control valves: supply or return

steamfitter

On large commercial hw systems I have seen control valves on either supply or return of heat transfer units.

On hw re-heat coils in the ductwork the cv's seem to favor the return, but on fintube baseboard I usually see them on the supply. What is the primary reason for the cv location on hydronic heating systems?

Do engineers differ in opinion on whether it is beneficial to locate the cv on the supply or return? Are there other reasons for the placement of cv's?

SWEI

Depends on the valve

If it's a 2-way, I've seen both positions on drawings but can't see any reason for one versus the other, as long as there are purge/drain provisions in all the right places.



With a 3-way, a lot matters.  Belimo explains it better than I could (can't upload the single page PDF I extracted for some strange reason, but it's on p.6 of http://www.belimo.us/media/downloads/Technical_Documents/Characterized_Control_Valve/CCVTechDoc.pdf)

Zman

Siegenthaler

Siggy advocates for zone valves on the supply side to reduce heat migration. This makes sense to me. As Swei points out 3 ways are a different animal.

Carl

SWEI

Just curious

where would the heat migrate from and to?  Not doubting Mr. Sig, but can't picture it in my head.

Mark Eatherton

One pipe thermosiphon...

THAT is why Siggy wants the zone valve on the supply. You can see it in an infrared imager much easier than your minds eye.



On a vertical pipe, hot water rises up the center core, gives up heat with cooler water coming down the exterior of the pipe.



I think the OP was referring to flow balancing/control valves, and technically it doesn't matter wether it is on the supply or return because it has a given pressure drop across the device which is the same regardless of wether it's on the supply or return in a given flow path.



If the OP was in fact referring to a ZV, then one advantage of placing them on the return is that it exposes the motor assembly to the lower temperature potential thereby potentially increasing the motors life expectancy.



In reality, I don't think that in the grand scheme of things it makes THAT much difference tho...



ME

SWEI

Zone valves

Makes perfect sense in that application - thanks.

hot_rod

return side

is sometimes preferred on solar for example, so the high temperature fluid does not hit the valve, or pump. The fluid passes thru the HX first.

ColoradoDave

Heat Migration

Sigenthaler also is an advocate for closely spaced tees for the secondary loop, which I'm a huge fan of.... but I'd think there'd be just as much heat migration regardless of which side the zone valve is on in that scenario.

The idea of putting them on the return for the lower temperature to potentially extend the life of the valve is also valid, but in theory the delta T from service to return side of a heat emitter loop should be 10 - 20 degrees depending on design, so IMHO the potential is relatively small.

I think it comes down to personal preference and in the overall picture, it shouldn't impact your system regardless of which side you install your zone valve (control valve) on.

However, men (some of them Dead Men) much smarter than me have been talking about this for years and THAT is why I lurk in the forums hoping to absorb this vast knowledge.

Jean-David Beyer

Sigenthaler .... but I'd think there'd be just as much heat migration regardless of which side the zone valve is on in that scenario.

When I was reading John Seigenthaler's big book, he showed a flow check valve in both the supply and the return of the zones. I could not figure out why, so I sent him an e-mail enquiring about why two. I pointed out that my installing contractor had not put any flow check valves in the secondary (system) loop other than the light spring check in the Taco IFC circulators.



He replied that the trouble with using only one flow check is that  you could get unwanted heat transfer with a single pipe (hot going up the center, cold coming down the outside), and that the reason I did not notice a problem is that most of one zone is only 1/2 inch tubing, where the effect is much smaller than with larger diameters. And the other zone (radiant heat slab) is about 5 feet below the boiler, so it is effectively heat trapped.



So what difference does it make where the zone valve is, since you may need a flow  check valve in the part of the loop where the zone valve is not?

icesailor

Zone Valves/Flow Checks:

I always considered most zone valves as a form of motorized valves.