Primary/Secondary Loop System

[Mancino]

Question for any of the experts here on a P/S loop system. I'm getting prepared to start my boiler install for my garage and it's been suggested that I do a P/S loop system for this particular boiler.


Are the primary and secondary circulators hooked to the same zone? And do they run at the same time? Or do they need to hooked up in some special way where the primary runs first and continues to run after the thermostat is satisfied?

 

[johnnyradiant]

A bare bones basic primary loop would be running at the hottest temp required for your secondary loop. The secondary loops would circ as needed. The boiler would cycle as much as required to keep the primary loop ready to go. You could control the primary from being on always but that is sorta the base rudimentary start point. I assume you are using a single boiler for a typical GJ garage shop, so what is the reason for a p/s system? Is it to get radiant loops down to a lower temp than boiler works best at, or is it because you have a couple different zone (secondary loop) temp requirements?

 

[Mancino]

Yes, single boiler only dedicated to running the garage. I have a 4 loop radiant system installed, and it will be the only zone. Each loop is approx 270' long, about 1200 sq/ft.

I'm only going on what the manufacturer suggested for their type of boiler. The plumbing supply outfit I bought the equipment from couldn't explain it well(honestly, I don't think they understand it well enough), so I figured I'd get more information here.

 

[Jackfre]

Does your boiler have the built in controls to operate one or more zones? Some do. If not, you will have your slab or thermostat wired to a Zone Control panel. I've used Taco's but forget the model number. That will wire to TT on your boiler. NOW you need to get the manual for your boiler and see what they say. Are you going with closely spaced Tees (no more than 4 pipe diameters) or a low-loss header? If CST's upsize the pipe at that point. It acts as a mixing chamber. You should not have to stage operation of the circs as the slab won't be fazed by temp swings. Does your boiler have a built in outdoor reset.
I don't do many boilers as I am retired out of the business, but I get to do one or two a year. The boiler I use has an internal modulating circulator. Its built in control panel will run two zones and it has a very robust outdoor reset control. All I need to do is select the secondary circulator and pipe the system correctly. Does your boiler have this equipment?

 

[Mancino]

That's a good point on the built in zone control...I'll have to look. It's a Navian NHB55

The manifold I bought as a package for these types of boilers and it's pre-assembled with the correct spacing, sizing, etc. I'm certain the boiler has an outdoor reset also. It came with an outdoor sensor. I also installed a sensor in the slab before I poured.

I know the NCB-210 I use in my house has an internal circulator. That's why at first I didn't think I needed the 2nd circulator for the garage unit.

 

[fitter30]

Radiant heat runs water temp 5* to 10*over set point. Cast iron boilers can't be run under 140* boiler will crack and fail. A condensing boiler will run with colder water. Cast boiler would need two loops primary and secondary with two pumps, one expansion tank. If your load is small enough a water heater can be used wth one loop. Thermostat with sensor for the floor works the best. For freeze protection if running propylene gylco 30% is a minimum for the additive.