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ASHRAE Design Guide

Paul Rohrs_14
Paul Rohrs_14 Member Posts: 80
I was thumbing thru the ASHRAE Designer Manual and came upon the advantages and disadvantages of Drainback and Closed-Loop systems.  No surpises there.

I was mildly surprised to not find any information (even cautionary) on Direct or Open Systems that are typical in South Florida.  In short, all potable water is piped thru the collector and then into storage where it is then backed up with water heating equipment and then a pressure booster package is then added to compensate for the higher pressure drop thru the collector.

Lochinvar systems are designed around the closed loop principle of "Forced Circulation" .  "Best Practices" or standard systems designs of SDW systems don't really list Direct/Open systems so I am wondering about the Industries mindset of the "responsibility" or legality of these types of systems.

Local codes must allow this but I am really questioning why they are so popular?  The cost of an external HX or a tank with a dedicated coil for pre-heat seems like it would mitigate the cost of a pressure booster package in spades.

Anyone out there have experience with these systems and can you tell me the resistance to move to a closed loop system?





  • hot_rod
    hot_rod Member Posts: 22,118

    is the driving factor.

    I remember a seminar at ASES where San Diego was looking at solar thermal DHW in the $4500.00 INSTALLED cost range. That pretty much limits you to those direct or ICS systems.

    They are common in the warm climates across Europe and in the Caribbean Islands.

    St Croix made me nervous, rain water sitting luke warm in cistrens then run through a direct collector. But maybe the collector provides the bacteria control :)

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Tom in Maine
    Tom in Maine Member Posts: 23

    I suspect that there are some other reasons for a preference to antifreeze or drainback systems.

    If you ever have a leak, the leak is limited to the volume of the antifreeze in the system or the volume of the drainback vessel.

    A direct system is going to leak water until someone discovers it.

    I suspect there is some issue with cross-contamination(?)

    We have had go rounds with code people in the US and Canada about our tanks using

    single wall heat exchangers in either application. Since we (or more correctly--our customers) do not use toxic materials in their solar loops, there is little risk of the kind of cross-contamination that code is concerned with. And there is an exception to that rule in the code.
    Tom Gocze
  • Karl_Northwind
    Karl_Northwind Member Posts: 139
    direct systems

    I've heard horror stories of collectors plugged with mineral buildup.  literally plugged.  too much so to even get de-scaling fluid thru them.  time to replace the absorbers. 

    lots of systems going in in florida are 40 SF panel with 80 gallon tank, direct active systems.  just a stainless circulator, and expansion tank if the house has a check valve on the water inlet.  there's no HX and no pressure drop issues.  the controller has a freese protection control (recirculation) with a dribble valve (Dole valve?) for a backup.  if you only get a couple of freezes a year, it's reasonable.  if you have more freezes and really hard water, an indirect system would seem to make more sense.

This discussion has been closed.