Dan H's Primary Secondary article

Buzz G.

Reread several times your article on looking down the pipe and water taking the easy way. BUT-one of the circuits has to be straight and the other has to be perpendicular to it. Does it make a difference which one is straight and which one is perpendicular. If so why? Or How Come?

DanHolohan

It makes no difference.

It's all about the differential pressure between the points where the water splits. The less pressure drop in the common piping the better.

Buzz G.

P-S loop

Finished install today and the P-S loop stayed cool while the DHW took time to heat up. Using 1" pipe a 10" P-S seems to do just what you described. Today Finally have heat and Hot Water-the OK icestorm passed thru but we did not lose power this time like so many.
They set the pressure to about 23 psi, but only two floors of rads above the boiler so I set back to about 16 lbs as that is what I got off the Wall. Left the exp tank at 12 lbs-should I add 4 psi to it? Cannot figure reason to have more pressure than what it took to fill the highest rads.

DanHolohan

You need

enough pressure to get the water to the high point, plus a couple of extra pounds of pressure. Measure the lift in feet, divide that by 2.3 and then add two or three psi. That's all you need.

Abro

Dan,

If you have time, could you give your opinion on another P/S matter that I’ve posted about on the Wall? How would you recommend achieving boiler protection on a primary-secondary system with multiple, different temperature zones and regular cold-start conditions?

I’ve gotten several helpful, but different, suggestions so far. Some people have said p-s virtually guarantees boiler protection Others suggest simply ensuring that the primary (boiler) loop pump exceeds the GPM needed by all secondary (system/zone) pumps—which would ensure some return flow to the boiler. Your article, if I understand it, suggests piping a bypass into the boiler loop and using balancing valves to ensure a safe return water temperature. Finally, some folks have suggested returning a similar bypass into a Danfoss TV piped right at the return boiler tapping. This TV would keep the boiler bypass open if temps are below 140f and opens the entire primary (boiler) loop as soon as temps exceed 140f, permitting hot water to flow into several secondary loops.

In case you need more details, I am repiping a 1903 gravity hydronic system with a 1989 Dunkirk boiler that was installed to pump toward the boiler. 177mBTU with a simple Honeywell aquastat and a Grundfos UP-15-42F. No complex controls. The boiler currently has no bypass piping or protection whatsoever. I’m in SW Ohio in a 3 storey house with a separate heat pump/ electrical resistance heating system on the 3rd floor. The boiler serves 675 EDR over 2850 square feet on floors 1 and 2.

I plan to repipe the boiler primary secondary pumping away. The secondary (system/radiation) loops will consist of (in this order) the old gravity loop, 2 zones of radiant flooring running at roughly 160f and a zone of radiant slab flooring running at more like 120. I could send a schematic picture if it would help.

Thanks very much for all your great resources and help so far. Coming here has been an incredible educational experience for me.

Jim Erhardt_3

Another fool proof method....

...of protecting the boiler is to install a reverse-acting (close on temperature rise) aquastat in the primary loop and use it to drop out the system circs when primary loop temp drops too low. Most of the multizone circ relay boxes have a set of terminals to make this connection a snap. The burner and primary circ will continue to run as long as there is a call for heat.

Tim_39

Thanks, Jim

Yes, I've heard that one, too, and like it because there's no valve to break down. Am I correct in understanding that I need a second aquastat--and that I will pipe it in just prior to the return boiler tapping? Or do I want to replace my existing aquastat with something that has more control options?

And then, if I get the idea, each secondary circulators is are set to run on a call for heat in the relevant zone (or perhaps in the gravity zone set to run more or less continuously?)--but the second aquastat shuts all of circulators off until the low limit is reached. This means, if I get the idea that no bypass piping is required--as the primary (boiler) loop will function as a bypass when the low-limit aquastat shuts down all secondary pumps.

Do I have it right? And do i run the gravity pump more or less 24/7 in the cold weather? Or only on a call for heat?

Jim Erhardt_3

It's easy...

You would install a second aquastat (such as a Honeywell L4006B) in the primary loop. I typically install the aquastat on the supply side of the loop, upstream of the supply/return connections. The reason for this is to avoid excessive on/off cycles of the secondary (system) pumps. Since primary loops on non-condensing boilers generally have a low delta-T, setting the aquastat to 140 to 150 degrees usually does the trick.

In terms of wiring, it's very simple - the additional L4006B aquastat gets wired directly to the circulator relay. Most of the multi-circ relay boxes I have used include a set of "ZR-ZC" (terminal designation will differ, refer to label inside relay cover) that come from the factory with a jumper installed. Remove the jumper and wire the L4006B directly to those terminals. When the aquastat opens (low primary loop temp), the power to secondary circs will be dropped, but the rest of the relay (and thermostats) will remain energized.

This wiring scheme does not "talk" at all with the operating control on the boiler, so burner function and the primary pump continue to operate normally. In this wiring scheme, a set of dry contacts on the multi-circ relay box (usually designated "X-X") are wired to the T-T terminals of the operating control so that when any zone calls, the burner fires and the primary loop pump runs. Since the additional aquastat on the primary loop is wired to the circ relay using the ZR-ZC terminals, the relay will remain energized and the X-X end switch remains closed on call for heat, maintaining operation of the operating control/burner/primary pump.

I hope this makes sense. It's actually a very simple method requiring only a $75 aquastat and a short piece of BX wire to connect the aquastat to the "ZR-ZC" terminals of the multi-circ relay box.

Abro

Man, I REALLY appreciate this! This is very helpful and very clear and sounds like a simple and cheap way of ensuring boiler protection without additional piping or valving.

If you wouldn't mind, could you suggest a multi-circulator relay box that you think would be suitable for a 4 - 5 zone job?

Once again, I am truly grateful for your help and your time here.

Thanks,

Tim

Jim Erhardt_3

Most of them...

..have the needed terminals and will work fine. Check before you buy to confirm.

Robert_22

Where can I find Dan's article on Primary - Secondary?