Question for the experts?

HDE_2

If I had a ModCon boiler and had the opportunity to operate it on supply water setpoint temp or return water setpoint temp, which would you choose and why?

Wouldn't return water temp operation receive better feedback of the heat emission?

What would you do and under what circumstances?

Keep in mind outdoor reset could/would play apart but for comparison, lets leave it out.

Unknown

I`ve thought about this too,,,

But EVERY zone(if there are more than one) would have to be balanced perfectly,, H/L and flow-wise,, if not(as most are) how would one govern(independently) the water temp for each, shy of TRV`s?

Some rooms with more windows, outside wall space, or facing different directions on the same zone may not need the same heat. How can this be applied to a "common" return water temperature?



If it were as simple as that, why not (on small systems) a buffer-tank on the return?



I wonder where ME is?,, he`s never at a loss of words for any subject!  :-)

Mark Eatherton

If I only had one eye...

In the land of the blind, I'b be KING!



And if I were a boiler operator, and I could only look at one thing, and I didn't have the ability to do an outdoor reset, I would limit the supply temperature with an automatic control set for worse case scenario, but would judge demand, and the need for bigger fire off of the return, because that is where LOAD will be reflected. In fact, that is the FIRST place that I would see a change indicating load, or changes in load.



But back to your question, if I had to limit the boiler with one or the other, I would probably chose the supply. Historically, we have always monitored the supply to insure that we are supplying the hottest necessary water to satisfy the theoretical load, but not exceeding the system design temperature.. With older steel and copper systems, that is probably not a major concern, but with the plastic tubing in use today, it is a BIG thing.



I have seen older systems that were intentionally piped backwards through the boiler, and by doing this, the limits are essentially running off of the return except they are still located physically on the supply, and it caused the boiler to put out SUPER hot water. But then again, that boiler (Ajax) didn't have the same mass characteristics or control sensitivity that the newer more efficient boilers have.



So, as with any hydronic system, the correct answer would be "It depends"...



Personally, I'd watch both the supply and the return, because I can anticipate increases or decreases in load based on what the return is doing. That is the way most low mass mod con manufacturers are doing it at present.



By the way, the daffynition for expert is, X being the enumerator/operator for the unknown, and spert being a drip under pressure, so an expert is an unknown drip under pressure :-)



Happy Good Friday/Passover to all my friends of all denominations.



ME

Unknown

Thanks Mark,,,

Some how I just knew you you would "chime-in" on this. ;-)

seabee570

reset

I have never seen it done on hydronics in a commercial setting...but....I have seen it done an airhandler,with a ddc(direct digital control) energy management system. I had a problem with an office space that had lots of windows on the exterior.the single large airhandler also served interior spaces.by the time the space temp sensors opened the chilled water valve,it was too late.so I looked at the return air,and as soon as it got warm,I opened the chilled water valve 100 percent.It seemed to work. you may be on to something,depending on application,and specifics.

HDE_2

?

Laars has been doing it for years, for example their Mighty Therm boiler. It only takes one trip to reset the hi-limit manual reset to realize that the operating sensor is in the return, thus setting it for 180, many times meant the supply could be 210. I know lot's of L series aquastats were returned for being "out of calibration".

I see the benefit of return water temp on an air handler/hot water coil, especially if it is V/S or has a VAV system.

Good point on the maximum PEX temps but on baseboard for instance instead of setting it for 180 supply and designing for 20 delta t, why not just set it for 160 return, with 20 delta t design and should dampers be closed, flow changed or whatever, the final baseboard sees what it needs?

Any other input to sway the mind and my thought process?