Weird Hydronic Anomaly Part 2 Confused in Pittsburgh

RayWohlfarth

Ok so I have had a couple of hydronic projects where the boiler seems to run forever without raising the temperature of the loop. The boilers are piped primary secondary. Once I close the flow through the boiler so the boiler temperature raises to close to 170-180, the loop temperature starts to rise. I posted something a few weeks backs about this. Well, I thought it was a fluke and the other day, I was at the job site and decided to experiment. I allowed the loop temperature to drop about 20 degrees below the setpoint of the controller. I opened the valve and allowed full flow through the boiler. The loop temperature never rose after 20 minutes. I closed the valve to the boiler so the internal water temperature of the boiler rose close to 170 degrees. Once I did that, the loop temperature rose twenty degrees within 15 minutes. Now, I am really confused. I plan on repeating this soon. Any thoughts?
Ray

Solid_Fuel_Man

When you say loop temperature, do you mean supply to the building? If so, what did the return temp from the building do? Rise, or stay the same?

RayWohlfarth

@Solid_Fuel_Man The temperature at both the supply and return were the same as the boiler temperature. Really weird

hot_rod

got a drawing on how it is piped?

RayWohlfarth

@hot rod_7 The takeoffs are about 12" apart

Solid_Fuel_Man

Ok so it's a typical pri/sec or as the engineers call it "injection".

So when you throttle the boiler pump the water slows enough that the delta widens and you get hotter water injected into the building loop?

What happens when it's just left on it's own and the boiler runs high fire? What is the target set point? What kind of radiation, and what is the boiler input? This is cast iron I assume.

And kind of control for variable speed anywhere? Or are all the pumps simply on/off and the boiler is low-hi-low?

RayWohlfarth

@Solid_Fuel_Man The boiler is a steel firetube boiler. The system has a traditional pump on off. If the boiler stays on high fire for 20 minutes it did not raise at all. Once I closed the valve on the secondary going to the boiler, everything worked well Weird They are trying to maintain 160 degrees F

JUGHNE

Could the loop pump be flowing in the wrong direction?

RayWohlfarth

@JUGHNE I checked that thanks

JUGHNE

I knew you would have.

Do you need P/S for this boiler?

DZoro

Just a thought on this. Being that the boiler tees are 12" apart, the boiler is actually getting full return water temp into it, and not a mix temp like it would get if they were close together. Even further exaggerate this is the higher gpm the main loop probably has. If the boiler is tightly sized it may struggle in your scenario you are putting it in. But probably working just fine when left alone and doing its thing??? If it were oversized like so many are you probably wouldn't see such a large delta....IMO
Food for thought

Gordy

Is it possible the tees are to far apart, and the system pump, and boiler pump are running in series. By choking the flow to get boiler temp up creates enough temperature, or flow differential to get an actual hydraulic decoupling going?

I’m learning on this one....

Pipe sizes?

Emitters?

ch4man

I'd like to see the flue gas temperatures of when the boiler heats the loop as opposed to when its "spinning its wheels"

the heat has to be going somewhere, no?
as you know, i too have seen this phenomenon

hot_rod

Do you have any way to measure or calculate flow rates on the two sides?

Here is the basic mixed temperature formula you could use to predict what is or should be happening. It works for primary secondary piping, or hydraulic separators.

T-mix = t.hot X flow hot + T.cold X flow cold ÷
flow hot + flow cold

EBEBRATT-Ed

I agree with @ch4man , heat has to be going somewhere. Finding that out will provide a clue to the problem.

If the boiler pump was over pumping the system pump the supply water (from the boiler outlet) would "back up the bridge" and the boiler would warm up. Since it doesn't the reverse must be true system pump is pumping more than the boiler pump.

For me its' balancing valves and a flow meter to figure it out.

My guess would be both pumps are over pumping and the temperature rise is very slow. If the burner input is normal, normal stack temp, and efficiency, boiler not sooted then it has to be some type of flow issue.

Zman

It sounds to me like the emitters are equal to the output of the boiler at the water temp you are seeing. It won't go higher because it does not have enough BTUs. How is the boiler combustion? Have you clocked the meter?

RayWohlfarth

@JUGHNE . There is another boiler in the loop down stream and this was piped P/S to meet the energy code here.
@DZoro I wondered that also and thought if the velocity was too high, the boiler temp would get high and shut off It seems like the boiler tempe matches the loop temp
@Gordy Im learning on this one too. The primary loop is 2". The secondary loop to the boiler is 1 1/2"
@ch4man I have seen this on two other systems. It seems to occur when the loop temp is low. Last time was on a snow melt system. The owner said it ran all night and never heated

RayWohlfarth

@hot rod_7 Since this is a service agreement, I am going to go over when we slow down and experiment. I will let you know Thanks
@EBEBRATT-Ed This is a head scratcher and will become an article once I can figure it out LOL
@Zman I checked the combustion and the system is about 85% efficient with a stack temp of about 350 degrees. I had one where they forgot to install turbulators in tubes and the stack was over 500 degrees. This s not the case.
I guess it will be a work in progress.
Thank you everyone I will keep you informed once I get some time to play with the readings.

ratio

I've been thinking about this since I read it. Interesting!

What kind of emitters? Any kind of individual or secondary loop controls?

If there's no visible ΔT at the boiler with everything wide open (& the boiler isn't cycling or dropping out on any kind of limit) then the flow has to be sooo fast that the discharge ΔT isn't noticeable. Cypher up what the flow would be with the nameplate input & like a 1° ΔT, is that flow possible in the system with the valves wide open?

I think @Zman's headed in the right direction, the heat is leaving the loop as fast as the boiler adds it. If there were some kind of e.g. control valve on the emitters, that would explain what's going on—they're wide open when the loop temp is low & the emitters suck all the heat out of the loop, but as the AWT climbs they throttle down & allow the loop temp to climb.

But, if there are no controls on the emitters, I guess my theory won't hold water.

RayWohlfarth

@ratio The boiler feeds reheat coils in the building and a couple of covectors at the entrances. There is a reset control on the loop. I plan on heading over there one day next week and playing with the system. The boiler ran continuously without any temperature change until I closed the valve to the boiler about halfway and let the temp inside the boiler rise

Zman

The energy is going somewhere. The questions going through my head are:
How much energy is going in? Clock the meter?
Where is it going? How many btus of emitter is out there?

Are there OA dampers on the coils that are stuck wide open?

Solid_Fuel_Man

What happens if you kill the power to the blowers on the reheat coils? While letting the boiler run and all pumps run?

Henry

I have seen a few times this situation. Each time it was over pumping the boiler. Check the pump specs and the DT on the boiler with an infrared. If the DT is 10 or less, you will have to choke the outlet of the pump to get 20DT.

RayWohlfarth

@Zman I am going to try spending some time there next week and do some more research. I will be using some better instrumentation and walk around and check the system. I kinda get like that dog with the old bone sometimes.
@Solid_Fuel_Man I will look into this next week Thanks
@Henry I wass thinking the same but would the boiler temp rise if nothing was being sent to system?
Thank you sirs! I have homework

jumper

Henry said:

I have seen a few times this situation. Each time it was over pumping the boiler. Check the pump specs and the DT on the boiler with an infrared. If the DT is 10 or less, you will have to choke the outlet of the pump to get 20DT.

Somewhere outside the boiler room is an uncontrolled pipe for "hydraulic independence" in case operators close the shunt in the boiler room. Primary-secondary was invented to sell more pumps. Originally for chilled water and there there can be worse problems.

RayWohlfarth

@jumper Our state follows the IECC code which states that idle boilers have to be isolated. I would rather use primary secondary versus isolation valves.

EBEBRATT-Ed

Just wondering if the system comes out of night setback and in set back mode it is unoccupied mode the AHUs not bring in any outside air. Now it comes out of set back and goes to occupied bringing in outside air and trying to raise the building temp at the same time. HUge load

The way to do this is to raise the building temp first and when the RA to the AHUs is up to 70 deg then bring on the OA. Morning warm up

Of course I am speculating about a system and I don't know what's their. But it sounds good.

ratio

I think we're all just speculating until Ray gets back out there. @RayWohlfarth, will you be able to post from your phone when you're on site next? My theory requires control valves on the majority of the emitters, be sure and check carefully for them.

jumper

RayWohlfarth said:

@jumper Our state follows the IECC code which states that idle boilers have to be isolated. I would rather use primary secondary versus isolation valves.

Delta T equals zero spells short circuit. I built multiple AHs CW systems that way. When there's no demand boiler or chiller has to shut down anyhow so that distant shunt makes no difference.

I too prefer pump/ check valve sets to isolation valves. I also have seen what Henry described. Incorrect circulator on boiler. My guess is that it's preferable to adjust a balancing valve beyond the farthest emitter than near the pump.

RayWohlfarth

Thanks for your expertise @jumper

Harvey Ramer

And do check for leaks. Condensate drains in the fan coils and such. It doesn't take much of a leak to start chewing up the btu's the boiler is producing.

RayWohlfarth

@Harvey Ramer Thanks never thought about that

Solid_Fuel_Man

Is the building heating well?

2" main loop with 1.5" boiler loop, sounds like it's not too big of a building. You mentioned boiler isolation, so I assume there is another boiler. What happens if you run both heat plants at the same time?

RayWohlfarth

@Solid_Fuel_Man There is an old Raypak boiler which is really really over sized. It bangs on and off every minute or so. We inherited this job. We cut the firing rate way down. the building heats well. I was just noticing this weird thing when I played with the boiler valves