pressure bypass valves

Mike T., Swampeast MO

Differential Pressure Bypass Valve:

What it does: Diverts flow from a source of higher pressure to a source of lower pressure at a rate dependent upon the difference in the pressure of the two.

How it does it: The moving portion of the valve looks something like the valve out of an internal combusion engine but is oriented so that higher pressure OPENS it instead of tending to CLOSE it in an engine. The valve is held against a seat by an adjustable force spring system.

Fluid is ALWAYS on BOTH sides of this moving valve. Regardless of the actual pressures involved if the pressure on the high side exceeds that on the low side by the amount of force the spring is applying to keep the valve closed, the valve opens and fluid now flows THROUGH the valve. (see illustration)

Where it is used: In forced circulation systems where the amount of fluid circulating continuously varies from high to low (or even zero). Proportional flow devices like TRVs or pneumatic valves do this. You can view TRVs similarly but instead of a FIXED adjustment, the amount of flow varies based on some temperature VARIABLE.

Why it is used: Flow in such a system can be reduced to near nothing (or even nothing). Circulating pumps DO NOT LIKE IT when they are "forced" to put their entire energy into an amount of flow much less than their total ability. Heat/pressure builds up in the pump and its life is either shortened greatly or the device itself is damaged (seals fail and it "explodes").

Setting: Specific instructions should be supplied with the device. Most have a human-readable indication of the current differential pressure across the valve. Setting amounts to determining this amount of "normal" differential pressure under full flow condition with the valve completely closed. You then adjust the valve to begin to open when this amount of pressure is exceeded by some amount (generally quite small).

If the device is installed in a gravity conversion system, the EXTREMELY low restriction in the piping will generally prevent a differential pressure sufficient to get a reading. Even when there is VERY little flow through the system you may STILL not get a reading!

My adjustment method in this case is "by ear" and "by boiler cycle time." If you are using a traditional (non-condensing) boiler you really want SOME bypass at all times. With the system up to temperature and under condition of VERY LITTLE system load (i.e. warm inside WITHOUT added heat) press your ear against the valve and adjust until the turbulence you hear when the valve first opens JUST disappears. Play with it a bit to learn the sound difference.

Now wait until it is VERY cold outside and the system under HIGH load. The outside temperature MUST be steady and the space temperature inside at MAINTENANCE level. Listen to the valve. If you hear no bypass open or close it until you JUST hear the turbulence.

Wait again until the outside temp is quite warm and the system under LOW load (outside temp will vary with your climate, but you want a "typical" near high temp on a "typical" day). Again, the space temp MUST be at MAINTENANCE level and the outside temp quite steady. Time both the burn time and off time of the boiler. You will find it VERY consistent. You can safely "expand" this to "total burn time in one hour" or similar convenient measurement of time. Make TINY adjustments in the valve until you get the longest individual cycle burn time with the least total burn time in an hour. YES, it actually seems to "work" this way. The more oversized the boiler the more sensitive this adjustment. If you make too large an adjustment you can easily "loose" your reference point and you have to start over.

If you think that there might be a chance that occupants (remember kids are naturally curious) will "monkey" with the setting, REMOVE THE ADJUSTMENT DEVICE and store somewhere near.

I've only worked with gravity conversion systems, BUT I do imagine that you could make similar adjustment to more modern systems. If you are using a CONDENSING boiler however, you only want bypass to save the circulator--you don't need to have bypass to keep the return temperature UP when the boiler is most efficient when return is DOWN!

Josh M.

presure by-pass

Can anyone explain presure bypass valves and how to set them?

Mike T., Swampeast MO

Did anyone notice the illustration error...

...in the "open" situation (as labeled and assuming the valve is properly sized)?

If you can see the problem you truly understand what the valve does in operation!

doug_10

Okay, I'll bite...

Mike, is it because you still show the valve actually still being closed? Looks like the one "end" should be at least in the middle of the elbow...

Take Care, PJO

Mark J Strawcutter

temp bypass vs pressure bypass

I think you may be trying to make the differential bypass serve too many functions.

Why not let it provide circulator (pressure) protection and have a second (temperature) bypass with something like an ESBE thermic valve to protect the boiler?

Mark

Mike T., Swampeast MO

Not the quick sketch

The bad numbers.

I showed a differential pressure of 4# with a differential SETTING of 3#.

This DOESN'T happen (but it does make it easier to understand the operation of the valve if you imagine it can do so).

Whatever force the circulator applies trying to achieve a differential higher than the setting is immediately translated into greater bypass flow.

The flow through the bypass is proportional.

I didn't really understand TRVs until I installed a differential pressure bypass valve and realized that it was proportional as well--just in a way that is easier to see.

Label one of the illustrations as follows:

High Pressure = BTUs Available

LoW Pressure = BTUs Delivered

Adjustment = BTUs LOST in the Space

The valve moves automatically doing everthing in its power to make BTUs Delivered = BTUs Lost.

It does this by assuming that if one piece of the system--the sensed element--no matter how small--DOES NOT CHANGE in temperature, nothing else in the space changes in temperature. If that device looses heat more BTUs are immediately delivered; if it gains heat fewer BTUs are immediately delivered.

This feedback is completely independent of the number of BTUs Available. The valve could care less whether the BTUs Available come from velocity or temperature because it puts this into an INDIVIDUAL and essentially INSTANT calculation of BTUs Lost. It will achieve BTUs lost among an infinite number of temperature/pressure combinations.

This is why the setting (or moving) of one TRV has no real effect on the others. One change in one TRV is translated nearly instantly into changes in all of the rest of the TRVs so that they ALL achieve their individual goals regardless of what the others are doing.

Eric Taylor_3

Apples and Oranges

The differential pressure bypass knows nothing about temperature or BTUs or anything else except the differential pressure across its ports. It will continue to do its job as long as the circulator is running even if there is zero heat being delivered to the space. I can envision a positive feedback situation with TRVs though. As TRVs close the flow through the diffpress valve increases and the system temp also increases and the TRVs close even more. Of course I'm assuming that the valve is a system bypass. This is not a bad thing provided the boiler temp is properly controlled and the circulator is properly sized.

Eric

Unknown

One slight change...

the TRVs won't close more, if they are sensing air temperature, as most do. The flow rate in that emitter will go up, and the room will warm, which THEN will modulate the TRV. Same effect, though.

Noel

Mike T., Swampeast MO

That detailed setting talk was SPECIFICALLY about a system with constant circulation, reset, TRVs and a SINGLE circulator. Remember that the only reason the differential pressure bypass is required is because the system is completely proportional and the circulator has NO WAY of knowing the current state of the proportional devices.

IN THAT INSTANCE the differential pressure bypass not only protects the circulator but adjusts the amount of bypass in the EXACT way you want it to happen--more bypass as less heat is delivered. It's just a convenient way that things happen.

What it does NOT do (nor did I in any way imply) is provide a MINIMUM RETURN TEMPERATURE. If the system requires a minimum return temperature you must do something else.

IT DOES HOWEVER, AFFECT THE CYCLING OF THE BOILER AT ALL TIMES!!!

My gas meter makes a loud pinging noise--I can hear it in most of the house and at least two doors away when outside. I hear it almost constantly--so often that small changes are IMMEDIATELY apparent to me. I know for instance if the hot water heater, boiler or combination is running. I know if someone has turned up a TRV in the house.

REGARDLESS of outside OR inside MAINTENANCE temperature I hear the SAME NUMBER (within one or two) of "pings" with EACH CYCLE OF THE BOILER. I CAN THOUGH change the number of "pings" in two ways:

1) Turn up a TRV significantly. This only affects burn time TEMPORARILY.

2) Change the setting of the differential pressure bypass valve. This changes burn time (number of pings) at ALL states of maintenance. This adjustment is EXTREMELY sensitive and the desired effect only happens in a NARROW range. Go outside that range and "up/more" and "down/less" loose their simple correlation.

I SERIOUSLY doubt that the differential pressure bypass setting optimized efficiency at ALL states of maintenance--just around the typical high temperature of a typical winter day. This was the best I could do with an oversized, standing-pilot, low water content, cast iron boiler.

A very well-balanced system using a boiler that modulates both heat output and circulation force wouldn't even require differential pressure bypass because it would go into warm weather shutdown at the instant bypass was needed...

Do you have to use a differential pressure bypass valve on a Vitodens when its built-in circulator is the only one in the system? Or is one built in? Or did the Viessmann engineers simply make the unit shut down by assuming that if the boiler is trying to "hurt itself" it is no longer needed by the structure?

Mark J Strawcutter

I inferred

>What it does NOT do (nor did I in any way imply) is
>provide a MINIMUM RETURN TEMPERATURE. If the system
>requires a minimum return temperature you must do
>something else.

Oops, sorry Mike. I (incorrectly) inferred such from the last paragraph of your original post.

Mark

Mike T., Swampeast MO

It WILL raise it somewhat, just not to a pre-defined minimum.

Mike T., Swampeast MO

I see what you mean

I was just imagining air as another object in the space.

If you consider how the air sensing element is intentionally shielded from radiation, conduction and drafts from OUTSIDE the space, I actually think the valve is trying to do this as well... TRVs don't achieve absolutely perfect control, just quite close...

hot_rod

Directions come with the valves

if you are talking about the basic PAB (pressure activated bypass.) The Oventrop brand have you adjust according to thee pump model.

The Honeywell Braukman have setting instructions, and the Danfoss brands I have also give you setting instructions. It's fairly simple, once you read and understand the concept.

The B&G website generally has excellent explaination and application info also.


hot rod