modular boiler woes

furnacefigher15

Went on a job today to do some boiler maintenance at a new customer. I did the maintenance, and that went fine. But here's the thing, the boilers short cycle.



The system is a Hydrotherm Multitemp  There are a total of 8 boiler modules each with a Gordon Piatt S4 power burner. The controller is a Heat timer 8 stage sequencing reset control (ESQ I think)



The boilers are piped as OEM instructions dictate, but there is no means of controlling flow through an idle boiler. The flow through each boiler is balanced by way of reverse return piping. 



There seem to be 2 things happening:

1) even with only one boiler firing, the individual boiler heats fast, and cycles off after 2 minutes 140 to 180, then off.

2) the loop temperature stays low, b/c the 180 output of the one boiler gets tempered lower due to the 7 bypass paths created by the non flow regulated idle boilers.



Is this just the Hydrotherm way, or am I just dealing with a grossly oversized heating plant?

ColoradoDave

Reverse Return?

I'm not familiar with Hydrotherm or the Hydrotherm Way as you posted....



But, I wonder about the reverse return in your system... wouldn't it be better and more predictable for you to do kind of a Primary / Secondary or perhaps it'd be considered injection pumping instead of reverse return?



Not having seen your installation,  I imagine that the flow is equal through all your boilers using reverse return, when in reality, I would think it'd be better to have a System loop pumping to the building and returning from the building on it's own loop.  Then use a boiler pump for each boiler.  That way when the staging control fires each boiler(s), their individual pumps draw through the boiler... it would elimate the "mixing" effect of having flow through all boilers all the time.

icesailor

Reverse Return, Direct Return:

Do you know the difference?

I'll bet that the 8 boilers are actually piped as direct return. Which can allow one boiler to do what you say. A True reverse return makes equal flow through all boilers. Same as radiation.

Mark Etherton had a great synopsis on direct return here, right now. Describing a relative who liked to water ski.

RobG

Short cycling

Fist thing to do is crank up the hi-limit to 220f (per manufactures instructions, page 17 of installation manual). Secondly, I would also check the flow rate, should be 30 gpm through each module. The system should not be over-sized (that's one of the benefits of modular systems). Check that the controller is set up properly and functioning accordingly. That should take care of the problem.

p.s. with the hi-limits set at 220 ensure that the system control is set at design temp. You're basicly doing high temp injection mixing.   

Rob

furnacefigher15

Reverse Return

Yes, I know what reverse return is.    First on the supply side is last on the return, and vice versa.



Colorado, I agree p/s piping would be best, but I am not in a position to re-pipe the boiler room. This is not a new install, but one at least 10 years old.



Rob- The operator on each module is currently set at 180. I saw in the manual the 220 setting, but then a few more pages in it showed a picture having the manual reset at 220 and the boiler operator at 170 to 175.  



So should I set each operator to 220? If so what setting should the manual reset be.

icesailor

Woes:

In some jurisdictions, you must have a high limit with a manual re-set, usually set at 220 degrees. The high limit operating control is set lower, The 220 degree limit is a safety.

In my opinion, and that doesn't count for much or mean that I am correct, if you have multiple boilers, and they are connected as a direct return, rather than as a reverse return, the bad flow characteristics won't be solved by P/S piping or a hydraulic separator.

I replaced three boilers in a building I do work in. It was no more work to pipe it as a reverse return. It is P/S with bridge loops to the seccondary. It has a controller that stages and rotates the lead/lag boiler. The boilers run (unfortunately) on outdoor re-set so they condense (oil) creating large amounts of kibbles and bits. At no time, will the gauges on the off boilers ever be colder than the boiler that is doing the work at the time. And all three boilers have equal amounts of kibbles and bits in them when I clean them. 

furnacefigher15

hydrotherm design boiler rise?

I guess what my question for Rob should have been is;



Are the hydrotherm boiler modules designed to be set at 220? Is there intention to have a high individual boiler temperature rise? like 60 to 80 degrees?

The manuals don't really say.



As it sits, I'm getting more than a 40 degree delta on 1 boiler / burner module.



return 140      supply at running boiler 180 and cycling every 2 to 5 minutes



Blended supply in header 142 degrees or so.



Any system I ever designed with multiple boilers would not short cycle in these conditions.

rich pickering

You "may" have flow problems

 We have a building that has 17 of these.  7 in one bank, 7 in another and 3 in third.  All feeding the same loop. There is supposed to be a high limit on the header, and operating limits on the boilers.   We use 3 tekmars to stage these.

Normally a 40delta T means low flow, it should have been designed for 20. But that is at design conditions.  What is the setpoint of the loop?  Without a load on the system, one boiler may be enough.

Mark Eatherton

Flow too slow or fire too high...

The only things that can cause the symptoms you describe are low flow or input that is extremely excessive, both of which I have experienced with this pancake boiler.



"Theoretically speaking, in a parallel reverse return scenario, ALL branches (boilers in this case) SHOULD see a balanced and equal flow. Notice I prefaced that last statement with theoretically.... If there is more rust in one boiler than another, then it sets up unbalanced flows and causes this scenario as well. It is not uncommon for the regulator serving the appliance gas train to fail and allow full main pressure into the orifi. Have you done a combustion analysis yet? That will tell you how to eliminate one potential (over firing).



I had a professor of hydrology tell me once that you never want a perfectly square grid on a radiant floor heating system piped parallel reverse return because it causes the water to get lazy and random. One time it flows well through this circuit, then the next minute its flowing well through another circuit, and the one that was flow well is not flowing for crap now...He said YOU must provide the grid with a "finite" pressure drop across the grid. The pressure drop through the cast iron sections of this boiler are less than 1 foot of head AT design conditions. Got other boilers with a LOW delta T?



How much flow do you THINK you have crossing over the boilers? Try closing isolation valves on all but the troublesome one and see what happens to its DT. If it remains high, its either a problem on the fire side, or a blocked flow (gate valves DO drop their gates on occasion). If it drops, then it is a random flow problem. Create a finite pressure drop by choking flow...



If you can isolate some of the boilers, do so (say half) and see how the system performs. It has been my experience that most, if not ALL heating systems are over sized by a factor of 2...



Let us know what you found.



ME

furnacefigher15

No Valves to isolate

Thank you for all the responses. Unfortunately I have not been back to that site to troubleshoot any further.



There is a auto reset operating high limit on each boiler set for 180, and a manual reset on the supply header serving multiple boilers set to 210.



Unfortunately there are no isolation valves to each module.



I did not have time to combustion analyze, but did go through and clean all the pilots, scanners, ignitors, and sail type air proving switches. I also checked the gas pressure settings, and they were all at 3.1 inches on the manifold upstream. I did find a bad gas valve causing one boiler to go into flame fail, and also an issue with a low water cut out.



I am definitely leaning toward a low flow. The problem seems to be universal. Regardless of which 1 boiler fires it short cycles just the same.



The loop Temperature setting is variable based on the outdoor reset control. At time I was there the demand was for 145 or so.



I can't hear any kind of percolation that I would normally associate with a fouling cast boiler.



There is an abandoned 3 way mixing valve just down stream from the boilers, that says its 100 percent open. My first instinct was too close the manual valve in the bypass line, but I could not get the plug valve to budge ( even with a 3 foot pipe wrench)



But in the situation I would not be surprised of gross over sizing (this is a government building)



Government building engineering moto = if little is good, more is better, and a lot more is the best.



The boiler plant in this building is 3.8 million btu and could likely heat the rest of the block.

icesailor

Low Flows:

Here's something I found and learned from.

I have this account that is full of queebs. When I took it on, the flow and system was strange. It had three of those Hydrosperm modular boilers like you have. The primary side had redundant Taco 1600 1/3 HP 1725 motors that went to a mechanical room where bridge loops sent water to the secondary loops. The bridge loops were ppiped backwards as they were shown on the plan. Always follow the plan, even if it is wrong. They used Taco 112 1/3 HP 3450 motors. The maintenance guy that had been doing the work figured out that he could save some money by buying motors without the frame and put the motors in the same bearing cradles. Because these motors were universal, they had a long shaft sticking out the opposite end. At some point, he must have needed a motor so he used what he had at the time. He replaced a 1725 RPM motor with a 3450 motor on the 1750 motor/pump. The motor got hot and burned up after a while. He also substituted 1725 motors for the 3450 RPM motors on the Taco 112's. I came along, and faced with strange problems, like these shafts sticking out of the other end, and finally realized what had been done. After installing the proper RPM speed motors, things got better.

Don't overlook the obvious. It's easy to do. There might be a valve open somewhere. I've always found that a careful perusal of the plans might show some mistake. Or at least help you figure out what was planned.

Good luck.

furnacefigher15

Update

I went back to this building yesterday. While I was there I did combustion analysis on all 8 power burners on the modular boiler set up. Each and every one of them was way over fired, and had likely been that way from day one.



Burners were running with 1% O2 in the stack. Made gas/air adjustments to obtain 4 - 5% O2 and 9 - 10% CO2 with CO under 30ppm. Most of the manifold gas pressures went from 3.1" down to 2.25"



I also reprogrammed the heat timer control to look for a slower reaction time. Now instead of expecting immediate changes in water temperature, it waits 3 minutes for a purge delay per boiler, and 5 minutes after that before looking for a change in loop temp.



System operates much better now. No more cycling on limit, and no more running 8 boilers when one will do the job.



I suspect the simple changes that I made will save them huge on the gas consumption.

Steve Whitbeck

hydrotherm boilers

Those boilers are known to plug up. The heat exchangers are horizontal like a stack of pancakes. The tubes tend to plug up with sediment and this impeedes water flow. Was there any percolation noise when the burner was on?

furnacefigher15

No

No percolation noise.