How to install system bypass piping

luna12 · March 11

I have a B&G Monoflo hydronic heating system in my house that was installed in the early 1950s. I have cast iron radiators throughout the house. There are two circuits, but only one thermostat, so the system isn't zoned. I plan to install a Weil-Mclain CGa-6 boiler. The installation manual suggests using system bypass piping in order to provide minimum130°F return water to the boiler. The diagram that seems like it would best suit my system is this:

However, the manual states, “Figure17 however, is not suitable for radiant heating applications because it does not protect the radiant system from possible high water temperature.”

I have two questions:

1) How would I create a system bypass for my particular system? A diagram or photo example would be helpful.

2) I have seen many photos and videos of boilers and piping, for example ads for second hand boilers, and I have yet to see one with system bypass piping. None of those installers seemed to find it necessary. So my question is, do I even need to do the system bypass piping?

4GenPlumber · 12:26AM

With monoflo you shouldnt need a bypass. If you zoned the 2 sections it would probably help your situation more. Monoflo is constantly mixing full temp water with water that has given up some temp out of the radiation. If you install an additional tridicator on the return of each leg, and install 3 speed pumps, you would, or should be able to adjust your deltaT and keep you return temp above 130. Faster pump speed would reduce deltaT.

bburd · 1:34AM

The installation note about "radiant heating applications" generally means radiant floors, which often need unusually low supply water temperatures; some floor finishes are subject to damage if they get too hot. It does not mean cast-iron radiators. They could certainly have been more clear in their instructions.

luna12 · 1:49AM

One thing that is frustrating me is the lack of information on installing a bypass. I can't seem to find a single photograph, 3D diagram, or video showing one. A line schematic won't fully cut it for me, as I need some real, visual, tangable examples to copy from. So, I'm left with a bunch of questions like, what specific valve should I use?? Can the bypass piping be smaller than the main pipe in diameter?? Thousands of Weil Mclalin boilers have been installed, and I can't find anything about this bypass… but thanks for the feedback so far. It's helpful.

EBEBRATT-Ed · 2:07AM

@luna12

IMHO you shouldn't need a bypass with the average Monoflow system. If you are concerned about it do the piping install as in the B & G drawing above but leave the bypass tees out. If you do get low return water temp, you can easily add a bypass valve.

Having low return water temp is a progressive disease. Your boiler is not going to blow up or self-destruct with low return water temp. The damage occurs over running a long time with LWT which can cause issues but it takes a long time.

hot_rod · 2:10AM

The bypass pipe or pump is not much more than a made up concept, which us why there is no math or science to support it. It can work under one set of circumstances.

Uf you set ut at cold start then when the boiler us hit, system yo temperature it is byoassing too much. You don’t need any gom bypass uf boiler and system is above condensing return temperature . So who goes down and closes the bypass at that condition?

You may notice some boiler manufacturers now caution that a manual bypass may not adequately protect a boiler from extended cold run cycles.

It all comes down to the mass and water content in the system. A properly sized boiler at design conditions can lift or increase the water flowing through it a few degrees.

So you want a “thermal clutch” A valve or device with a temperature sensing function to disconnect the load from the boiler

Perhaps the simplest device found on many euro cast boilers was an aquastat that turned off the boiler pump when return was too cold. Not an elegant solution as it cycled the burner on/ off frequently until it caught up with the load

A thermostatic mix valve is a simple 100% solution

How to install system bypass piping

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