Boiler bypass valve

Bridgestone

I have a boiler with with 3 zones and 3 thermostats. Attached is an image of what I believe is a manual bypass valve, half closed. What is the purpose of the valve, should it be open fully, how does it influence water temperature when the boiler is operating. I'm a bit new at this so thanks in advance for any guidence.

offdutytech

That's typically installed as a means of protecting the cast iron boiler from condensing and keeping return water temps above 140 Deg. The valve is usually adjusted and left in a position to blend supply water into the return of the boiler. It's not a perfect way to do it, but is a can work. When I find them on a customers boiler I will usually take the handle off the valve and zip tie it to the pipe. That way nobody accidentally closes it. Newer cast iron boilers come with a control module that can be programmed to delay the pump on to accomplish the same thing. 

The Steam Whisperer

Besides the above purpose, it also will tend to help greatly to protect the boiler from cold shock. If an off zone comes on shortly after another zone shuts off, the boiler is very hot and the off zone water is probably at 70F or cooler. This slug of cold water can cause a rapid shrinking of the castings which can lead to cracks in the boiler castings. Mixing the hot supply water with the cold return water will help bring it up to temperature before it gets to the boiler.

Bridgestone

Looking at the image, would it be correct that the water flows right to left, thus from the supply line to the return line? Does the level of mixing which helps bring water up to temperature before it gets to the boiler have much impact on how fast radiators will start heating? Might it increase the chance of boiler short cycling when 1of the smaller zones is operating alone?

hot_rod

The biggest issue with a manual bypass is it is set for one specific condition. Cold start it may work fine to blend up return.
Once the boiler catches up, return warms above 130F, the bypass really should be closed. If not you are needlessly blending still. You could get a temperature differential gauge, one sensor on supply one on return and watch what takes place.

That is why a valve that can know and respond to return is going to be the most accurate, keep the correct amount of flow going to the system as the boiler warms. A valve or pump with a temperature response function.

The manual bypass, or bypass pump is a small bit better than nothing. But always a guesstimate.

Teemok

When I find them and they have been closed, I set them and take the handle off. They are an old compromised way of doing the needful. They reduce boiler output flow and thus total capacity but most boilers are oversized enough that it doesn't matter. Thermostatic valves or motorized 3 and 4 way mixing valves are better but costly. Zoning configurations and load types can create more of a need for boiler flow and temperature protection. I've seen many single zone, cast iron, high mass systems with no protection at all last 40-50 years.

hot_rod

Teemok said:

When I find them and they have been closed, I set them and take the handle off. They are an old compromised way of doing the needful. They reduce boiler output flow and thus total capacity but most boilers are oversized enough that it doesn't matter. Thermostatic valves or motorized 3 and 4 way mixing valves are better but costly. Zoning configurations and load types can create more of a need for boiler flow and temperature protection. I've seen many single zone, cast iron, high mass systems with no protection at all last 40-50 years.

What is your procedure for setting them? On a design day, cold start up, boiler running 10 minute or longer cycles?


I'd wonder for every boiler that has worked fine without protection, you could find one or more that corroded away, cracked, sprung a leak prematurely, rotted out the flue or diverter hood.

I don't know of any boiler manufacture or rep that does site visits to find that cause of every failed cast boilers? Boiler manufacturers warranty data would shed some light. Doubtful any would share that

Teemok

I see them mostly on copper fin-tube Laars with one pump. So it's a minimum flow thing with all trvs closed or the smallest zone calling. No pre-kettling sounds but restricted enough to get good system flow. Cast iron warm up is a different game. Design days? We get one or two of those a year. I quote min. return temperature scripture to the owners of the 1959-60's boilers to no result. It's CA and the ground isn't that cold and nor is the air. These homes have lots of glass and heat loss. Eichlers homes if you want to check them out. Un-insulated single zone copper tubing slabs on grade. I maintained a primary only first gen Muchkin that's still burning in one. Long warm up firings and then regular blips of warm slab maintenance and a relatively short heating seasons. Replaced single wall flues to 3 ft of asbestos through the roof and out. Bonnets, bases and burner seem to last but they might have been replaced. Easy living, that's my theory on their longevity. The old coiled copper r2d2 50's enameled robots and cast irons are mostly gone but every once in a while you find one still going. Owned by the elderly child of the original owner who was taught how to maintain it. I still haven't turned on the heat in my house just 40 min. north.

hot_rod

When the copper tube was the go to for radiant, I recall some Raypak had a cross over tube with a ball valve to adjust. It was a fairly small tube, maybe 1/2”. So that eliminated the worry of someone having it wide open. That seemed like a compromise solution to a full pipe size bypass.

Then again many copper tube salesmen told us to direct pipe to low temperature radiant, the boiler could handle cold return with no problem. That didn’t always prove to be the case.

Teemok

Yup, Raypaks too. Just absolute min flow necessary to prevent kettling noise before an off cycle. CI wet legs take just a bit longer to heat up than the water and tubes. The one zone thing helps all boiler types in these Eichler like applications. There's no cold shock like when the boiler is already hot and a 2nd very cold high mass zone comes on line.

HVACNUT

Looks like a CGi.