Radiant Heat - Why Boiler Loop and Primary Loop?

[BronYrAur]

I saw a system the other day with 4 "stages" of pumping. There was a non-condensing "boiler loop" with a pump that fed a "primary loop." The "primary loop" had a pump that just circulated around in about a 15' perimeter. Pulling off of that were injection pumps that dumped into a "secondary loop". The "secondary loop" contained the manifolds and all the individual zone pumps. So, there were 4 different "stages" of pumping. I understand that the injection pump process is to inject small amounts of very hot water into the lower temp radiant loops while at the same time not allowing the boiler return water temp to get too low. And I understand the zone pumps. What puzzles me is why was there a "primary loop" seperate from the "boiler loop". Since the injection pump system is designed to protect the boiler, it would seem that the "boiler loop" and the "primary loop" could be one in the same. Any idea why they should be seperate; it just adds a pump to the system?

 

[lilliput1]

It may be that over the years modification have been performed and that each engineer of each project chose to play it safe by adding a pump and secondary loop for their system to make sure the primary system do not run out of gpm. Nowadays with better control, two way valves and VFD pumps you can eliminate other pumps and secondary loops. Just make sure you also design a bypass for minimum flow.

 

[GMcD]

If it was a non-condensing boiler, then the primary/secondary setup insures that the boiler always sees a high return water temperature (above 160F), while the secondary circuits mix down to the required secondary loop temperatures. Radiant heating systems where the floor is used as an emitter want to be operating at surface temps of less than 90F, so that means water temps in the 95F to 115F range depending on the type of radiant system.

 

[lilliput1]

GMcD you are not supposed to design secondary systems as you said. The return of each loop should be equal to the the boiler design entering water temperature. It is the supply temperature to each loop that is varied.

 

[Jabba007]

This thing sounds like it had 3 primaries plus the zone loops?

Most gas fired non condensing boilers will handle down to 140F return water temp. Oil fired is higher. The system sounds like a Joe Bob abortion to me. I'm sure it works... but it sounds like it has a lot of unnecessary complication and pumping and thus failure points.

Jabba

 

[GMcD]

Lilliput1 - Not sure how you read that into my post. Absolutely the returns from the secondary circuits have to be matched to the boiler return water temp. A good source of further flavours of boiler circuit set-ups is John Siegenthaler's "Modern Hydronics". Lost of good info there.

 

[RossABQ]

There is no need for the boiler loop separate from the primary loop, but I saw a system that was somewhat similar because it was designed for addition of solar panels in the future; the panels would dump heat into the primary loop whenever available, and if that wasn't enough the boiler would kick in.