Primary Secondary w bull headed tee

[cdxx139]

I have an installed Primary Secondary Hot Water system. Attached is a diagram. Upper portion shows how the boiler manufacturer recommends the piping be installed, and the Lower portion shows how it is actually installed.

The complaint is the boilers are not recovering in time on morning warmup. I feel the issue may be boiler controls, however the client is convinced that the primary loop is not transfering water flow with the secondary loop through the decoupler. The controls contractor sold them that the reason this is happening is the piping configuration. The decoupler is hitting a bull headed tee, instead of going straight thru as shown on the manufacturer's recommended piping. He states the bull headed tee is causing unnecesary pressure drop (the decoupler is technically supposed to have no/low pressure drop to work).

I cant understand how the bull headed tee can cause this. Does any one feel it is the piping configuration that could be the issue?

Thanks

knowledge is power

 

[317069]

when you have a flow rate flowing through a run of a Tee, it will cause pressure drop less than if the sam amount goes through the branch of the same Tee
also, in your sketch, it looke likes the water circuits(supply and return) in primary loops of both the manufucturer and actual installation are not equal, you might have to contact manufucturer and discuse with him.

 

[cry22]

I see more b"bull-heading" on the proposed design than on the installed configuration any way.
I do agree with you that piping configuration does not have much to do such problems.
But what is the problem exactly? what does it mean "not recovering in time on morning warm-up"? are all boilers operating at that time? is it because the secondary loop is too cold during morning warm-up? is the secondary pump turned OFF at night?
You could re-program the morning warm-up start time, insure that te secondary loop is warm (i.e run secondary pumps during night set-back control), Bring the stand-by (is that the case?)boiler "ON" during morning warm-up.
Does yous SHWP have a flow meter? if so, you should be able to tell if water is going thru the decoupler or not. Besides, if the water is too cold, all the water must be flowing thru the SHWP (VFD's at 100%) because all the control valves would be fully open if they are receiving colder water.
Can you delay SHWP start untill the boiler LWT is at 180 or whatever max temperature?
Does your primary GPM match your Secondary GPM?

It is highly questionable that the piping arrangement is the culprit here in my opinion.
You need to look at your sequence of operation, boiler and pump star/stop, boiler staging, lWT/EWT control, etc..

 

[ChrisConley]

Pressure drop in your decoupling link will cause a percentage of secondary flow to run through the primary loop. I can see this creating control problems but not really the issue relating to morning warm-up.

 

[walkes]

The only thing I can think of is that the night setback may be too low. If your building has a high mass your recovery time may be long.

The way things are piped, I assume that your secondary loop flow is equal to or less than your primary loop, ensuring that the secondary loop temperature is the same as the average discharge temperature from the boilers. There may be instances where primary flow is lower than the secondary flow.

Are there boiler isolation valves? If you get flow through a boiler not firing your supply temperature will be reduced. If the boilers are isolated then the flow may be less than the secondary loop flow, the secondary loop will get mixing and the temperature will be lower.

Can you command all boilers to be on in morning warm up such that the correct supply temperature is seen by the building?

 

[cdxx139]

thanks everyone.

The morning warmup is something the client is complaining about but doesnt completely understand so is blaming anything he can. I am trying to grasp my head around the exact issue.

To keep the loop "warm" at night, we will open two control valves on either ends of the building at 50%, this will keep flow going.

My secondary flow equals my primary flow at design. Primary is 50 GPM per pump, which always run in the winter, since the boilers all ramp up together. They equal the 160 gpm of one secondary pump (both are redundnat), which is controlled on VFD, so a majority of the time less than 100%.

The morning warmup is supposed to be "optimized" so it should "know" what time to start to make setpoint when "occupied".

The main thing they teach abour a primary secondary system is "what goes in must come out" So im going to assume the piping is fine and focus on controls for now.

What is a good way to simulate a large load in summer for testing? Can I reduce the water setpoint to 100 degF the night before and increase to 180 degF in the morning? Would that simulate a potential warmup load?

Thnak you

knowledge is power

 

[pipesnpumps]

Also a good rule of thumb on crossover piping is to keep the pressure drop low, like 1 ft total pressure drop.

Also separate each tee in the crossover by at least 4 diameters so the building return doesn't get sucked back into the secondary, by swirling inside the pipe..

I agree with others, it is most likely NOT the crossover pipe. People will blame everyone but them, and the only way you can figure out the truth is get data... strip chart, field measurements, data logs, etc.. That is the path to the solution.




Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.

 

[317069]

Have you spoke with the designer or checked the original design, building heating load, and total boilers size?

 

[cry22]

CD

Your boiler LWT should be controlled either of two ways:
1. Outdoor temperature reset - required by ASHRAE 90.1 anyway.
2. Flow + delta T (secondary loop).

I do not see any reason for going from 100F to 180F arbitrarily. why 100F?

I'd look into simple answers, such as early firing of morning warm-up, monitoring water loop temperature and the likes.

I actually question the "concern" itself (very vague), so I would not go into extreme scenarios to solve this "concern".

Yes, like others said here, collect all your data, flow, temperature, AND DEFINE THE PROBLEM (I see this as the main problem, the issue is not clear) - You seem sharp enough to analyze data, I am sure you can figure it out by rejecting the non-sense first, and then make numbers talk.

 

[cdxx139]

@ 317
I am the origianl designer over two years ago. I am resolving this so I can build a portfolio of clients I can use for references in the future, as I build my career.

@ cry
I am controlling my boilers off w OA reset, and the sensor is on the secondary supply pipe (inlet of secondary hot water pumps).

The suggestion to drop the temp to 100 the night before, and increase to 180 that morning is my way to "simulate" morning warmup in the winter during these summer months. Do you think that will be somewhat accurate? or is there another way. I dont want to wait for winter. (actual design temps is 140-100 degF.

Thanks

knowledge is power

 

[DrRTU]

I agree with others. The as built piping should work ok and the warm-up issue is a separate problem.

A few things to watch: Make sure you do not have reverse flow in an off boiler/pump combination. In the future, install an elbow on the leaving tee to change the direction – the cost of the 5” welded elbow is not worth system performance. Keep both close spaced tees on the main LT 12” apart. I would have built a straight sub-header out of 5” to tie all pumps into very close to the main. A few of the condensing boiler mfgs like to pump into the boilers. Hydralic separators work great but are expensive.

http://www.caleffi.us/en_US/caleffi/Details/News/files/HydSep102508.pdf

Measure the flow via the balance valves on the boilers in various boiler combinations vs system pump % - boiler flow should not change.

 

[317069]

let us take it this way:
the owner want his building to have 22C for example at 7:00 Am,
the building operator starts the boilers at 6:00Am and that does not bring the building to 22C at 7:00 Am. an extra amount of heat must be added to the building.
your idea is to provide this extra amount over the night
cry has suggested to provide this amount over certain time( less than your time) before 7:00 Am
both solutions have the same meaning, but the difference is the operation cost, if you check this point you might find the much suitable idea.

 

[ChasBean1]

You don’t have a bull-headed tee. Bullheading is where water streams collide head-on in the tee branches and are forced together down the stem.

If there is ever a need for heating off-hours (a building zone drops below 60°F for example) the primary loop should run 24/7 at a value such as 180°F. The secondary loop might vary 140-180°F based on OA temp and could even be reset to 120°F off-hours, but boilers should NOT shut off and the primary plant operation should not be altered.

If the primary boiler plant is being shut down at night for energy savings (kudos to you for greenness!), that’s why you’re experiencing your problems.

 

[BronYrAur]

I think this point has already been driven home, but i can see no way that the piping is causing this problem. The piping should have been installed as designed, but i don't think the actual installation is hurting you. You may get a very small amount of undesired primary/secondary mixing, but not enough to the worry about.

 

[cdxx139]

Thank you all for the input. I absolutely agree, even though the response from ther manufacturer was:

"the piping is not per XYZ's P/S arrangement and you will not have a balanced heat load"

I think they are just covering their a$$.

Im going to start with getting the controls to where I think they should be, and update the piping as Plan D.

knowledge is power

 

[SAK9]

If you have motorised ventilation dampers on the air handlers,you should command them to fully shut during the morning warm up cycle.It can make a big difference to your warm up load.

 

[cdxx139]

Thanks Sak9, unfortunately, it is a school building and the classrooms are served with 100% OA Energy Recovery Units and VAV units w reheat coils as the terminal units.

The ERU's do not come with a return air bypass damper for morning warmup and I did not think ahead enough to ask for them during design.

We do have radiant panels serving the perimeter and are part of the warmup sequence, but hey are only sized for skin losses. With 140 degF EWT, we are guessing they dont have the capacity for morning warmup.

(Radiant panels do not have great capacity output, especially at 140 EWT) I may have been to aggressive with the minimum water temperature, which I did more maximum boiler efficiency. We may try raising the EWT to 160 degF next winter.

knowledge is power

 

[SAK9]

If it is a junior class room check what the local codes prescribe as a "safe " skin temperature for the radiator will be.Another possibility is to have a higher discharge air temperature or have the HHW valves opened 100% during the warm up cycle.