Critique requested for boiler repiping plan

Tim Melley_2

Tim Melley_2

Repiping Boiler to Add Radiant Loops to Gravity System

I’d love to get any comments or critiques of my current plan to repipe a gravity system in preparation for adding 3 small loops of radiant flooring (2 full bath floors and a kitchen floor).

I have 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.

My thought is to repipe as primary-secondary. I hope single pipe primary will be adequate. I plan to pipe the old cast-iron system as the first zone off the primary loop (i.e. on the hot side) This zone serves most of the house. After the cold return from this zone, I would place the staple up radiant zones and then finally the slab (kitchen) zone, each of which will be tempered with 3 way mixing and piped with ½’ pex. Here’s a visual schematic of the plan as I currently see it. (Note: I could add TRVs to all radiators but won’t get to it this year.)

Can you all tear apart this plan to point out weaknesses or oversights?

In addition, I’d appreciate any advice in response to the following questions:

1. Do I want flow check valves on both sides of all 4 zones, including the old gravity designed zone? Or should I build a thermal heat trap for the old gravity zone (Zone #1 in the diagram) and run the Zone #1 pump continuously?

2. Advice on selecting circulators? Using Steamhead’s circulator sizing charts (thank you VERY much!), I calculate I need a maximum of 15.9 GPM on the old gravity zone. If I stick with Grundfos, it seems I’m looking at a UPS15-58FC—which gets me flow check, three speeds and up to 17 GPM for the same price as my existing UP 15-42F. Is that the right choice? My radiant loops need less than 1 gpm at design temp. (and run 200-250’ of ½” pex each). Do I choose 15-58s for them, too? What about my primary loop? Can I re-use my 15-42 for that or for one of my radiant loops?

3. If I wish to incorporate outdoor reset, can anyone recommend appropriate equipment? We currently use a programmable T-stat resetting nighttime temps down to 60 deg. Is outdoor reset going to cause problems with an 8 degree setback?

4. I worry about short cycling and residual heat loss. Any ideas to prevent it/maximize efficiency?

5. Any comments or advice on controls? Do I want a two stage thermostat as I’ve seen sometimes described in these pages? Any other controls?


Finally, let me say in advance how incredible this site is. Obviously, I don’t know much, but for me I’ve been able to learn a whole lot here by studying this site.