Boiler Protection: Automated Bypass Valve Needed?

bdubya · August 2022

Q: Should I install an automated bypass valve that monitors return water to the boiler? If so, what are my options i.e. 2 way or 3 way valve or another pump in the bypass?

I have an 82% efficiency cast iron SE140 slant-fin boiler piped for two zones with each zone having its own circulator piped on the return of boiler. My boiler is controlled by a Hydrolevel 3200 Plus with outdoor reset. The zones are controlled with a Taco zone control unit. This aquastat stops the circulators if the return water in the boiler drops to condensing temps and restarts circulators after the water temps rise. While this is better than no protection at all, I am realizing that this type of boiler protection starts and stops the circulators way too often during cold starts which takes forever for the radiators to start warming the home. I have a tee installed on the supply side leading to the return side with a manual valve on it but have never used it to manually throttle the bypass flow during cold starts. Is it better to have an automated bypass monitoring return water temps and tempering bypass valve to open and modulate to keep return water above condensing temp or is the Hydrolevel enough protection and my worries are needless? See pics - sorry if they seem sloppy but wanted to help you make sense out of the piping rat's nest. Thanks in advance.

mattmia2 · August 2022

The bypass should be partially open, that would keep the circulators from short cycling. A thermostatic valve would be better but the aquastat and fixed bypass are acceptable.

bdubya · August 2022

Thought I should add that when I do open the manual bypass valve there is flow going from supply into return but I have no idea what the mix temperature is going into the boiler. Also should add that I am adding CI baseboard (replacing fin tube) in zone #2 (have upright column CI in zone #1) and I'd like to take advantage of the ODR function more and let the boiler fire down to the minimum return temperature without condensating to allow a lower CI temperature in shoulder seasons but protect the boiler at the same time. I feel that the Hydrolevel doesn't really do a god job of that and I worry that its circulator hold function doesn't protect the boiler enough. Just a worry I have - no proof it isn't protecting as that proof usually shows up when it's too late.

mattmia2 · August 2022

You would need to make significant changes to the near boiler piping to do outdoor reset with a cast iron boiler, you would need somewhat separate systems to protect the boiler return water temp and to mix the system water based on an outdoor reset control. Primary-secondary piping with an electronic mixing valve that controls the heating water temp is the first thing that comes to mind but there are other ways to do it too especially with a CI boiler that doesn't have minimum flow requirements.

EBEBRATT-Ed · August 2022

The only way to protect the boiler for sure is to have a sensor on the boiler return pipe. There are many was to pipe this. See the attached. You can download the whole series of Caleffi manuals there is about 25 of them

hot_rod · August 2022

A couple of determining factors for adding or upgrading return protection methods.

1 if the boiler can see a large slug of cold, room temperature water. Old gravity conversion systems, radiant concrete slabs as examples, lots of high water content cast radiators.

2 if the boiler has to run more than 10 minutes or so to get the return above 130F. Same issue caused by high mass emitters or boiler sized close or below design load condition maybe.

I suppose you could group the various methods in to good better best.

The bypass pipe and valve or a bypass pump are certainly better than nothing. They get set at some arbitrary point and protect at that exact condition. So they may be over or under mixing most of the time. That could slow heat delivery to the radiators if they are blending down SWT.

An aqua stat that holds or turns the circulator off as return temperature drops. Now you have added some intelligence into the decision making, a good thing. Depending on loads, flow, boiler output they can bang on and off every few minutes under a big cold return load, kind of annoying and not ideal for the circulator. It needs to be on a boiler that can safely operate without flow, cast iron, not for copper tube type boilers

You still see it used on some of the price-point Euro cast boilers.

Very best is a control, valve, or device that watches return and smoothly modulates the mix of supply and return. Think of it a slipping the clutch in your VW bug to start with a heavy load going uphill.

The very simplest is a high Cv thermostatic mixing valve. Use one intended for that application. Most off the shelf 3 way thermostatic have low Cv for DHW mixing use.

A variable speed circulator properly applied is another clean method.

If you need to mix down temperature for loads a 4 way motorized can do both mixing and return protection with an actuator and control, tekmar, HBX, Taco I-valve for example.

If you can observe the boiler from a cold start, maybe your first heat call this fall. That will be one typical condition. The other is first start on a design day where it is trying to catch a design load, and get its return up.

Or coming out of a frozen home condition

STEVEusaPA · August 2022

Looks like you bypass is only for the one zone. Best would be a proper repipe, with proper supply/return manifold piping on the wall, with a 3 way bypass that @hot_rod mentioned. Then when it's time for a new boiler it's an easy swap out.
Your not-to-code flue pipe on the water heater scares me.

EBEBRATT-Ed · August 2022

If it was me, I would use a 3-way mixing valve driven by a Johnson 350 0-10 volt output but not cheap

bdubya · December 2022

Everybody, happy holidays and please accept my apologies for dropping out for a few months. I sincerely appreciate that you took the time to address my thread. I've had a renovation from hell going on that is finally wrapping up and with work travel, I've had no time to put into the recommendations - but I did take some time to interview some local HVAC companies to look at my system and advise on their solution/s. I guess according to each, I should spend thousands to repipe my entire home to smaller piping than 1.25" and install new pumps which would slow the flows to the liking of these newer efficient boilers with smaller tanks. While I didn't disagree with their assessment, I find that repiping as their only solution to be short-sighted and non-achievable with the fact that we don't intend to stay in the home much longer than 5 more years.

Addressing some of the responses to my thread:

Mattmia2: Your 2nd post had me thinking about removing the outdoor reset wires to allow economy feature to take precedence with thermal targeting. Would this be recommended? Would this stop the 3200 Plus from allowing the return temps to drop to 114F before de-energizing circulators? Their manual does not describe the sequence in such a way.

Hot_Rod: We have the 3200 Plus that does what you explained but see below for the issue it creates. Basically, it's what you described. Also, we see the below condition play out - especially as the temps are at design the last few days. I am condensing no doubt because of the 3200 Plus control sequencing allowing the temps to drop to 114F before de-energizing the circulator/s. This condition is almost constant and the fully open at all times 3/4" bypass helps to slow the temp drop by a few minutes but doesn't stop it from happening. I'm guessing that the recommended 3/4" bypass is insufficient for this application. Would the higher CV automated bypass (perhaps 1.25" line size) fix this condition or would a different control than 3200 plus be better option or both? Seems that Hydrolevel's technology is not designed for programmability for all applications/conditions and is limited to a "canned" circulator cut-out programming.

EBEBRATT-Ed: if a fully open 3/4" bypass (recommended to be half supply side diameter) will not keep the 3200 plus from allowing the temps to drop to 114F - which seems to be the issue, do you believe that an automated 3 way mixing valve and additional control investment would solve the issue? I guess the same question posed to Hot_Rod above.

STEVEusaPA: The way the 2nd zone return is piped into the manifold next to zone 1, even if zone 1 is not calling for heat, the bypass flows hot water with just zone 2 calling for heat. I have tested the temps on this pipe and the bypass pipe and return gets to same temperature with either zone calling for heat. It just doesn't stop the low return condition from playing out.

This winter we're seeing the same old issue play out with the boiler, the Hydrolevel 3200 Plus is allowing the return temps to drop to 114F before de-energizing the circulators, the temps raise up to 130F and the circulators are energized again and within a minute or two the temps are back down to 114F. Seems that the 3200 Plus is keeping the temps too low - regardless of what my system is doing. Take away the fact that my system is a gravity conversion (before I bought the house), why would the 3200 Plus allow the temps to drop to condensing conditions - even if they believe it's a "temporary condition" - which in my case it's not?

The sequence happens like this: The bypass valve is wide open to allow supply water to be mixed into the return. Either or both zones call for heat, the boiler (Slant Fin Sentinel CI 120,000 btu) fires up, the water temperature gets to 130F, one or both circulators (depending on zone demand) get energized until the water temperature drops to 114F then the 3200 Plus de-energizes the circulator/s. Doesn't matter if one or both circulators are energized, the above-described condition plays out and the temps only stay above condensing if the demand is long enough for the boiler to stay fired for longer than 30 minutes. Is the 3200 Plus a bad choice for this application? Is there a way to program the circulator to energize and de-energize temps? Should the OS100 outdoor reset be disconnected to allow economy and thermal targeting to play out? Nothing in the 3200 Plus manual describes situations like this and how to address it with programming.

I'd be willing to invest in the larger automated mixing bypass if that will fix this issue. Repiping my entire system is not an option though as the investment is just too great for the time left we have in this home.

Thanks for your patience and your time in assisting with my issue. Merry Christmas everyone! I hope your 2022 was blessed.

bdubya · December 2022

Forgot to include the fact that the two zones are getting satisfied but not without the issues described.

HomerJSmith · December 2022

I would put a bypass if there is significant operation with the return water under 130 deg or in your case a start and stop cycling till the boiler get up to temp.

I would use a mixing valve with a sensor. I like and use Taco I-series. Your situation is a little different. You have two pumps that can be operated independently. You're pumping into the boiler rather than away from it. So, I think you would need a three way valve with sensor.

Boiler Protection: Automated Bypass Valve Needed?

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