ME bail me out; P/S quandary

copperhead

Houston,I believe we have an anomaly... it looks as if I'm about to build a mechanical room for a 4 story building that is mostly radiant-floor heated...the cartoon/blueprint shows 2 banks of 3 Hydrotherm boilers supplying 2 120 gal. DHW heaters , 4 not-so-large cabinet unit heaters , and approximately 18 radiant floor manifolds @ 180'F and 6 radiant floor manifolds @ 140'F. The concern is for the 3 "peripheral" primary loops that supply the 6 ea. manifolds @ 180'F. My foreman's concern is that 6 P/S connections @ 180'F in 8' horizontal will create a hot supply shortage downstream of the first couple of connections....sounds plausible...but I don't posess enough data to prove it either way...it seems to me,from my tyro way of thinking that if the P loop were adequately sized and so was the circ.,that it might perform as drawn....however,his solution to this perceived problem seems even worse than what is drawn,....which is to drag the returns across the mains. I just read JS article in P/M which speaks to this,showing each circuit across the mains w/it's own P/S connection,unlike my foreman's idea which would leave it to the circs to duke it out. what is right? it seems the origanal might work...? I now know the books I need to answer these little puzzles,but I'm doing this myself,so moving forward is painfully slow at times.

[Deleted User]

Got drawings???

I think I can see it in your minds eye, but am not 100% sure. Can you make a schematic shooiwng whats what and wheres were? Something like this would work fine.

We'll get you on track some how.

ME

[Deleted User]

Possible fix...

I have prepared a drawing showing how I would perform this if it were my charge. Notice that the boilers all have one common connection to the primary loop. This insures that all boilers will see the lowest incoming water temperature potential and keep their efficiency as high as possible.

The first load coming off the primary is the DHW load. This can be electronically prioritized if need be, but is inherently prioritized mechanically, as the first load on line.

The next load coming off the primary would be the high temp radiant floor branches. I've shown it as having one pump. If you have one pump per manifold, then you will first want to make sure that all pumps are sized the same, otherwise, the big'uns will win. If there are multiple pumps, simply make the connection supplying all these pumps the same as I did for the boiler connections.

The next load coming off the primary would be the cabinet heaters (fan coil units I assume). These will work quite well with 160 degree F water.

The last load coming off line is the low temp RFH loads. This load is protected by a 3 way tempering valve. Not the best temeprature limiting device, but none the less, functional.

The whole key to all of this operating correctly at design condition is that it assumes a 20 degree F drop in temperature for each load. Excluding DHW, which should be the only load on line when it IS on line, the water starts out at 180 degrees, drops 20 to 160 degrees. This goes into the fan coil units and comes out at 140 degrees. This 140 degree F water enters the 140 degree RFH zone and comes back at around 120 degrees F. This is assuming some pretty big load parameters on my part, and the possibility of it actually happening in the real world are slim to none.

If the designer IS concerned about providing fluid temperatures below 135 degrees F to the boilers, he/she can incorporate control strategy to shunt load and allow the entering water temperatures to come up before allow the secondary loads back on line.

If the secondaries are piped with individual branches, as I gathered from your post, then your associate is correct in his asssumption that the down stream taps would see a cooler inlet temperature. Essentially, piping it this way (numerous secondaries in a row) is akin to piping it in series, which is NOT a good idea.

The reality of the matter is this, the heat loss calculations that were done on this building assume ZERO input from any other sources, including body load, solar fenestration, lighting load and electrical usage in general. To say nothing of the internal mass flywheel affect. You could incorporate the controls necessary to protect the boilers from low entering water temperatures, but in my field experience, they probably would not be necessary if the system were sized "per usual".

Hope this bailed you out, or at least got the wheels of change a rollin'...

Let me know if you or anyone else has any other questions.

We're ALL here to learn, and I have only presented one mans way of doing it.


ME