Pressure Differential Bypass still useful

SpeyFitter

So I was helping a buddy who owns his own company recently with some design work on the hydronics side as well as specing the boiler, equipment, etc., for a house he's doing. The homeowner/his client in question has a small downstairs radiant demand (about 7 loops, of which these loops are designed into 2 seperate zones, off of 1 manifold - the rest of the house is heated with a furnace/heat pump combo as it's basically a major reno plus he wanted cooling upstairs), plus a 75 gallon Indirect, and a potential future hot tub load down the road (aka S/S heat exchanger off of the boiler, etc.). The boiler I specified was the new IBC SL20-115 (20,000 to 115,000) which has a fire tube heat exchanger much like the Triangle Tube and Lochinvar Fire tube Knights. 

Anyways, I hummed and hawwed back and forth and the biggest issue in my mind was trying to maintain minimum flow rates through the boiler when only one of the zones was open (aka 3 loops). The boiler specs suggest 2 to 14 GPM as the ideal flow range with anything lower than 2 GPM's causing the boiler to shut down due to lack of flow.

Up until the more widespread "push" of fire tube heat exchangers, primary secondary has reigned supreme for various reasons of which many of you already know. But I thought about it and this being the first fire tube style mod-con I've spec'd/dealt with I decided instead to go to a 2 pump parallel pump system with one pump for the radiant, and one pump for the indirect, with a differential bypass on the radiant loop.

My rationale with the differential bypass (other than bypassing water to avoid pushing too much water through the radiant loops when only 1 zone is calling for heat) was it would allow some return flow into the boiler to keep minimum flow rates, and it also saved a pump on top of numerous fittings. This allows me to take advantage of the lower head loss of the fire tube exchanger, plus less the electricity usage of one pump, in working with the small radiant load.  In comparison to a primary/secondary system, there would still be some return flow going to the boiler inlet anyways, as not all of the water from the boiler would be diverted into the radiant secondary loop during lower demands. So in a sense it should work out to be similar. The dymamics, however, of the differing radiant loop lengths versus the perssure differential bypass setting could be somewhat interesting, but probably not too much unlike one of those Delta P pumps with a constant head setting.

If all goes well, the boiler should be getting fired up by early next week and we'll see how things work.

meplumber

Hydraulic Separation is the answer.

By piping the primary/secondary through a hydraulic separator, then you are separating the boiler flow needs from the system flow needs.



http://flopro.taco-hvac.com/products/Hydronic%20Accessories/index.html?category=377



Take a look at this.  It is a very simple but effective form of hydraulic separation.



Also agree that the Delta P circ on the system side is the answer. 

The Steam Whisperer (Formerly Boilerpro)

Minimum flows

Look carefully at those specs.  I bet the 2 gpm mimium flow is at full fire.  The TT units will modulate down as the delta tee through the boiler starts reaching the maximum (60F IIRC).  At minimum input of about 35,000, the minimum flow rate drops to a little over 1 gpm with a 60F delta tee.   What you may need to watch out for in this case is very uneven heating of the floor.  Radiant floor tends to be  inhernetly power hungry for pumping due to the need to keep temp drops reasonable to keep floor heating even, unless you design the tubing layout accordingly.

SpeyFitter

Hydraulic Seperation

Correct me if I"m wrong, but the entire point of hydraulic seperation is to reduce or minimize the potential of "ghost flows" from your primary "boiler" pump moving heated boiler water into secondary loops that do not need heat. There are various strategies that have come avail to do this such as low loss headers/hydraulic seperators, or primary-secondary pumping strategies with various rules for seperation between the secondary supply/return tees and relationships to other supply/return tees, as well as check valves, flow check valves, etc, etc. etc.

But then this goes back to the fact that primary-secondary pumping did not become THAT popular, or critical until the advent of high head loss modulating condensing boilers where it was prohibitive for various reasons to use one pump to push water into a bunch of on or off radiant zones. The head loss of the often water tubed heat exchanger, combined with the head loss radiant loops, on top of the changing environment that a radiant system requires (opening and closing zone valves/actuators requiring various flow rates), along with the fact that these boilers often require minimum flows means that a one pump parallel system for a decent sized radiant system would potentially be quite expensive in one pump. So a 2 pump system was the answer to keep minimum flow rates through the boiler, as well as keep pump costs down (2 smaller pumps were cheaper than the 1 much larger pump required on top of the factors mentioned above).

But the entire point of my thread was that the mod-con industry has some big players that seem to be moving over to fire tube style heat exchangers. Triangle Tube has been around for a long time, now Lochinvar, and IBC, are moving in that direction. Lochinvar sells A LOT of condensing boilers - their Knight has been extremely successful, and IBC doesn't have the volume, yet, but they do have a cult like following. Many also praise the Triangle Tube as well.

The commercial sector for floor standing mod-cons is also moving the way of fire tubes - Cleaver Brooks Clearfire is a fine example of a top of the line fire tube (they are built very well), as is the new Lochinvar Crest.

So the big question amongst contractors is with the low head loss heat exchanger of the fire tube, can we drop down to one pump now? There seems to be a lot of confusion or uncertainty in this question. My understanding is on bigger systems the answer is still closer to no, but on smaller systems the answer may be yes.

As for flow rates - 2 GPM is the limiting factor for this boiler on whether I can drop down to one pump or not. I will have to contact the factory to understand their rationale, but judging from the boiler internal layout, it appears to me as though the flow rate would be determined by temperature difference between the top of the heat exchanger and the bottom, not an internal flow or pressure switch or differential measuring sensor.