Pumping away with two boiler set up

Mosherd1

Does this piping arrangement look correct for a two boiler set up. Existing electric boiler to remain for redundancy for when the new LP modcon has an issue and parts aren't readily available. The second pump in the supply side is merely a back-up pump for when the main pump fails. Installation is in the midwest--propane is significantly cheaper here than electric comparing BTU to BTU, and dont even suggest oil in these parts--its all but dead and gone around here.

STEVEusaPA

I’d move my boiler circs over to the left pipe and pump into the boiler instead of pumping into the closely spaced tees.

EBEBRATT-Ed

In addition to what Steve said (it is usually preferred to pump against the boiler resistance) It depends. If you're not going to run both boilers at the same time your drawing is ok except for moving the circs.

If your going to run the boiler together then see the attached drawing. If running them together you want the boilers to get the same return water temp.

Mosherd1

@EBEBRATT-Ed only one boiler will run at a time. The LP ModCon will do all the heating, unless it malfunctions and they need heat. The electric boiler is exisiting and is just going to stay since they already own it. The control power will be interlocked with a toggle switch so that they have to manually flip the switch to change from gas boiler to electric boiler.

hot_rod

Can you assure enough flow to pump the mod con direct? Skip the P/S? Then parallel the electric?
Put some flow rate numbers on the two zones and see how it looks

Mosherd1

@Hot_water_fan I feel more comfortable doing the ModCon P/S, and had considered doing the electric boiler in line rather than putting it in its own loop. I could put some valves around it and make a bypass in case it starts to leak or needs to be removed, but I personally like to know I can isolate it completely from the system for trouble shooting. They already have two old (~ 5 yo pumps). These pumps will become the pumps for the electric boiler and the spare system circulator, with a new pump for the Mod/Con and a new ECM pump to feed the new zone valves. The current system has two pumps on the zone return lines. Updating to 1 ECM pump and 2 Taco I Series zone valves on the baseboard side of the P/S.

Mosherd1

@hot_rod if I stick with the two boilers piped P/S as shown, are all the key components in the right spots? Dirt mag, expansion tank, fill valve, air eliminator, service/purge/drain valves?

hot_rod

The ps piping may cost some mod con efficiency as, depending on flow rates, you may be blending return up and lose some condensing.

With fin tube, what is the highest temperature that you will need to run?

Nice about fin tube or panels, you can sometimes spread the delta wider, maybe 30, the lower return and maximize the condensing operation.

If p/s is the final answer, a Sep 4 type of device sorts out air, dirt, mag, hydraulic, and the ponoc question. In one device🤔

Mosherd1

@hot_rod unfortunately the efficiency is going to take a hit either way. The house pretty much needs 180* water at design temp. I know this because I’ve worked on the electric boiler many times and when they loose 1 of the 5 elements it isn’t enough to keep the house at temp. I’m hoping that with a decent reset curve we can keep it condensing as long as possible. The house has no chimney, and no good place to make a chimney chase, so we’re going the mid/con route for easier venting, and I know at the hi output temps we can’t use regular pvc to vent. I’ll look into the sep 4, I know that will change the piping layout a little bit. 

hot_rod

Something along these lines. This shows zone pumps,it could be a single pump and zone valves on the secondary

offdutytech

As others said primary pump into the boiler. Typically you see zone valves on return from system. If you want secondary pumping redundancy maybe consider a lead / lag control. I/O controls makes an inexpensive one. This will rotate the secondary pumps to equalize run time. If the lead pump fails, the lag pump starts. Just make sure the pumps have check valves on them.