary piping requires both pumps to operate in order for the system to work. The other undeniable negative results are the increased parasitic loss of a second pump, and when installing condensing boilers the unavoidable and equally undesirable increase in return water temperature to the boiler.

Most manufacturers "recommend" primary/secondary piping for their condensing boilers but this actually is to overcome installations that might compromise the boiler, causing it to short-cycle and flash to steam for lack of flow. But a well designed hydronic heating system, as with with any mechanical system, should be designed with the minimum of material and operate at optimum efficiency. This is rarely achieved on a residential scale with primary/secondary piping. Some manufacturers mandate the use of P/S piping e.g. Buderus with their GB142 model--but not with the new "Bosch" GreenStar series--and Weil McLain Ultra--but again, they do not mandate it with their new ECO series boiler.

In fact we just installed a Weil McLain ECO using an ECM pump on a radiant floor retrofit. One boiler, one 45 watt pump, multiple zones and a return water temperature 20°F lower than it would have been with P/S piping. And yes, we removed a condensing boiler "properly" installed with P/S and two 220W pumps!

https://www.mge.com/images/PDF/Brochures/business/CondensingBoilersForYourBusiness.pdf

In the unlikely event that hydraulic separation is required or beneficial, a hydraulic separator with integral air separator and sediment separation is more efficient than any "closely spaced tee" configuration. We buy them from Viessmann, Caleffi and others.

With regard to Weil McLain we always say never and with regard to hydraulic separation and adhering to Siegenthaler best practices, we always say always. Experience is only a positive attribute if it is based on good knowledge and good practice. There is no shortage of highly experienced boneheads in the construction and HVAC industries.

Or sharp-tongued sycophants for that matter. Best stick to your knitting.

Primary/secondary piping has a lot of great applications. but if you are in an application where the range of flow rates and pressure drops presented by the emitters is within acceptable spec for both the pump and the boiler in all conditions, then all primary/secondary does is increase first, replacement, and operating costs forever by adding an unnecessary pump and its parasitic energy usage... even if it improves efficiency of the system circulator it would never be by enough to justify the second pump in terms of energy usage.

Systems that work include single zone systems as well as zoned systems where the smallest zone guarantees flow rates above the heat source minimum requirement, and obviously where the full flow capacity can be handled by the boiler/circulator combination as well. This is one small part of the reason I prefer firetube boilers with their lower pressure drop across the heat exchanger... many more homes then become accessible for single pump design.

"Always" and "Never" are words that really don't have any application in engineering.

Borst, that description of hydraulic separation in your first post is probably the clearest and most succinct I've seen out there. Nice work there.

Badger, I have to disagree with the hydraulic separator comment. with any pipe junction water in must equal water out. In a P/S application, if boiler flow exceeds system flow, you'll return water and elevate return temp, period, no matter what you're using for a separation device. I can see an argument for labor savings but given the steep cost of most packaged separators I've seen I don't think I can justify their existence in most cases... I've come to regard them as pretty overly hyped. Can you elaborate on your reasoning? the separation is superior but I have to think the quantifiable difference is pretty minimal.

Posted By sailawayrb on 20 May 2014 05:30 PM
Wow Dana, even with you pointing this out, I still struggled before seeing that John was the presenter. Thanks, I feel much better about this :-)

So you are saying you where not comfortable with the systems but because John says it, it must be so?

What bothers me about any of the layouts (other then true primary/secondary) is the chance of very cold water coming straight back to a very hot boiler.

Systems that work include single zone systems as well as zoned systems where the smallest zone guarantees flow rates above the heat source minimum requirement...

I believe that this is an argument for systems where zones never go off completely. Ie, why restrict flow all the way down to 0% when you know that most zones will always require some heat? Unnecessary restrictions just add to pumping losses.

as for the hot boiler, the same thing can happen on or after a heavy domestic demand as well, and it can still occur in primary/secondary piping in some cases too. I don't regard that with much concern. in low temp heating systems (such as should be used with most mod/con boilers) it should be a absolute non-issue as the boiler should almost never be that hot. the times you'd exceed the 30 or 40 degree max design dt across the boiler heat exchanger that most boiler MFGs recommend would be pretty rare and pretty short lived given the lack of mass in the boiler in any case.

jonr: to maintain minimum flow through a boiler would, in most cases, overheat the zones. on/off means you can ensure that when you DO run, you're over minimum flow rate, as opposed to flow modulating zones where all bets are off and primary/secondary is an absolute requirement to avoid flashing to steam in the boiler under very low flow conditions.

Rob is right, welcome back.

If you will recall the old days--before ModCons--when P/S was used to hydraulically separate system flow from boiler and distribution, the primary purpose was to protect an atmospheric boiler from return water temperatures that may bring the byproducts of combustion to the dew point of the particular fuel you are converting-- roughly 130°F or NG and 140­°F for oil--and sustained flue gas condensate. Ultimately this would corrode the fireside of the boiler, leading first to combustion issues and ultimately to premature death. Yes, there are large residential and commercial systems that will require pumps zones, circuit setters et al. But we are in a residential forum...I think?

Fast forward the the 21st Century and low-mass condensing boilers. As you stated what goes into a tee must come out, but the mix of temperatures depends entirely on the flow between the heat source and emitters determined by pipe and pump size. I agree, if the flow is balanced there is no temperature difference between boiler supply and system supply, nor return for that matter. My condensing boiler and I are happy. If however you adhere to multi-zoning with pumps ( I know you don't) then you can easily change the perfect balance in either direction. In systems we diagnose for lack of performance, we often find P/S pumps in the 4-800% over-size range. It is not unusual, in fact common, for the boiler pump to be over-sized with a 5° Delta T and the Biblical 20­°F Delta T on the system side. In this situation the boiler return water temperature is raised frustrating any attempt to condense the byproducts and recover latent heat. You have a very good 88% combustion efficiency boiler, but not the 99% you really want.

When I installed my first condensing boiler back in the late 80's P/S was well known, but the early condensing boiler did not have the benefit of tach driven fans or neutral pressure gas valves, so one had to learn how to design systems that were well balanced on paper or learn the hard way when a short load (micro-zone for those who only read "the book") cut the flow in the boiler until it is flashed to steam or short-cycled in less-than-minute fire cycles.

Commercially produced hydraulic separators can save labor, depending on the local market , but we rarely use them in any form with the notable exception of multi-boiler systems where the combination of hydraulic separation, air elimination and sediment collection make sense.

We use both fire-tube and water-tube boilers. If you do the math the circulator will be right-sized and boiler, emitters and occupants satisfied.

Low-mass condensing boilers can not suffer thermal shock.

jonr: to maintain minimum flow through a boiler would, in most cases, overheat the zones.

Yes, if no zones need heat, then all flow and the boiler should stop - no need for pri/sec loops. But completely shutting off a zone that could predictably use some (not full) heat is an unnecessary flow restriction and may force the up-sizing of loops. And when one zone calls for heat, there are many cases where that prediction is possible (for some other zones).

I know- it was a bit on the subtle side...


















...which is a bit unusual for Ziggy, who is usually blowing his own horn pretty loudly.