To use or not use the temperature sensor on the return?

Frederick

On a properly insulated and vented return pipe there is a temperature sensor that is not currently wired into the control circuit, i.e. thermostat, pressuretrols, etc.   My question is should this be rewired into the control circuit?  Is the temperature in the return pipe a good indicator of the radiator temperature?  Would the radiators and return pipe cool off approximated at the same rates? 



I guess what I am asking is, does this work:

a) flawlessly,

b) reasonably with lots of tweaking and finger crossing,

c) nice to have as a high temperature safety.



Thanks in advance,



Fred

Brad White

It may be

part of a Heat Timer or other form of cycle timing control which varies the number of cycles per hour based on outdoor temperature. Look around and see if you can find any other remnants of that system.

Rod

Sensor

Hi- The Tekmar 279 uses a sensor on the return similar to what you describe.

- Rod

 http://www.tekmarcontrols.com/literature/acrobat/d279.pdf

Frederick

No outside temperature control on the old system.

Previous to the current boiler being installed there was no outdoor temperature sensor.  The previous thermostat was a simple threshold, no clocks, etc.



This is a mercury temperature sensor attached to the slowest return pipe.  On a different system there is the same model, which is attached to a wet return.  As far as I can tell it has not tripped, at least not when I have watched that other boiler, which leads me to think that this is some kind of safety control.  Although I have read that some people control their boilers using a temperature sensor, they did not refer to it as a thermostat thus I assume they were referring to something similar to what I have sitting there.  This temperature sensor on this system is attached to a dry return (not a wet return like the other system), thus I assume that it will get considerably hotter than a wet return.  If the dry return pipe heats to steam temperature and stays there a long time because the pipes are insulated, I question the temperature of the dry return pipe being a good indicator of the radiator temperature(which are uninsulated), thus the reason for the question.



Does anyone use the a temperature sensor like this to control the boiler?



Thanks,



Fred

Steamhead

It was probably

part of a control system that was abandoned. We sometimes see this in apartment buildings. What type of buildings are these? 

Frederick

12 unit apartment building / Chicago

So I assume you do not use these return pipe temperature sensors/control?

Steamhead

Au contraire

an outdoor control is definitely the proper choice for this type of building. We like the Tekmar units but there are plenty of Heat-Timer fanboys as well.



It is essential that the sensors be properly placed, as they determine how the control cycles the boiler. In general, and subject to manufacturer's instructions, the outdoor sensor goes on the North side of the building where the sun won't hit it.



The "steam established" or "condensate" sensor placement should be close to the furthest radiator from the boiler. This way the control knows when the pipes are full of steam and can begin the countdown to where it will stop the burner. The length of that countdown determines to what extent the rads heat up.



Some of us like to use indoor sensors as well, but we've found that with proper steam distribution and even heating throughout the building, they aren't really needed. But with some setups, you can use them to monitor temperatures throughout a building from a central location.

Rod

Condensate Temperature?

Being located on the return, the sensor would be probably have been measuring the temperature of the condensate possibly to indicate to the controller that steam was present in the system. 

As you are in Chicago, if you're ever looking for a good steam pro, Dave Bunnell, (312-202-0511 and 815-857-2339) would be the person I would get. He's in the "Find a Contractor" section at the top of this page. His steam work is top notch and he is very experienced with different controls and specializes in getting the maximum efficiency out of a steam system. Here's an article he published on optimizing steam systems.

http://www.heatinghelp.com/article/11/Hot-Tech-Tips/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell

- Rod

Steamhead

From the Tekmar 269 manual online

"The control will not start to time out the Heating Cycle until the Condensate Return Sensor reaches the temperature set on the "Steam Established" dial. When this occurs, the "Steam Established" light will come on and the control will operate the boiler until the on time of the Heating Cycle has elapsed. This function ensures that steam has been established throughout the system before the Heating Cycle is started.



In a two pipe system, most installers will place the Condensate Sensor on the condensate return line of the radiator furthest from the boiler. In one pipe systems the best location is usually on the pipe just before it enters the last radiator. These locations are usually good, but in some larger systems with long runs it may be necessary to place the sensor closer to the boiler so that the first rooms in the system do not overheat.



Again, it is important to ensure that the system components are all working properly, since in a properly vented and drained system, steam is established quickly throughout the system during each cycle, and sensor placement is not as critical as in some "problem" systems."



Kind of ambiguous, but there it is. Not sure why you wouldn't want to have the sensor pick up when steam is approaching the last radiator on a 2-pipe system, rather than when it has filled the rad which could lead to overheating on a mild day. This placement on the return also makes it possible for someone who has partly closed the valve on that last rad to screw up the system's cycling.



We put our sensors on the steam side on all our systems, right before the last rad, and this gives us much better control.

Dave in QCA

Error in Tekmar manual perhaps?

Steamhead, I installed a Tekmar 279 a couple of years ago.  It was before I had found this site and read the books, so I was still pretty much a novice on low pressure and atmospheric steam.  I studied the sequence of operation and concluded before I even placed the order, that putting the condensate sensor on the "outlet" of the furthest radiator would cause the system to do exactly what the Tekmar control was supposed to prevent, overheat!  The sensor is supposed to determine when steam is established throughtout the supply mains, not when the system and all the radiators are fully heated.  So, I installed the sensor on the dry return of the steam main, as it loops back to the boiler room where the vent is located, and it works perfectly.  It gives me satisfaction to know that my conclusion coincides with your installation practices!  



It just occured to me now, as I am writing, that Tekmar's instructions would probably work well on a 2-pipe system with fintube convectors.  They heat up and cool down very fast.  Having the "steam established" function determined by the condensate outlet of the furthest fintube convector might actually work well.  But, in a cast iron system, I could not work, and the instructions are in error.



It also occurs to me on this thread, that terminology can cause confusion.  From the writer's original post, it is not clear to me if he is referring to a dry return on the return piping of a 2-pipe system, or the dry return "end of steam main" on a 2 or 1 pipe system, or on a wet return. 



Many years ago, when trying to assist in the marketing of a historic hotel Mt. Pleasant, Iowa, I came across something quite puzzling and it took me a while to figure it out.  The hotel had been owned for many years (1930s-1987) by a man, who's brother operated the municipal electric plant, and was quite an engineer.  I suspect that his brother had helped to solve problems on the hotel's old one pipe steam system.  The system was controlled by one thermostat located on the first floor, somewhat in the center of the building, in the lobby.  It had some degree of anticipation in it, and that can always cause cycles to occur that are so short that the far reaches of the building do not get any steam at all.  So, what was done?  They installed a strap on T'stat on the furthest lateral coming off of the end of the steam main.  It was set at about 200F.  The system was set up so that when the thermostat called for heat, it engaged a latching relay.  The latching circuit was tied through the contact of the pipe mounted T'stat, and was held in the on position until steam got to the end of the system.  At that point, the latching relay dropped out.  If the thermostat was no longer calling for heat, the burner stopped.  If the thermostat was still calling for heat, it would continue to fire until satisfied.   At was all rather ingenious I thought!  I had been in the building many times through the years, they had a great dining room!  One thing I noted was that the temperature was always confortable; never overheated, never cold.

 

Steam Heat is Great!

Frederick

Hmm

After the boiler was replaced an outdoor sensor and four indoor sensors were added and a RD14xx thermostat controller was added.  I do not see in the manual for the RD14xx that it has inputs for a condensate sensor.



The temperature sensor in question (on the system in question) is left over from the previous boiler and is a mercury switch sensor connected to the dry return of the slowest returning, i.e. at the end of the supply loop next to the boiler, of a one pipe system.

Comparing this to another system in another building that uses the same model of temperature sensor, albeit in this system the sensor is connected to the wet return of a one pipe system next to the boiler.  In this system the temperature switch is connected in series with the indoor/outdoor thermostat controller.  So I would assume that when the temperature of the wet return exceeds a threshold the burners shut off until the temperature in the wet return pipe falls below the threshold.  This wet return pipe is uninsulated.  



Sooo, in the system in question (sensor connected to the dry return of a one pipe system) if I connect this sensor in series with the call for heat, from the indoor/outdoor temperature controller, should I expect the dry return pipe to cool down at relatively the same rate as the radiators in the apartments, i.e. is the cool down time of the dry return going to be in the same ballpark as the 'control"s countdown?  I guess the main reason I am asking is if others use this type of sensor for controlling the boiler and consider it of value I will put forth the effort of trying to get it to work, if not why waste my time and have to listen to the tenants complain while I am debugging...



Without indoor temperature sensors how do to set the indoor temperature?



Thanks,



Fred

Frederick

Compare and contrast the use of a condensate temperature sensor on wet vs dry return of 1 pipe system...

The temperature sensor in question on the system in question is on  the dry return "end of steam main" on a 1 pipe system.  I would like to use this sensor to shut off the burner if the radiators are hot.  I was comparing the system in question to another system that has the same type of sensor on a wet return.  Is this an apples to apples comparison or is one a lemon?



Here are some specific questions:

a) Is there a safety concern if the temperature of a wet return rises above a certain temperature threshold (set below 212 F).  i.e is there a need for a safety cut off on the wet return line?

b) Should the temperature of a wet return be considerably lower than that of a dry return?  Wouldn't the longer the wet return the lower the temperature of the pipes contents closer to the boiler and thus using the temperature of the wet return would not accurately reflect the temperature in the radiators?

c) Is there a safety concern if the temperature of a dry return rises above a certain temperature threshold (set below 212F)?

d) Should the radiators cool off at approximate the same rate as the dry return pipe?  Are there ball park estimates for either of these rates?



Thanks in advance,



Fred

Brad White

My take:

To your questions, Fred:



a) Is there a safety concern if the temperature of a wet return rises

above a certain temperature threshold (set below 212 F).  i.e is there a

need for a safety cut off on the wet return line?



A. Your pressure controls are your best safety device. If you do have elevated return temperature, that usually indicates (on a 2-pipe system, not yours), a steam trap stuck open and passing steam directly to the return line. While abnormal, it is not a condition I would use to shut down the boiler.





b) Should the temperature of a wet return be considerably lower than

that of a dry return?  Wouldn't the longer the wet return the lower the

temperature of the pipes contents closer to the boiler and thus using

the temperature of the wet return would not accurately reflect the

temperature in the radiators?



A. Yes, the return temperature would be lower and yes, the longer the return the greater the cooling effect. When return line temperature is used as a proxy for system cycle completion, this temperature is adjusted for what represents the "peak". I can see why Steamhead suggests using the supply pipe temperature in lieu of the return. Makes sense and no "skew" nor "Kentucky Windage".





c) Is there a safety concern if the temperature of a dry return rises above a certain temperature threshold (set below 212F)?



A. Only to the extent that you have a bypass of some sort and this is not a 1-pipe steam system condition that I can see. Not a safety issue that would not be preempted by your pressure control that I can see normally.



An exception or example though:  If you *did* see temperatures well above 212F, say 250F or more, that would indicate a runaway boiler but your pressure controls should see that first. If the pressure high limit failed, the temperature sensor could serve as a second tier but frankly, I have never seen it done.



d) Should the radiators cool off at approximate the same rate as the dry

return pipe?  Are there ball park estimates for either of these rates?



A. The cooling off rate would be the same if the pipes and the radiators are in the same ambient environment. But consider that what is warming the dry return is what has already passed through the radiators.



The rate will be proportional to the temperature difference to the ambient, but the return will start out a degree or two below what is in the radiator and drop from there. For example, with a room at 70F and the steam at 215F under pressure, you have a delta-T of 145F. Once the condensate passes through the trap, that condensate will be, for discussion purposes, 210F or less. However, the basement is likely to be a few degrees colder than the room above it., so academically speaking, the delta-T might well be the same and so shall be the rate of cooling.  Make sense?



 (As an example for discussion, in a 2-pipe system, condensate passing through a trap will have a very minor degree of "flash" due to the lower pressure it sees on the downstream side of the trap.  This flashing -exchange to vapor- cools the remaining condensate by expansion or adiabatic cooling and so you lose a degree or so, it depends. On a one-pipe system, you also go through a slighter pressure drop after condensing, basically hovering on the saturation line. The net result is still a drop in temperature of at least two degrees by the time you  are clearly in the dry return. In fact I have never seen the temperature above 205F that I recall.)



Regardless, I think it will be "close enough" of a proxy value, but will never be the same as what is in the radiator.

Frederick

Thanks

This is the conformation I needed to brave this adventure.



I will report back on success or failure.



thanks,



Fred



P.S. And thanks for the new phrase...

nicholas bonham-carter

multi-unit steam control

i think this control is one which has been recommended by boilerpro of chicago, and maybe a search here will bring up his thoughts on the subject.

this seems to be their website:

http://www.rdcontrolsystems.com/steam-1400-series.php

do you now have the complete rd steam control system, or only the remnants?

bear in mind that these controls cannot compensate for any lack of venting, over-pressure, or other deferred maintainance. hopefully your new boiler is now skimmed, properly main-vented with a back-pressure of 2 ounces, and a cutout of 12 ounces,  with steam arriving at all the top floor rads simultaneously, and then you can start tuning the control!--nbc

Dave in QCA

Sensor on 1-Pipe dry return / end of steam main

Fred, I think I understand what you are explaining. One thing is still unclear, do you have a sensor or a thermostat.  A thermostat has a set of contact points and is usually adjustable.  A sensor is usually a thermister with 2 wires coming out of it.  It is not adjustible, and is used only connected to electronic circuitry that is designed to read the resistance of the thermister and translate that to a temperature reading, and then utilize that information as the controller is programmed to do.  I suspect that you probably have a pipe mounted thermostat.



As far as your desire to have this sensor turn of your burner when the radiators are hot, that is perhaps a misguided thought.  For example, when it is very cold, you not only need the radiators to get hot, but you may need them to stay hot for a long period of time.  The burner would need to be able to continue to run and make steam for that to happen.  Your room thermostat is the best control for the boiler.  Of course, there are other limit controls that come to play as well, such as the high pressure cutout, low water cut out, etc.



Regarding your questions,

a) Is there a safety concern if the temperature of a wet return rises above a certain temperature threshold (set below 212 F).  i.e is there a need for a safety cut off on the wet return line?  

NO



b) Should the temperature of a wet return be considerably lower than that of a dry return?  Wouldn't the longer the wet return the lower the temperature of the pipes contents closer to the boiler and thus using the temperature of the wet return would not accurately reflect the temperature in the radiators?

Temperature of the wet return will be lower than the dry return / end of the steam main.  Just how much cooler depends on how long the wet return is.  It could not be used to accurately indicate conditions in the steam side and the radiators.



c) Is there a safety concern if the temperature of a dry return rises above a certain temperature threshold (set below 212F)? 

NO.  The dry return / end of main is going to be at, or very close to the temperature of the steam inside the pipe. 





d) Should the radiators cool off at approximate the same rate as the dry return pipe?  Are there ball park estimates for either of these rates? 

Maybe, Sort of, Not Really.  The piping does not have the same mass as a cast iron radiator.  A fully heated radiator will probably hold its heat longer than an uninsulated pipe.  However, if the pipe is insulated, that will keep it hot longer.  If the space where the piping is is cold, it will cool it faster.  The room temperature will also have an effect on how fast the radiator cools down.  You cannot reliably read the temperature of the radiator via a sensor or thermostate mounted at the end of the steam main.  The sensor or thermostat at the end of the steam main can read the temperature of that point only.  A sensor at this location is usually used to indicate that steam has been established, and to allow a controller such as Heat Timer or Tekmar 279 to begin the timing of the heat cycle.  Neither of these systems use the sensor to turn off the burner.

Frederick

Mercury switch not thermistor

Sorry if I was unclear.  The temperature measuring device is a switch capable of conducting 120Vac.  I believe that it is intended to be attached in series with the pressuretrols and low water level, etc, sensors that turn the boiler off if the conditions are not acceptable.  A similar device on a similar system is connected in this fashion.



I do not intend to use this sensor instead of indoor/outdoor temperature sensor/controller.  I assume that boiler will cycle on and off as long as the 'call for heat' is being sent from the indoor/outdoor sensor/controller.  I assume that generating steam while the radiators are hot is a waste.  When the radiators fall to being merely  warm and the 'call for heat' still exists, turn the burners back on.  If the radiators  go from hot to warm in 5 minutes and the dry return pipe goes from hot to warm in less than 10 minutes I would assume that the apartments would stay warm fine.  What I was worried about was if the pipe took 30 minute to cool, or possibly varying time, then the apartments would never reach the desired temperature.  Or there might be other murphy's laws commonly experienced with this setup that others had suffered that would convince me to abandon trying this.



My other concern was that this was a safety and should be set to a higher value.  It seems to be a general consensus that this is not intended to be a safety device.



Thanks,



Fred

Dave in QCA

Depends on your objective

Putting the device in question into the control circuit will limit the firing time and reduce the ability of the system to heat.  If the building is overheating, and you do not otherwise have an effective control system, this may help you to reduce overheating.



However, your assumption that generating steam while the radiators are hot is a waste, is not correct.  If the space is overheated, it is a waste, but if the space needs heat it is not.  Keep in mind that a radiator emits 240 BTU of heat for every sq ft of surface area while at a mean temperature of 215F.  So, an average radiator with 40 sq ft, emits 9600 BTU/HR of heat, and as it does that, it will require a steady supply of steam and it will condense 9.6 pounds of steam per hour.  As a radiator sets there, seeming to do nothing, it is actually busy condensing steam to water, the water is running out, and more steam is coming in.  The heat content of the steam coming in is equal to the heat being given off.  Once you stop the flow of steam, the radiator immediately starts to cool down.  The only heat that it can give off is the residual heat contained in the cast iron.   If that is enough to heat the space, fine.  If it is very cold outside, it probably will not suffice, depending on how the radiators were sized for the building in the first place.  However, keep in mind, supplying steam to a hot radiator is not waste, because the radiator needs a steady supply of steam to remain at or near 215F and continue to give off 240 BTU/Sq Ft.



Perhaps a poor analogy would be a baking oven.  If you're supposed to bake a meatloaf for an hour at 350F, would you preheat the oven to 350F, then turn it off and put the meatloaf in and expect it to be properly cooked in an hour?  Of course not! 



At any rate, continue to tinker with your system and see how it works.  Let us know, we'll be interested.



By the way, for a building like yours, I would highly recommend the Tekmar 279.  Mine works great, and produces very even heat.  The temperature fluctuation with the cyles of steam are imperceptible.

Frederick

Hysteresis not single threshold

Thanks, tid bits form a guru is worth days of experimenting . . .

But the only way a fool becomes a guru is by experimenting . . .



I suspect the cooking analogy is slightly more complicated and alludes to my idea. note: we will treat the oven's burner as on/off and not variable intensity, like a steam boiler.  You set the oven to 350F, i.e. call for heat,  the burner comes on and the temperature rises to 349F and then oven turns off the burner.  The temperature continues to rise to 351F due to delays between heat produced at flame and convection to sensor.  Temperature begins to fall and falls below 349F and the burner comes on again.  When the temperature rises above 349F the burners shut off.  This continues until the oven is shut off. 



Reading the values off the graph on page 5 of the Tekma 279 manual http://www.tekmarcontrols.com/literature/acrobat/d279.pdf

If the system described by this graph was shut off at a condensate temperature above 190F and turned on below 190F the boiler would cycle about every 10 minutes, 5min on / 5min off, (until call for heat was satisfied).  It would seem as though the radiator would stay within 10% of it maximum temperature.



hopefully this works,



Fred

Frederick

Only the remnants?

I have the controller, 4 indoor and 1 outdoor sensor, but no alternate clock,  I obtained it used.  What else would there be?



Over pressure was fixed last week.  The vents should be perfected today.  I will subsequently wire in the condensate temperature switch and sit and watch the boiler.



Thanks,



Fred

nicholas bonham-carter

steam control experiment

i will certainly be interested in the results of your experiment. was this controller removed from a demolished building, or was there some other reason for its retirement? is this a home-brewed device, or from some current mfg?

i presume that this controller uses the usual method of establishing the run-time for the boiler, based on outside temperature; and starting the timing of the duration of the burn when the condensate sensor has detected the proper temperature in the return pipe.  you say that the clock is missing, but hopefully that is not the burn timer.--nbc

Dave in QCA

Bad example, I know......

OK, I admit, the oven was a bad example.  I was affraid you'd catch the flaw in my logic!  However, keep in mind that when a boiler is firing and the system becomes completely heated, the pressure will begin to rise.  At your hi pressure cutout, hopefully set very low, the boiler will stop firing and will restart when the pressure drops.  Just like the oven example, the boiler firing over time will match the system's ability emit heat.



I don't mean to throw a wet rag on your experiments.  By all means, I think you should proceed and see what happens.  Keep in mind, I think you will find that your dry return / end of steam main will get hot the same time that the steam ARRIVES at your radiators, not when they are fully heated.



The example on page 5 in the Tekmar manual, as I am sure your realize, the condensate sensor does not shut the boiler off, but causes the timing of the heating cycle to begin.  If the boiler is cold, it takes a lot longer to establish steam than it does if it fired an hour ago.  As the graph indicates, the system determines the length of System ON Time based on the temperature reading of the outdoor sensor plus the reading of the indoor sensor (and whether space temp is at set point, below set point, or above set point).   If the system is set for 1 hour cycles and the the conditions calculate a need for a 25% cycle, when the previous cycle's off time has completed, the boiler will begin firing.  This portion of the cycle is not part of the on time, the Tekmar Panel will read "Establishing Steam".  Once the steam gets to the end of the main, or to the furthes radiator, the so called, "Condensate Sensor" will read the temperature.  When it gets to the adjustible condensate set point, (mine is at 190F because I want to assure that steam has also gotten to the tops of the risers) the Heat Cycle begins, and from this point, the boiler will fire 15 minutes, (25% of one hour).  At that point, the burner will shut off, and will stay off for 75% of the cycle time, or 45 minutes.  At which point, the boiler will begin to fire again and repeat the cycle unless the outdoor temperature has reached the warm weather shut down set point. 



In my system, if it fired an hour earlier, it will take about 3 minutes to begin to produce steam.  It will take another 3 minutes for steam to reach the condensate sensor.  At that point, steam is just starting to enter the radiator supply valves. It will take an additional 25 minutes to fully heat all of the radiators.  And so, in any cyle that is less than 42%, the radiators will only partially heat, and that is desirable in maintaining an even space temperature.



Again, let us know how you make out.  I apologize if I am explaining the obvious, just trying to be helpful. 

Frederick

History of sensor

I was told the sensor was removed because the building it was in had the boiler and all ancillary equipment replaced.



From a cursory read of the RD1400 manual it seems as though they alter the indoor temperature threshold when it gets really cold and does not change cycling periods.



The primary clock is there.  There is some Alternate settings controlled by a alternate clock, which is missing.  The primary clock has 4 periods of control, so I do not see the need for the alternate clock.



This controller is not cognizant of the condensate temperature, i.e. no inputs or setting for a condensate sensor.



Fred

nicholas bonham-carter

condensate temperature sensor missing?

without some sort of "steam established" sensor, how will the controller know when to start timing the burn?--nbc

Dave in QCA

Strictly just a thermostat

It appears from my skimming of the product information on the RD1400, it operates as a thermostat, averaging the temperatures from several inside sensors.  The outdoor sensor is used only as a Warm Weather Shutdown.  Therefore, it does not calculate run time, and does not need to know when steam has been established.

Dave in QCA

I WAS WRONG AGAIN!!! Darnit!

On page 12 of the RD1400 Manual, there is a little bit of information that sheds some light on this discussion.   http://www.rdcontrolsystems.com/manual-downloads/om_1400c.pdf 

It appears that when the RD1400 is switched from automatic to manual operation, that it will cause the boiler to run continuously except for steam pressure cutout unless a jumper is removed and an additional device is installed.  One option is a return line aquastat, (BINGO!!!) or a time clock (probably an interval type).  Either of these devices only come into play with the system is switched from AUTO to MANUAL.  It would appear, based on the terminology (aquastat) and the author's description of a similar installation that the return line aquastat would be connected to the wet return near the boiler, and thus would indicate (if above a certain temperature) that the system was really cookin and it was time for a shutdown, as opposed to running wild.

sreja

tekmar condensate sensor location

The points that are being made about where to place the condensate sensor for the tekmar 279 are very interesting.



I agree completely that it seems very odd to be testing for the establishment of steam by measuring CONDENSATE line temperature instead of far away steam main pipe going INTO radiator.



However, two things are worth noting:



1. The tekmar manual is *REALLY* clear about putting it on the condensate line in a two pipe system, with pictures of cast iron radiators, etc.  and after all it is actually CALLED a condensate sensor.



2. There are actually *TWO* functions of that sensor -- one is to determine when steam is ESTABLISHED by measuring a RISE in condensate temperature.  But the other is to tell when all of the latent heat in the radiators is used up, by measuring the DROP in condensate temperature (Lockout Differential in manual).



It still seems to me that having the sensor on the farr away steam main makes more sense for establishing steam detection as many hear have suggested.  But for the second task, measuring loss of stoed heat, you need to be on the condensate line.



Perhaps the way to think of it is that if you put it on far away main instead of condensate line, you will get much faster cycle times between steam established and tempoerature bottoming out and being ready for next cycle.  Wheras if you put it on the condensate line, you will have MUCH slower cycles -- much longer to build to where it thinks it has established stream.. and then staying off much longer because it thinks the system still has heat to lose before turning on again.



Does that make sense?

sreja

condensate sensor

Having just installed my new tekmar 279 and posted about the *THEORETICAL* pros and cons of attaching the condensate sensor to the last condensate line vs the last steam main input line, i think practicality has to trump theoretic.. the condensate line was just not rising in temperature enough to be of any use at all.



putting it on the last steam line at least shows a big rise of temp on steam production followed by a reasonable fall after heat is turned off.. so i suppose i'm a convert to those who put the sensor on the steam line.



now the question i have is, why does the manual say to do one thing when all of us are doing another?

Dave in QCA

It makes sense, but....

Sreja, your line of logic of why it might make sense is certainly reasonable.  However, I will make 2 points as to why the manual is wrong. 

1. As you say, it will make very long cycles, and therefor wild swings in space temperature.  Remember, the indoor sensor never directly shuts of the boiler, it only provides feedback that affects the cycle "ON" time.  So, if the Tekmar fires the boiler for 45 minutes to establish steam, and then an additional 15 minutes (the time of a 25% cycle), it will cause tremendous overheating on what should have been a light cycle. 

2.  The placement for the condensate (steam established) sensor on a one pipe system is on the steam inlet of the radiator furthest from the boiler.  If we assume for a moment that the placement as shown for the two pipe system is actually correct, then the corresponding location on a 1 pipe system would be directly mounted on the last section of the furthest radiator from the boiler.  This of course, is not what the manual shows. 

I still maintain that the technical writer made an error, and it should be pointed out to Tekmar.

Frederick

Results

The Aquastat (label inside the temperature switch) requires the pipe to be at 200F for about 5-10min before tilting the mercury to disengage.  It takes about 30minutes for the dry return pipe to cool down to 130F (device set at 130F) and tilt back to re-engage, and about 15 minutes for 180F.  The temperature in the apartments continues to rise, about 0.7F during this period of time while the Aqustat has shut off the burners.  The apartments stay at the requested temperatures, as before using this device. 



The pressure gage at the pressuretrol max out  about 1psi and the pressure gauges at the air vents at the end of the dry returns pipes do not even register.  Hopefully this will save enough in gas bills to pay for all that pipe insulation.



Is the hysteresis in the Aquatat normal our am I lucky this old device degraded in this way?



Thanks,



Fred