short cycling in shoulder season, fact/fiction/improper design ?

jp_2

in houses with FHA heating, I never see this as a problem.  early fall, late spring, furnace will run in morning, be off most of the day and run in evening, but they do not short cycle. short cycling- series of on/off on/off's in relatively short time periods. 



  so why do boilers apparently have this problem? improper design? improper set-up?



flaw in outdoor reset control strategy?

Charlie from wmass

Thermal mass of heat transfer media

There is a store and release when using water as a heat transfer media that air does not have. You just send more heat up the flue with the furnace in the shoulder months.

Jean-David Beyer

Short cycling with mod-con boilers.

Recall, that I am not a professional.



I have a mod-con boiler that can short cycle, especially in warmer weather.

I have figured out how to set mine up to reduce short cycling. I cannot answer your questions, "improper design? improper set-up? flaw in outdoor reset control strategy" directly, because they are all interrelated. And in the case of improper design, there are economic and practical limits to the designs.



The main problem I experience is that the modulation range of 5:1 that my boiler has is not enough to handle the range of loads the boiler sees. My total heat load (both zones) is about 35,000 Btu/hour when it is 0F outside, but design day around here is 14F. My boiler is rated to consume 80,000 BTU/hour and modulate down to 16,000 BTU/hour. It is the smallest mod-con the manufacturer makes.



Since my two zones have greatly different heat loads (one is about 26,000 BTU/hour, the other is about 6500 BTU/hour), if the small zone is the only one wanting heat, I am almost guaranteed short cycling because the boiler cannot get down that far. Is this design improper?  If they made a boiler half the size, it would work a lot better, but how many would they sell? Would it be enough for the manufacturer to recover the design costs? Would it be enough to warrant an additional production line to make the things? I suspect that it might be worthwhile in my neighborhood to have a smaller one available, since all the houses are about the same, so should have similar heat loads. If they designed it for a 10:1 modulation range, that would be fine too. I wish they did.



Improper set up? The installing contractor set it up to the manufacturer's defaults, that are not good for me at all. It meant it would heat, but with wide swings in temperature (downstairs is radiant with copper tubes embedded in a slab), upstairs ran 180 degee water when 135 was enough (so no condensing), etc. It is easy to set the reset curves for my system (separate curve available for each zone). So I made some changes there. I set the maximum and minimum points for the reset curves to what I calculated they should be, and that was an improvement. I then diddled them once a day for the first heating season and got pretty close. I found that the minimum temperatures I wanted for the water were too low in the sense that the heat given off by the emitters did not sink enough heat from the boiler, so it went into on-off mode. Raising the minimum temperatures helped. Running 75F water into a fin-tube baseboard just does not put out enough heat to use up all the energy even when the boiler is running at 20% modulation; I raised it to 110F and this helped a lot. I also set the range from +- 5F to +|- 7% to lengthen the cycles, and I set the maximum firing rate for the small zone to 55% instead of the 94% default rate. So now it is a question of whether it is short cycling or not. The slab zone does not short cycle. The baseboard cycles something like on 5 minutes, off 10 minutes except when it is very cold out, putting 135F water into the baseboards. So was it improper setup? Improper design (not enough baseboard). I do not know.



I do not believe there is a fault in the idea of outdoor reset, at least for houses like mine in New Jersey. If I had lots of large windows admitting lots of sunlight and heat during the daytime, I could have a problem requiring some kind of indoor reset, but not here in New Jersey with houses designed in the 1950s. I have a thermostat in each zone works pretty well even when it is warm out. Of course, it is now 57F outdoors and the heat from my computers (one is very big) is enough to heat the place. So the boiler did not run today.

jp_2

slight correction

I 've thought about this some more, the programming modification should be made at the boiler, or in the boiler control programming and not the ODR controller as in the tekmar.



I wouldn't really call it faulty programming, just an improvement that could be made.



this problem/complaint  of operation has come from the pro's here, I just thought I might have a solution.   more later...........

Jean-David Beyer

ODR programming modification

My boiler has all the ODR stuff as part of its built-in controller. If I did not use one of the thermostat inputs for DWH, I could have three outdoor reset curves. As it is, I have two zones each with its own reset curve (one radiant slab, one baseboard). All in the boiler's controller. So I have no need for a Tekmar or something like that.

CMadatMe

Trick Question

This is a trick question. There are too many causes to apply it to one. Let's not forget maybe one of the main causes. The dollar bill. "You pay for the Indian who fires the straight arrow, You get by with the arrow that grazes."

Fortunat

What IS a short cycle

A related basic question:



What constitutes a 'short cycle' in various boilers? High mass, low mass, mod con?



I've seen some data on cast iron oil boilers and how long they take to come up to steady state efficiency? what about mod cons?



If my boiler is firing at minimum input in the shoulder season and shuts off after 90 seconds and then starts again 60 seconds later and repeat on an off for 30 minute heat call, is that actually bad?



It's not like the boiler is starting from cold? other than a bit of extra heat loss during the pre and post purge cycles, what's the harm in the cycling?



If it is bad, what is the limit? 2 minutes? 5 minutes? 10 minutes?



If we had that number we could design around it....but I've not seen a credible number anywhere.



anyone ?



~Fortunat

CMadatMe

Definition

I would say anytime a heating appliance does not make "steady state" it is in short cycle..That's how a piece of equipment gets their AFUE. Now define steady state in a modulating piece of equipment running at different rates of modulation and water temp... 



This in itself brings to that fact how a mod/con achieves the ratings of over 90%. THE TESTING IS DONE 140 SUPPLY 120 RETURN! The boiler is condensing and NOT AT ITS FULL LOAD. Last I knew they don't set a heating curve and hook up an outdoor sensor in a cooler and force the boiler into full load with 180 degree water. If they did the boiler would only be rated at 87% AFUE. 

bill nye_3

Short cycle ?

? Just be glad the thing is short cycling, Who would want it to run all the time @ almost $4.00 per gallon ?

Jean-David Beyer

What constitutes a 'short cycle' in various boilers? High mass, low mass, mod con?

I have a mod-con with outdoor reset. It is a bit oversized for my house, especially in warmer weather. This thread has me thinking more about what short cycling is.



My boiler is cold start, so it really does start from cold or very close to it, since the boiler's heat exchanger holds only about 3 quarts of water. When running when it is cold enough outside and at least the large zone is calling for heat, the water temperature stays within the +|- 5F range. The firing rate wanders up and down within this range, and usually stays pretty constant. It can run 12 hours straight this way, firing at close to the minimum possible firing rate (20%). When running only the small low-loss zone, it must be very cold out to get even close to this. What I would need is the ability to fire it at about 5%, and it will not do that. So when that call for heat comes in, it starts at about 50% firing rate, and the water temperature rises very rapidly to the upper set point. The speed of the controller to respoind is too slow to get the firing rate down as much as it needs to, so the water hits the high limit and the firing stops altogether (bang-bang mode). Since the thermostat is still calling for heat, the water temperature gradually drops until it hits the lower limit and the boiler fires again. If I remember correctly, the total period of one of these cycles was a little over 5 minutes. I decided this was rapid cycling. 10 to 12 times an hour.



Since I could not get it to modulate any lower, and could not change the constants in the PID controller, I had to figure other ways to combat the problem. The best I could figure was a storage tank with an additional circulator. The existing thermostat and circulator would take hot water from the tank and heat the zone. The new circulator would keep the tank warm. That would not fit where my boiler and indirect hot water heater are, and would cost quite a bit of money, so I abandonned the idea.



Instead, I increased the spacing of the limits to +|- 7F and that helped a little. I did not want to go any further because the expansion and contraction noises increased as the differential increased, and that was enough. An interesting feature of the controller in my boiler is that I can set the maximum and minimum firing rates (within reason), separately for each zone. So for the zone where things happened too fast, I reduced the maximum firing rate from 94% (default) to 55%. This meant it took quite a bit longer for it to hit the upper limit, giving the PID controller more time to lower the firing rate. It now takes about 10 minutes to go through a cycle instead of about 5. I decided that is not rapid cycling. But it is not really steady state.

Jean-David Beyer

Who would want it to run all the time

I would, actually, as long as it modulates down low enough. Mine happens to burn gas that is less than $4/gallon (equivalent) these days. I can get it pretty close (12 hours a day) when it is fairly cold outside. To do that, I have to adjust the outdoor esset fairly closely to supply just the amount of heat required. There are three benefits that I can see.



1.) The house is kept at a much more even temperature.



2.) The return water temperatures are lower, so more condensing is obtained.



3.) With nearly constant water temperaturees, the expansion noises in my baseboard zone are less.



There is a disadvantage. My setting allow me to keep the house at 69F, which is where I like it. When I die, a friend inherits the house. If she wants it warmer, it probably will not go much over 70F, and they may think the boiler is too small or not working. What they would need to do is diddle the outdoor reset curves, but I do not think I could explain it to them.

Mark Eatherton

Its for real...

I can sit here in my own house, and hear my boiler short cycling directly below me in the basement, and my boiler is intentionally UNDER sized by a factor of 50% compared to my theoretical heat load calculations. It drives me NUTS, but I've made all the adjustments I can make within reason. I have a combination DHW/buffer tank, and I am going to have to instill a control logic that brings the buffer on line anytime the outside air is above 50 degrees F. That won't STOP the short cycling, it will only delay it and stretch it for longer on/off cycles.



The attempt to compare forced error system to a multi zoned hydronic system is no where near fair. Can your forced error system handle micro loads? Does your forced error system have a 5 to 1 turn down ratio? Does your forced error system have thermostats that are located in different loss zones, or is it one stat centrally located in the core of the home? See, no comparison... Apples to Oranges.



As for negative operating cost, the hardest thing you can do to ANY mechanical system is start it up. Once running, if you can keep it running, the efficiency is much higher. Every time a mod con boiler goes through a start up, it has a pre purge and post purge cycle that is not only chewing up KWH, but also blowing paid for thermal energy up the stack.



My personal definition of a short cycle in an OFF to OFF cycle of less than 10 minutes in duration. Anything less than that is considered a short cycle in my book.



A part of my short cycling is due to the manufacturers required minimum run time at a higher than minimum run speed for the burner to establish a stable flame before idling back, and that is NOT an adjustable feature of any of the gas fired appliances I've been exposed to. By the time it gets around to idling back, it has already over shot its ODR calculated temperature, and shuts down and post purges, constituting short cycling.



The same thing happens on larger multi family systems. In a lot of cases, the DHW system is the root cause of the short cycling, and there is not a lot that a person could do for that, other than setting a LOT smaller boiler to handle the standby losses of the DHW distribution system.



The only thing that I can see that would help alleviate shoulder season space heating short cycles would be a true indoor feed back loop, instead of a conventional on/off thermostat. We are dealing with a lot of theoretics, which if confirmed through solid state controls, could push the reset curve down, eliminating short cycling during shoulder seasons.



It is still an evolving science, but it is a LOT better than it use to be.



ME

Jean-David Beyer

Can you do what I did?

"By the time it gets around to idling back, it has already over shot its

ODR calculated temperature, and shuts down and post purges, constituting

short cycling."



I had short cycling in my small baseboard zone. My WM-Ultra 3 starts a cycle (cold start) at about 50% firing rate and fairly rapidly increases the firing rate to about 94% with the default limits. This results in overshoot before the PID control gets enough time to reduce it. Since I need nowhere near the maximum capacity of the boiler to heat that zone (even 10% firing rate would be too much), I lowered the maximum firing rate (for that zone only) to 55%. I did not know what would happen if I set the maximum to less than the ignition firing rate, so I did not dare. That kept the rate of increase in firing rate low enough so that the PID could catch up, reduce the firing rate down to 20%, and about double the cycle time. This would depend on the capabilities of your control system and might not be practical with what you have.



Of course this is not the same as downfiring the boiler, luckily, because I would not know how to do that, nor do I have the equipment to make the necessary tests were I to attempt it.

jp_2

eliminating short cycles, modifing ODR

ME I agree, you can't do much about DWH cycling, with recircs and the mini smart things get worst.   but machine cycles are the facts of life, you need to design them in, its really not that hard.  think of a punch press that cycles all its life.  or your poor truck that has to run from -25F to 100F with 170,000 miles on it.  think of the poor electronics that work in those conditions and then worst, think of an airliner flying from fairbanks to hawaii , thats a thermal cycle.



micro zones are the fault of the design, why in the world would you hook up a 500btu load to a 100,000 btu boiler?  a better design would be to try to move water from higher mass loads to the micro load between cycles, similar to your idea of moving water from south face rooms to north facing.  I understand if a customer wants a tstat for every radiant panel thats what you do, but thats not what this discussion is about.



to me, the fault in ODR/constant circulation is in the shoulder season, when loads are say 25-35% below minimum fire, system should switch over to a modified indoor tstat bang bang operation.  this will fluctuate indoor temps more, but in the shoulder season it should be hardly perceptible.  this also assumes your sizing is closer to actual.



I no longer think outdoor temp sensing only is the way to go, adding an indoor sensor and compensating  ain't rocket science







trying to keep the boiler running constantly in the shoulder season is a poor design when the FHA system stays off for hours at a time.  which would you call efficient? isn't a system thats off 100% efficient!!!

Unknown

Short Cycling Boilers - Would ECM Pumps Help?

Curiosity killed the cat...



Reading this post I am wondering the effect of a variable flow ECM pump during shoulder season heat loads (the toughest time to maintain comfort in my opinion).



In theory, boilers with excessive flow rates cycle more - makes sense that they cycle less with lower "proper" flows - one of the benefits of ECM pumps.



Do any of you have a project that was converted from constant speed circs to variable flow and a comparison of cycling times during shoulder seasons?  I would think ECM (variable flow) would reduce cycling but don't like to assume anything when it comes to heating.

John Mills_5

Forced air side of things

Being a dry head, I can answer the furnace part of it. They aren't designed to cycle on limit. Those $3 insertion limits tend to stick open if they have to open very often. The duct system should be designed to handle more air than the furnace needs to move to not overheat. Of course as homes are getting more poorly built, that sometimes isn't the case. So a forced air furnace will come on, run til the thermostat satisfies and shut off. So it isn't constantly cycling on high limit but especially if oversized, it may be short cycling just due to the high capacity and low needs of the house in mild weather. Also, probably 95% or more forced air jobs aren't zoned.



Boilers can cycle on the high limit of the aquastat repeatedly and usually do, especially since so many boiler installations are zoned. Or if they have outdoor reset, that is keeping the water temp down on a mild day. Just spent a couple hours with my favorite wet head and we agreed that boilers tend to be more oversized than forced air installations. He was consulting with us on our first mod-con quote, boilers being rather rare around here. His recommended size was lots less than I would have gone with personally but I sure trust this guy.



Speaking of oversized boilers, he advised a job with us years ago taking out 2 boilers totaling 550K input and replaced with a single 175K and proper piping for copper tube in floor heat. Customer said the house has never been so comfy.

jp_2

apples and oranges, nah...

""""""The attempt to compare forced error system to a multi zoned hydronic

system is no where near fair. Can your forced error system handle micro

loads? Does your forced error system have a 5 to 1 turn down ratio? Does

your forced error system have thermostats that are located in different

loss zones, or is it one stat centrally located in the core of the

home? See, no comparison... Apples to Oranges.""""



ME, i gotta disagree a bit here.  just because you can make a micro load,  doesn't make it a good idea, and the bucking modcon lets you know.  most boiler operated systems I have seen only have 1-2 tstats.



 how well does an iron boiler take these loads? my guess it acts similar to the FHA or its hiitting its high limit, which in turn is no good either.



 still doesn't make sense that the FHA runs once an hour and the modcon bucks and kicks trying to run constantly- a fault of ODR in mild weather..



i see no reason why not allowing it to cycle once every 1/2 hr (for example) and give it a 1/2 hr break would be a bad idea?  as you say on/off/on/off just sends heat out the vent and overshoots its set point, so you're loosing your constant circ modcon efficient then.