Installing a HE gas boiler on a 1.25"monoflo system

Fungohitter

I logged on to the HH forum a couple of months ago with the discussion titled: "68 year old Monoflow distribution: Keep or replace?" Well, the monoflo is staying but my son and his wife have made the decision NOT to make extensive chimney repairs or make upgrades/adjustments to the existing 35 yr. old oil fired boiler (I certainly understand the reasons to stay with it, but it's no longer a question.) Instead they've chosen to install a HE natural gas condensing boiler. One question... your ideas will be welcome. Besides removing any residual sludge in the return flow... What concerns are presented in the marriage of a HE natural gas system with a 1.25" (main) 1/2" (branch) monoflo distribution system with 1950s cast iron radiatiors? (No TRVs will be considered.) Thanks!

EzzyT

Definitely install a dirt/magnetic separators along with Demineralized water and system inhibitors.

Jamie Hall

Yup. Just clean in up well before hooking the new boiler in.

Robert_25

I would clean the system before removing the oil boiler. Might as well let anything that breaks loose settle in the old boiler.

Check out this old thread for info on sizing a circulator for a monoflow system: https://forum.heatinghelp.com/discussion/164843/circulator-and-1-pipe-monoflo-tee-system

mattmia2

I was going to say that you need to be careful about having enough flow in the monoflow loop for the diverter tees to work properly. Primary-secondary piping would be a great idea.

Fungohitter

mattmia2 said:

I was going to say that you need to be careful about having enough flow in the monoflow loop for the diverter tees to work properly. Primary-secondary piping would be a great idea.

I've been trying to read up and wrap my head around primary-secondary piping. It hasn't sunk in yet. This is a single loop main with one basement, five 1st floor, and two 2nd floor radiator on the branches. Would the secondary piping involve a return loop, or something altogether different? The main loop is simple... one lap around the basement, with all diverter and return T's accessible. With that in mind, would a return loop be worthwhile?

flat_twin

Your existing piping and radiators are the secondary piping loop and will have their own circulator. Your boiler will have it's own piping loop and a dedicated circulator within that loop (primary loop). The primary and secondary loops are joined in a way that neither influences the flow of the other. They are hydraulically separate. The pumps in each loop can operate at different speeds. This way the boiler always has the flow required to keep the heat exchanger happy and at the same time your radiators and mono flow system can get the ideal flow they need. The two loops join and exchange water through closely spaced tees or a manifold. The manifold can also be piped with a sediment / magnetic collector.

Fungohitter

flat_twin said:

Your existing piping and radiators are the secondary piping loop and will have their own circulator. Your boiler will have it's own piping loop and a dedicated circulator within that loop (primary loop). The primary and secondary loops are joined in a way that neither influences the flow of the other. They are hydraulically separate. The pumps in each loop can operate at different speeds. This way the boiler always has the flow required to keep the heat exchanger happy and at the same time your radiators and mono flow system can get the ideal flow they need. The two loops join and exchange water through closely spaced tees or a manifold. The manifold can also be piped with a sediment / magnetic collector.

Thanks for the explanation. What I take from that is to make sure the installer has the knowledge and background to identify the appropriate pump to handle the secondary loop, and to join the primary and secondary loops properly in order to keep them balanced, achieving the desired heat distribution.