Primary/Secondary question

Jed_2

Check out the attachment. What's wrong with that? Whoops, the schematics didn't Paste. Go to PM Mechanical, Nov.2001, An Indirect Question, Fig. #3.

Jed

Bob Murray

Primary/Secondary question

Gentlemen (Ladies too),
I am about to pipe a dual boiler system using recommended primary/secondary piping arrangement from our man Dan. I plan on using flow checks on the secondary loops. Do I need one on the primary loop? Do I need one on the boiler loops? I could not find anything specific on this in Dan's books other than the secondary loops.

Thanks all,
Bob

John Ruhnke1

Flow Valves

When we piped this job in primary secondary, we installed flow valves on the secondary loops only. The reason is that the boiler is shut down when the primary and boiler loops are shut down. This means the boiler cools down and so do the primary loop and boiler loops thus no need for the flow valves. On another job we did we installed a flow valve on the boiler loop because the boiler had a coil in it and needed to mantain temperature.

JR

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Boilerpro

Some words of warning....

There are several P/S multiple boiler piping techniques that are causing problems for boilers. One of the poorest and most popular is running a single primary main and piping each boiler secondary off that main, one after another. These are the most serious problems I see. 1) One boiler preheats the water for the second further down the primary main. The second boiler runs much hotter than necessary and the high limit must be set higher than the system high limit in order for it to provide higher temp water to the system at heavy load. 2) The first boiler in line tends to suffer from cold return water shock. If the system is operating and then all zones shut off and then a large cool zone kicks on all this water now goes into the much smaller lower water content boiler. 3) The second boiler to fire is usaully cold at start up (unless it has a pilot and stack damper), So when the pump kicks on the cold boiler recieves full flow of hot water causing shock.
Also, many manaufacturers and other sources are saying the delta tee across the boilers has nothing to do with the delta tee across the system. I strongly disagree, unless it is a low temp system, using high temp boilers. If the system needs 200F supply and returns 180 to the boiler room and the boilers are set up at 50F delta tee, the boilers will see at minimum a 180 at the boiler return and 230 at the outlet. Also, with any higher mass boilers, it is important to start the lead boiler pump with the staging control instead of the boiler relay or else you will probably have short cycling (fire times measured in seconds) in warmer weather. These are some of the things I've seen kill nice Burnham 806 807, an 810B's in as little as 7 years (sorry Glenn). Just a few things I've picked up from putting in alot of multiple boiler heating plants.

Boilerpro

John Ruhnke1

Everyone wants to save money on pipe......

Install a bigger primary loop. I like BIG and short primary loops they work well for me. That will drop the delta tee and most problems just seem to disapear. Try a delta tee of 20 or better yet 10 on the primary loop. Also wire the job for boiler protection. Wire the secondary pumps through zc and zr to the boilers. This will shut down the secondary zones when the boilers drop below 140 and run them again once the boiler water gets higher. Of course if you upgrade to Tekmar controls you can get boiler protection from them too plus boiler staging control. The Tekmar staging control can equal out boiler run times. Tekmar recomends tieing all of the boiler feeds together and all of the boiler returns together. This way each boiler gets the same temps. A kind of reverse return helps too. The first fed becomes the last to return, that balences things out even better.

JR

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Floyd

This works great..

Tekmar control to stage boilers and all supplies into one pipe
and all returns into the other pipe.
Also Weil has boiler protection built into their boilers....
Takes alot of time and materials....
but there's nothing like the satisfaction of a job that works well!!!!

floyd

Boilerpro

Tekmar only protects against cold shock, not hot shock..

in most applications, and this is only when using variable speed pumping or mixing valves. Alot has to do with the piping.

Boilerpro