Reasons for Using Both Return Lines and Bypass Line

The reason for returning water to both sides of the boiler, the balancing valves and the bypass loop, are because there are 41 gallons of water in the 520 boiler. Returning to only one side will create zones of varying temps inside the boiler.

If the single return line is on the same side as the aquastat, the cold water is hitting the aquastat probe and shutting off the circulator, causing short cycling, while the other side of the boiler remains relatively warm. If the single return line is piped to the other side of the boiler, the aquastat doesn't sense the drop in water temperature, so the burner doesn't come on soon enough and the domestic water coil temperature is affected. In both cases, a single return line causes a short circuit through one side of the boiler.

The bypass line, in conjunction with the bypass valves will force some of the cooler return water back into the heating loops, thus moderating the heating zone water and helping to avoid the shock of cold return water flooding the boiler. The valves have to be adjusted through experimentation so that the circulator doesn't short cycle, the domestic coil isn't overly affected by the cool water, and so the boiler can sense the need to produce more heat.

In idle periods, the bypass loop will also help to avoid stratification of the internal boiler water temperatures by using gravity to naturally circulate the water inside the boiler.

Shown is one way of piping it.

Good explanation,that should clear up a lot of questions.

RY

Thanks Stoker-man. This answers many of the questions I have with the bypass that was installed on my boiler.

One more question to help me understand fully.

Figure 7 “Force Circulating Hot Water Piping Layout” in the EFM 520 manual shows the diagram using both returns, the bypass pipe, and two balancing valves. Currently my return is only going to the bottom left with my aqua stat on the top right. I have a 1 ¼” bypass line, but I do not have balancing valves at this time. I am planning on having my plumber change it to use both returns and install balancing valves.

It’s my understanding the balancing valve installed after the bypass pipe and before the return input to the boiler can be “closed a bit” causing more of the cooler return water to flow up the bypass pipe and back into the system, preventing “shock” to boiler and causing short run time of the circulator. This should allow the boiler to maintain its temperature for a longer duration and in turn make the circulator run longer to distribute heat throughout the home more evenly. Please let me know if I’m correct in this thinking.

My question is why the balancing valve is used in the bypass line? It seems to me that you would want this valve open all the time. If the bypass valve is closed down (even just a small amount) wouldn’t that make more water flow into the boiler causing “shock”, and during periods that the circulator is not running slow down the gravity flow thru the boiler which could cause varying temperatures in the boiler? What is the benefit to “slowing” the flow in the bypass line? I’m having a mental block on this one!
Thanks,
IceDog

It’s my understanding the balancing valve installed after the bypass pipe and before the return input to the boiler can be “closed a bit” causing more of the cooler return water to flow up the bypass pipe and back into the system, preventing “shock” to boiler and causing short run time of the circulator. This should allow the boiler to maintain its temperature for a longer duration and in turn make the circulator run longer to distribute heat throughout the home more evenly. Please let me know if I’m correct in this thinking.

Yes, that is correct.

Your system would dictate how you adjust the valves. If you normally had alot of cold water returning to the boiler because you lowered the thermostat at night, you might want to shunt more of it into the feed line, but if the temperature of the return water was fairly consistent, you might want to feed most of it back to the boiler. So the option is: How do you want to adjust the valves? One may be open all the way and the other restricted or any angle in between for both of them.

If Big Jim wasn't chasing deer, I could further elaborate.

Thanks again for the information.

Does anyone know why plumbers install bypass lines without bypass valves? My coal and oil boilers are both done with a 1 1/4" line that run from the output of the boiler back to the return input (without valves). I saw other boilers done this way and I believe a member of this forum (bighouse) indicated his was done this way and his plumber called it an "equalization loop". I think this is what prompted Stoker-man to post this thread to help clear things up.

The odd thing for me is that I never noticed a problem with this when running my oil boiler. The return water would be the same volume and temperature returning to the oil boiler as it now returns to the coal boiler. The only thing I can think of is that the oil boiler is able to heat the water much faster than the coal boiler does and therefore I didn't notice the "short run time" of the circulator.

When a bypass line is installed in this manner without valves just how does the water flow thru the line? When the circulator is NOT running does the hot water at the top of the boiler flow "down" the bypass line and back into the return? Then what happens when the circulator IS running? Does any of the water flow from the return line "up the bypass" and back into the system, or does the hot water from the output of the boiler flow "down the bypass" and mix with the cooler return water which makes that warmer going into the boiler?

The old saying is, "Gravity Sucks". I guess this is true in physics and when you're trying to understand it!!! I guess the bright side is it's wonderful if the power goes out and you're counting on gravity to distribute the heat.

Stoker-Man,

I wanted to take the time to thank you for all the information you provide on this forum and on this topic in general.

I explained my issues to my plumber, and he was very helpful in addressing this issue. I was fearful I would get a "run a round" and excuses, but instead he was at my house 3 days after I called him and put valves in the bypass line and connected the 2nd input for the return.

I am now able to bypass water and keep the circulator running to distribute the heat more evenly and not shock the boiler.

Thanks again.

IceDog

If you can remember, in a few weeks, let us know how the entire system is working.

stoker-man wrote:The reason for returning water to both sides of the boiler, the balancing valves and the bypass loop, are because there are 41 gallons of water in the 520 boiler. Returning to only one side will create zones of varying temps inside the boiler.

The twin return ports should be plumbed from the boiler with a nipple and then a TEE with the branch facing down and a pipe plug at the outside port. Plumb the two branches together from there to a central point and the return from the house. This allows you to remove the plugs and rod out the sediment that accumulates over time. Most systems will not have a big issue with this, but some, especially with well water may. Over time the sediment will, if it gets high enough, damage the steel itself due to the intense heat with no water behind it to remove the heat.

Here's a shot of what coaledsweat is talking about.

It's been a few weeks now since I had the bypass valves installed and "both" returns connected. After some trial and error with the valves I was able to "dial" it in and everything is working fine. Thanks for all the help.

cArNaGe wrote:Here's a shot of what coaledsweat is talking about.

DSC03867 (Large).JPG

That's correct. Except the TEEs should be branch up to the return. This allows you to uncap the TEE and go in and rake or "rod" out the sediment in the boiler's belly.