Vitodens short cycling

Dave_4

My Mod-Con is normally modulated by an external reset control which would modulate up fairly rapidly to target temp and ramp right down and shut off. I used two relays and a fixed voltage source to override the external control with a voltage that produces minimum modulation at the boiler. The override control is bypassed and the external reset control resumes modulation duties if a second stage call for heat comes from the one (1st floor of 3) thermostat in the house that calls the most for heat.

The override greatly increased cycle length and keeps the Mod-Con efficiency high per the low fire/heat exchanger ratio and longer cycle times. The second stage usually doesn't kick in until about the 13 to 18 degree outdoor temp range.

DHW is more than adequate at low fire, but I could add a relay to allow the external control to manage DHW modulation if I wanted.

For ease of service, if power is removed from the override control(s), the external reset modulation control operates normally.

The override function I have described could be implemented with a relatively simple software modification to the external reset modulating control, should the manufacturer desire.

Ron Gillen

Vitodens short cycling

In reading the posts in "Mod/con choices for small heating needs" a lot of emphasis is put on short cycling. This morning it is 32F and the Vito runs for about 5 seconds every 5 minutes.I don't think this is particularily inefficient but would seem to over use parts of the boiler. Should there be a concern for life expectency or is it just annoying?

Mike T., Swampeast MO

Viessmann tells me that such is considered "normal" operation when the load is very low. I call it "pulse mode"--could likely be described as "batch mode" as well.

What surprises me most about that mode of operation is that efficiency remains very high. Depending on how my TRVs are set, the boiler will heat the house solely via that mode down to the mid 30s to low 40s F.

I was attempting to eliminate as much of that sort of operation as possible via solar integration, but I can't find a space for the panels and my idea of using neighboring public University property was nixed by their lawyer (but approved by everyone else).

Mike T., Swampeast MO

For the 2005-06 heating season I conducted my "Eurocave" experiment. I kept the unoccupied areas 50-55F and the occupied about 60F and found a reset curve that could BARELY maintain such space temperature.

I have the boiler and piping connected to a datalogging system for various temperatures.

The boiler would "pulse" down to the mid 30s F. This about 3,300 sq.ft with 1,700 sq.ft. of basement. 1903 home with LOTS of original windows, "reasonable" but very well installed insulation (mainly fiberglass and cellulose) and hideous north exposure.

I could actually capture the pulses via the datalogger. Am not looking at my numbers right now, but if memory serves it operated in pulse mode for about 40% of the season with somewhere around 13,000 pulses.

After accounting for the reduced space temperature, efficiency as measured by therms per degree day--for the season as a whole--increased significantly.

Certainly some of that increased efficiency was due to the lower reset curve, but with "pulse mode" accounting for about 25% of operational time with more normal room temp compared to about 40% that season it seems reasonable to assume that "pulse mode" does not take much of a hit on efficiency.

I examined the direct spark ignition elements VERY carefully during cleaning before this season. Some very small but noticeable spots of discoloration but utterly no pitting. The perforated metal "dome" around the burner unit remains pristine. As I once said in a review (used to be here under "On the Job" but the entire category seems to be gone), "the direct spark ignition system is built like a tank".

I must agree with you that this "pulsing" mode of operation seems troubling. Even though I'd like to reduce it greatly all I can say at this point is that the Vitodens is designed to operate this way to maintain efficiency and that it's built to take it.

Glenn Sossin_2

Efficient ?? I don't think so

Cycling equiptment excessively shortens life span of the components. Realistcally, there is a finite number of times a relay will close, or a fan will start. Something doesn't make sense here.

Think of a light bulb rated for 2000 hours of useage. What would happen if you turned the light switch on for 5 seconds, then off for 30 secs and then on again for 5 sec and so on. Bet that bulb won't see 1,000 hours of useage.

I never got good gas mileage in stop n go traffic. Never knew anyone who claimed to either.

Mike T., Swampeast MO

That was certainly my original feeling as well when I first observed "pulsing" operation. After careful observation and measurement however it truly seems to be an efficient way for the Vitodens to deliver heat. As to the duty cycle of the components involved all I can say is that it gives all appearance of having been designed with this type of operation in mind. At least the circulator stays running...

jp_2

well then

we might as well stop all this talk about smaller mod/cons if they run bang/bang as efficient as steady state.

I can not see where a 5 second burn can effectively and properly heat the burner for best radiant heat transfer to the HX.

jp_2

how long

are your pulses? 5 seconds like the above post?

Mike T., Swampeast MO

3-5 seconds once the "burner on" LED comes on. 6-8 seconds from the first relay click when the blower comes on [sure sounds like full speed] to clear the combustion chamber.

Boiler temperature will indicate about 10F below target when the cycle begins. When "pulsing" the boiler temperature will change ONCE to a value at least 10F higher than target within those few seconds.

The first few seconds of firing of the Vitodens always appear to be at full input. Hard to clock the meter for such a short event, but by the quarter foot dial, full input [seems] right and the sound is identical to test mode which the documentation states is at full input.

After those first initial seconds of [presumed] full input I ALWAYS hear a drastic drop in fan speed when the boiler modulates down to minimum input (I've clocked the minimum input MANY times).

Mike T., Swampeast MO

Does any other mod-con operate in this manner?

From what I can understand it's the radiant nature of the burner that allows surprisingly high efficiency during the "pulses".

hr

fancy terms for a \"short cycle\"

I can't see how that could not effect component wear and longevity. Even start and stopping the combuster fan seems like inefficient use of KWs.

I'd swear the Munchkin Contender I have in the shop starts on it's lowest fire and stays there if the loads are small. Although the fan does "high speed" for a period to clear the combustion chamber.

Seems starting at full boiler fire would increase the short, I mean pulse, cycling?

That was always my biggest gripe with the MZ's. They had that large diameter blower wheel that took what seemed an eternity to rev up, then seconds later the same long wind down time.

Hearing that several times a minute really drove me crazy. Plus all the related relay clatter, and gas valve solonoid clicking involved.

I'm thinking that tank style mod con HTP is producing may be the ideal ticket for micro loading.

hot rod



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CC.Rob_3

temp rise

I find this discussion fascinating.

I want to make sure I'm following correctly. In five seconds, the boiler temp has a 20F temp rise?

heatboy

I can't wait............

........for a high mass mod/con to arrive (Viessmann has a beauty across the pond). I have never understood the reason for small boilers, which aren't any more fuel efficient than bigger ones. Buy low mass and then add mass to make it work. Silly.

hb

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Xc8p2dC

My Ultra's does similar

at very low needs when running constant circ. I describe it as "Space Shuttle control burns"

When my minimum modulation is reached [16K Btu] and 16K is still too much, it fires for only the time needed to keep needed circ'd water supply temp. Usually 15-30+ secs

Mike T., Swampeast MO

I certainly agree that it has to affect component wear and longevity in a negative manner. I've talked with a number of people at Viessmann on numerous occasions. All say that such operation is "normal" for a system with a single, directly connected, TRVd circuit.

Nobody has confirmed that the initial few seconds are at full input, but I'm almost positive such is the case.

I'm just glad I listened to you and others and used the 6-24 instead of the 8-32 technically called for by the heat loss calculation!!!

I've done everything I can possibly think of to eliminate this behavior, but nothing works. When the load is very low it's just the way it heats the house.

Mike T., Swampeast MO

Yes, at least a 20F rise in that short period of time as reported by the boiler via "boiler temperature". There is however only ONE "update" of "boiler temperature" during this period. My datalogs show a peak rise of about 5F but I'm sensing supply temp on the surface of black steel about 3' from the boiler.

When I first observed, I thought that all of the TRVs were closed and I was getting full or nearly full bypass (either through the outboard differential pressure bypass or the small one that's built-in). I adjusted the outboard bypass such that bypass was impossible but still wasn't positive about the built-in bypass

While I still don't have a flow meter the "Eurocave" experiment showed me positively that there was significant flow through the system when the boiler was "pulsing". First it kept the house heated solely via this mode as long as the temp didn't drop below 35 or so at night and even if daytime temp didn't rise above the mid 40s. Granted I kept the house cool, but not that cool! Also, I measured radiator temps MANY times. A few were virtually ambient. Most were near 72F with extremely low delta-t--say 1-4F. I kept finding 72F throughout the supply side of the entire system.

200+ gallons and 1,049 sq.ft. EDR of cast iron radiators. It would [seem] impossible for there to be a 20+F rise in the boiler in just a few seconds given even a couple GPM of flow. Given the low velocity due to huge original gravity piping I don't see any reasonable way for the water that's heated to physically make its way through the shortest flowpath in the system back to the boiler.

One guess I have is that if the boiler senses much (say just a few degrees) of temperature rise in those first few seconds that it generates a "false" signal with a significantly higher "boiler temperature" to force a shutdown.

Once the load increases, the boiler will fire (usually for at least an hour) at minimum input (verified MANY times). If load increases further it will begin true modulation and may stay firing for days at a time interrupted only by a brief, once-daily self test. If load stays below minimum modulation supply temp will eventually rise to some degree (around 10 it seems) above target and then the boiler will shut down. Depending on how close the load is to minimum input, my logs show this taking anywhere from about 40 minutes to about 5 hours with about 2 hours typical.

Steve Ebels_3

I'll second that

Cycling is the Achilles heel of ALL low mass mod/con boilers. (not great for high mass either) I'm not saying that hordes of these units will die an early death because of it but some will, especially where the installer is not well versed in the proper application and sizing of these fine units. I couldn't agree more that having to add system mass (buffer tanks) is somewhat backward. Why not start with a higher mass product that is designed to diminish the tendency to short cycle? For me, nirvana would be a boiler that turns on and off once a year. Probably impossible but that would be the holy grail of mod/con operation.

ALH_4

mass

I agree completely with that statement. Efficiency and longevity are somewhat separate issues from each other. Even if the Vitodens is efficient in "pulse mode", components are wearing out.

The use of buffer tanks to make low mass boilers does not make sense to me. It points out to me that, absent of very high turndown ratios, mass is needed for these boilers to keep from short-cycling (or "pulsing") themselves to death. For this reason, I prefer the fire-tube designs like the Prestige to water tube designs like the Vitodens and Munchkin etc. Not to say I prefer the Prestige to the Vitodens, but I do like the water content. If only TT had 80MBH and 50MBH models, and Viessmann had a 50MBH Vitodens...

Mike T., Swampeast MO

Stupid me thought that my big gravity piping would act as a buffer. Actually it [appears] to do so, but just not large enough.

See why I'd truly love to integrate solar to my system?

Barring the solar, I'm almost at the point of considering a small electric water and using a three-way valve to drop it in as a 2nd heat source in series with the Vitodens when outside temp is below some level. The heat it provides will keep the boiler temp from dropping thus preventing it from "pulsing".

Mike @ Capco

Hey bud, try going to coding 2 and change address 045 value 065 to value 100. This will give your pump its full operating parameters of 700 to 2700 rpm's.

Good luck. Later

The iceman

Uni R_3

A few questions/obervations...

Does the Vito ignite on these brief firings at the same ignition fan speed as it does on higher load cycles? If it does, it may be simpler for the controller just to hold the fan at ignition speed for the short burn time. Fans take a while to spool their RPMs up and down.

At full / 90K input for only 5 seconds every 5 minutes, the boiler would only be adding 1.5 MBH at the absolute most (or about 4 100 watt lightbulbs worth of heat). Essentially, operating as described it would be doing a dozen infusions of 125 BTUs or less each hour.

How does wear and tear on ignition devices on boilers in general compare to gasoline engines? Most gas-powered vehicles do 15 ignition cycles a minute per cylinder just idling. Compared to an automotive ignition system, 14000 cycles would typically be less than 10 hours of engine operating time.

There may be several reasons why Viessmann took this approach. As I think you’ve said, it could be testing the attached heating system to see what’s happening with the token amounts of heat being added. In addition, it may simply be a matter of trying to apply a very small amount of heat as evenly as possible while sacrificing a very marginal amount of lifespan of the ignition components in return for optimal comfort rather than adding larger volumes of heat each cycle.

Have you seen any efficiency graphs for the 6-24 for very low loads? The ones that I have seen usually cut off right at 30% of maximum load.

----- ----- ----- ----- -----

As for other modcons, the Prestige adds heat in 10 minute long burn cycles when it is below the minimum modulation threshold. It then varies the waiting time between these 10 minute burn cycles. That’s about 5000 BTUs each ignition cycle as opposed to 125 BTUs or less; very different operating characteristics between these boilers under low load conditions.

jp_2

I disagree, politely...

high mass low mass, when you are 10-20Mbtu's over your heat load, you are going to short cycle.

seems to me, thats why the iron boilers appear to have such low efficiency compare to a mod/con- shoulder season.

and then, whats the difference between a high mass boiler and a low mass boiler with a buffer? with a buffer you can select when you need it, no choice with a hi mass boiler.

also, though in princple I agree components wear with cycling, I assume most of these components where initial design for manufacturing machines. machines designed to cycle all day long, often 20 hours a day, 50 weeks a year.
think of bottling machines, paper machines?

so, the components should be able to cycle several million times without failure. a boiler will short cycle a short time of the year, off most of the year.

ALH_4

cycling

I'll buy that, except cycle times will always be increased with increased mass. It gives the manufacturer of the boiler some control over whether the appliance will destroy itself or not. The mass never hurts. If deemed necessary, using a buffer tank as the low loss header is a good idea in my opinion given the mod/con choices available currently.

What I was talking about was eliminating the 5-second "pulses". If the boiler will run 2 minutes instead of twenty-four 5-second pulses, the potential wear is decreased very significantly.

These boilers may be designed for several million cycles. I have never seen this number from any of the mod/con manufacturers, but I am sure it exists somewhere.

jp_2

It is!

just an unisulated buffer, unless all your pipes have insulation. anything between the boiler and radiator is a buffer, perfect insulated pipe or an un-intentional radiator.

jp_2

i agree andrew, but

where does the heat go during those longer heat cycles in high mass?

if it goes where you want it, you over heat the structure, if it goes elsewhere you've wasted it.

but i agree 5 sec on cycle doesn't seem sounds as far as efficiency, maybe control wise I see it.

Mike T., Swampeast MO

The only part of the piping that's not nicely insulated is along the north end of the basement. There's a two-car garage down there and the north wall (40') is completely above grade. I left the mains and branches in the garage intentionally uninsulated to give a bit of heat down there.

The rest of the piping has 3/4" minimum Armaflex and is then wrapped with heavy rubber 3-M "Arc-Proofing" tape from a surplus auction.

ALH_4

mass again

Constant circulation with outdoor reset wouldn't be as bad with high mass, although on domestic call the mass could be pretty hot. As long as there is a post purge into the DHW tank, it might be ok, but that would depend on how hot the DHW tank was being run.

So maybe 5-gallon low loss headers would be a happy medium? It might lengthen the 5 second cycles into a few minutes. I'm picturing something that looks like a small vertical expansion tank with ports in the top, sides, and bottom.

Mike T., Swampeast MO

It's impossible to visually verify burner operation so I have to go by sounds and the green "burner on" LED.

When a cycle begins you hear a quick "click-click", short pause then another quick "click-click". You hear the blower fan come up to speed quite rapidly--I think the blower starts after the first "click-click".

Then after a brief period I'm fairly certain I hear the fan ramp down a tiny bit and I might be able to hear the gas valve open. A few seconds later the green "burner on LED" lights. If this is a "pulse" the burner will shut down about 5 seconds later. If NOT a "pulse" you hear the blower ramp down.

See the attached operational sequence.

According to Viessmann literature, the ignition spark is initiated 0.2 seconds before the gas valve opens. Combine the <5 second "safety time" AFTER which the green "burner on" LED is lit and the 5 second "flame recognition" phase and it looks like the burner can be firing for nearly 10 seconds.

Also according to Viessmann literature, "...after the safety time has elapsed and the flame recognition has taken place successfully, the burner control unit modulates from its <I>ignition load according to the currently selected setpoint value. [Emphasis added]

No mention is made of the actual "ignition load", but it wouldn't surprise me if such is greater than maximum input during modulation.

Sorry for yet another automotive analogy, but it's almost like starting a cold carburated gasoline engine (press accelerator to floor, let up half way then crank) combined with the initial "heat the glow plug" phase of a Diesel. There's even a very curious element of the system that looks exactly like the choke on a two-barrel carburator. Of course you can only watch its action with the combustion chamber cover removed and considering that such chamber is extremely well-sealed I have a sneaking suspicion that the action you see when the cover is removed is different than when it's installed.

The time I observe between "pulses" varies considerably. I've seen as much as 15 minutes and as few as 2.

As to graphs of efficiency below 30% of maximum load, the ONLY place I've seen such is in that "Condensing Technology" white paper. You'll find it in the section under "condensate production". It shows increased efficiency but I've always thought such was more theoretical than actual... BTW, a rather large "plop" of condensate comes out at the beginning of each pulse when the combustion fan first ramps up. Perhaps I should measure accurately...

Mike T., Swampeast MO

Questions for Original Poster

Ron:

Does your system use a low-loss header?

Does it use the RS remote control?

Mike T., Swampeast MO

\"Pulses\" and Mass

No need for a "post purge" with a directly connected Vitodens as the circulator is running constantly anytime that the outside temperature is below the sun or moon dial setting (whichever is active).

The LLH volume comment made me think--again--and as always dangerous...

Since there is no thermostat connection to the Vitodens and it ALWAYS fires to maintain the current target as closely as possible--even if forced to "pulse" to do so--the ONLY way it "knows" to fire is if current boiler temperature drops to some level below the current target. This will be true regardless of whether or not the LLH header is used.

Say the LLH header is used. The primary system volume will be quite low--it's intended to be quite low. Unless the Vitodens itself is controlling the secondary circulators (unlikely in "American-type" systems) it has NO IDEA of the secondary load until it looks. How does it look? It initiates a firing sequence when the boiler temp drops to some point below target. If the boiler temp rises rapidly it assumes "no load" and shuts down--otherwise even a short period of minimum modulation would exceed the safety limit.

Now say this is a DIRECTLY CONNECTED system. The Vitodens--at least initially--has no idea of the volume of such system so it might make the same assumption if the boiler temp rises X degrees in X seconds of intial "safety" and "proving" operation.

While I'm convinced that an "educated" Vitodens DOES have information regarding the mass/volume of a directly connected system I'm not sure that programmers have found a way to separate such from the "noise".

Even though I'm "System Type 1--one directly connected circuit" the Vitodens [might] be treating me as if I were connected to a low-loss header...

The alternative is that the "pulses" are actually the most efficient operational mode.

jp_2

how on earth

could the system 'figure out' the volume of your system?

don't you just set a reset curve value? doesn't the boiler just chase after the target?

heatboy

Stored Energy.............

........certainly isn't a bad thing. Vitola boilers do it all the time and make plenty of hot water while doing full boiler reset, providing the 300 tank is used. I have yet to see a Vitola short cycle at anytime or with any load, or lack thereof. Off cycle losses with a Vitola are also negligible, even when hooked to a chimney.

I want a high mass, condensing, modulating matrix burner boiler. Simple and bulletproof. Too much to ask?

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Mark Hunt

Whacky thought here


At some point if a mod boiler was able to mod down to the 10k point, wouldn't the mass of the connected load be enough to negate the need of a buffer?

One other thing. If the heat loss of a structure is 10k or less, what are the odds that heat will actually be needed OTHER THAN the heat gain from other appliances in the structure?

Finally. What if the WWSD was set to a lower number, say, 50°? An alternative heat source could be utilized on days from 65° to 51°. Like............. a sweater.

I have had MANY customers tell me that they do not turn the heat on until the end of October. (Upstate NY climate)

I would like to see smaller mod/cons too but only because I run into buildings that only require the lowest firing rate currently available on the coldest day. I just finished a heat loss on a 1300 sqft. home. Design day requires just under 34k/hr and that is with all of the built-in fudge. Does it make sense to build a mod/con that goes from 10k to 40k?

Mark H

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ALH_4

pulses and efficiency

So you are saying that the volume is designed to be low in order for the boiler to have knowledge of the rate of temperature change of the boiler loop so the firing rate can be adjusted accordingly? That water volume could vary dramatically. Would a long boiler loop with a relatively high water content or a buffer tank confuse the Vitodens?

I cannot imagine that 5-second pulses are the most efficient way to operate. They might be relatively efficient but cannot be the most efficient. Continuously variable burner firing rates have to be the most efficient. Otherwise why are we using modulating boilers in the first place?

ALH_4

small boiler vs. high turndown

Does it make sense to build a mod/con that goes from 10k to 40k?

I think so.

Mike T., Swampeast MO

Would a long boiler loop with a relatively high water content or a buffer tank confuse the Vitodens?

Possibly. I can see how it could, but I do not know. That's the question I raised.

Perhaps all of this was thought of with the "uncompromising" boiler.

Mark Hunt

Thanks Andrew!


Care to elaborate why?

Mark H

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Mike T., Swampeast MO

TIME!

The volume/mass of the system will change the time required to "chase after the target".

Ron Gillen

Mike T.

Hi Mike. I have a 6/24 with low loss header and vitocell. Main floor staple up with plates, tube in basement slab with mixing valve.All wired for stats in every room but none installed. Don't know if that will ever happen as I'm running on reset and with no adjustments since last fall the house is always 70F up and down. 1677 sq.ft. walkout bungalow of ICF construction, Edmonton, Alberta.We're supposed to have some Viessmann guru's coming to the foothills conference next month and I'm hoping to get some further insight from them if they have time for me. I really appreciate your sharing your experience with this great machine. Thanks, Ron.

Mike T., Swampeast MO

Ron:

From everything I've observed and studied regarding the Vitodens your system is operating and learning exactly in the manner it's designed.

Presuming you're pleased with the comfort, leave things be, enjoy the energy savings and don't even think of installing the on-off thermostats.

If you find any need/desire for daily setback do so ONLY via the built-in timer.

The pulses/micro-cycles/short-cycles or whatever you want to call them are both expected and normal.

Yes, it's a great machine--the most amazing machine I've ever had the pleasure of observing. From all reports here there is nothing else like it so it's VERY difficult to compare.

hr

Make the buffer serve multiple functions

instead of just "parking" excessive btu's until needed.

If it were a well insulated reverse indirect, for example, it could also heat, or at least preheat the DHW.

If it were to serve as the LLH, air seperator, DHW preheat, and even a mounting stand it would be nice.

Sounds a bit like the early Voyagers, if they had modulation

If Aerco can build a boiler to run from 100K to 2 mil...

Surely a small step modulated could be built with 5K step each to modulate.

Basically a row of Bic lighters, I suppose

hot rod

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