Primary & Secondary

LD

Can anyone give me a simple description of what Primary & Secondary refers to in boilers? What would be a typical application?

I read about some boilers requiring this setup and am not sure if it applies to my setup (in-floor radiant heat with indirect DHW tank).

Aidan (UK)

In short, it's a system with multiple circulation circuits and pumps. Usually, although not always, the flow through the boiler is the primary circuit and there are secondary pumps serving the secondary circuits which might be DHW, radiant circuits, baseboards, etc..

The flow rate through the boiler will remain constant, at the optimum flow for the boiler, whilst the flow rates and temperatures to the secondary circuit can be varied to whatever is required.

See http://www.heatinghelp.com/newsletter.cfm?Id=37

hot_rod

If you ain't got closely spaced tees

you ain't got primary secondary. I have seen some pretty creative approches to PS. And I have bastardized a few myself. The key is the tee spacing.

Dan's book "Primary Secondary Made Easy" helps see this, order one from Books and More at this site

Here are a couple examples of a few p/s arrangements. All have pros and cons and need to be applied correctly. Boiler return protection is always a main concern. Take off of the various temperature loads is important with series P/S as each load in series "sees" lower temperature water. In this case parallel may be a better match.

Multiple boilers would be piped differently than a single. Jobs with all zones requiring the exact temperature (ie fancoils) would be better piped parallel.

Learn about all the various methods and apply them to the job accordingly. An excellent PS article appears in PM Engineer this month, check it out for some better drawings and explanations

hot rod