Vitodens 200 technical question on modulation

larry

I was going to essentially ask this same or similar question. When I've watched the boiler temp through some cycles it often seems rather bizarre. As an example your heating curve might be set for 129F at an outdoor temp of 25F, and as you watch the cycle the boiler control seems to always start some kind of learning over and over again as it might start out slightly below your target temp and then goes down to 85 to 90F before slowly ramping up to a sustained output near the target (plus or minus a bit). Is this the normal post "learning" (60 cycle) behavior? If so, why doesn't the control bring you up to your expected target more quickly?

My specific issue is that I'm using a Vitodens driving a pair of variable speed air handlers whose own control logic is to operate the water pump towards a goal of 20 degrees delta T and the fan at the slowest possible speed such that setpoint is reached in 15 to 18 minutes (the air is warm, but the air velocity is so slow it is not drafty in the least). Right now I'm operating on a heating curve with slope of 0.5 and a 22C offset. Last night was the coldest it's been since installation almost 2 years ago going down to 2F (design temp for this area is 0F). For the most part this setup is working well, the only exception being that if an air handler heating cycle exceeds somewhere between 15 to 20 minutes, the AHer goes into a catch-up mode where it starts ramping the blower speed to try and blow more BTUs off the coil and reach setpoint with the downside being increased noise and higher air velocity. Every once in a while this happens, and wonder if the Vitodens reached operating setpoint more quickly if this relatively minor annoyance could be avoided. Any thoughts on a better way to tune this setup?

Dave_4

modulating up


Does someone know how the Vitodens actually determines that it needs to modulate up? It seems like the boiler always tries to reach the target setpoint using the lowest modulation even if it takes a very long time. At what point would the boiler determine that it needs to modulate up?

Another related question is, how does the boiler determine that it needs to shut down? I used to think that it was when the temperature in the LLH exceeded the target by a certain number of degrees, but I now think it has something to do with how quickly the temperature in the LLH rises. If the rise if very slow (output slightly higher than load) then the boiler will keep heating and reach a higher temperature in the LLH than it would have had the temperature been rising faster.

Glen

your questions are related -

Viessmann modulating controls look at temp rise and loss very carefully. It is my experience that the rate of temp +/- outside is more important than other factors. Almost all Viessmann Mfg Co Inc controllers are programmed to mod up slowly, and the hystersis is the foundation. So your observations are correct - some temp overshoot can happen and sometimes it seems that the boiler will sit at a temp 2-3 degs lower than expected. Fear not - long slow burns are more efficient than a quick blast.

Mike T., Swampeast MO

The Vitodens measures and records information for 60 full operational cycles in its ODS (operational data store). If DHW is being provided it also measures and records information regarding DHW production. Just what information I do not know but it seems reasonable to assume that it's looking at deviations from target and boiler temperature change as a function of time.

After data from 60 operational cycles is collected the boiler is considered fully "educated". I do not know if subsequent cycles replace (or modify) stored data. I do however know that changing the heating curve via programming flushes the memory with learning beginning from scratch. The sun and moon dials are "special cases" and changing their settings does not appear to cause the memory to be flushed.

I also know that the Vitodens avoids making gross changes in modulation as Viessmann considers such to be inefficient.

My best guess is that the Vitodens uses this stored information to calculate a "base" level of modulation that should be just sufficient to maintain target temperature regardless of outside temperature. (Obviously this "base" level of modulation will change with changing outside temp.) It likely then tweaks the modulation in small increments should actual boiler temp (or its rate of change) vary from either target or some "acceptable" range of variance from such. I suspect the circulating volume of the system has much to do with how accurately the boiler "tracks" target with higher volume allowing for higher accuracy. In this regard, the boiler [could] have at least a relative idea of the volume of the system.

As to what causes it to shut down, I can only say from observation that it [seems] to be related both to some amount of target temp overshoot and the rate at which such overshoot builds. Both [appear] to change with time as the boiler "learns".

Note that with a low-loss header that "boiler temperature" is unlikely to be equal to the temperature maintained in the low-loss header (the target). When flow volume on the emitter side exceeds flow volume on the boiler side, the "boiler temperature" must exceed target to maintain such in the low-loss header.

Mike T., Swampeast MO

I see two distinct but related problems:

1) The operating philosophy of the air handler is in opposition to the operating philosophy of the Vitodens.

2) Neither the Vitodens nor the air handler know what the other is doing.

Problem 2) could only be solved if Viessmann made a controller for the air handler or someone else made an interface that controls the Vitodens based on the operation of the air handler. I'd say the chances of either are remote.

Problem 1) however might have some possibilities:

First, I would suggest that you disable the "boost" function of the air handler that [it operates] the fan at the slowest possible speed such that setpoint is reached in 15 to 18 minutes. If this cannot be disabled perhaps it can be set to take effect after a significanly longer period of time or perhaps reduced in magnitude.

The Vitodens does not want to satisfy the load anywhere near as quickly as 15-18 minutes! Both the original poster and I operate our Vitodens with the sun and moon dials in control. I know it sounds improbable, but the Vitodens can control room temperature without sensing room temperature. This means (provided load is above minimum modulation) that a heat call is continual with the burner operating at just the required level to prevent room temperature from either falling or rising significantly away from the sun dial setting. Such heat "calls" can last for days or even weeks!

Given the operation of your air handler and your heating curve, I'd say the Vitodens is doing it's best and that the variances you see in actual and setpoint temperature are "normal" for your system at your curve.

Your curve is quite shallow and has a lot of positive shift. From your observations, such curve is sufficient to meet thermostat setpoint within 15-18 minutes in most weather conditions. This tells me that the boiler is operating at a quite high output level--much higher than would be required if it took significantly longer to meet thermostat setpoint.

I have a feeling that the boiler is operating at a steady (or very nearly steady) and quite high rate of input during a heat call. If natural gas is your fuel, you can verify this by clocking the meter a few times during a heat call.

I also have a sneaking suspicion that even when your air handler starts ramping up blower speed when the thermostat isn't satisfied in 15-18 minutes that the boiler does not increase output too much.

I say this because it's "education" tells is that ramping up output significantly only forces an end to the load (thermostat satisfied) much more rapidly when it really wants the heat call to last as long as possible.

Again, if you can disable or modify the air handler "boost" function, do so. Then increase your reset ratio to 0.8 or so and reduce your shift to 15C or so. As long as you have comfortable operation wait for DAYS before making any further changes to the curve. During this time pay attention to the length of heat calls--you want them as long as possible.

Do you use daily setback for either or both air handlers?

Mike T., Swampeast MO

Vitodens Output Changes

The previous is based on my observation and conversations with Viessmann engineers when the boiler is modulating to maintain space temperature with the only change in the system being target as computed via the outside temperature.

I know of three ways to produce a gross change of modulation level:

1) Press the "party" button. (upwards modulation)

2) Significantly change the setting of sun/moon dial (whichever is active at the present time). (upwards or downwards modulation)

3) Significantly increase load. In my system that means raise TRV setting(s) significantly. (upwards modulation)

There is probably:

4) Change operation mode between normal (sun dial) and reduced (moon dial). I do not use timed setback so have not observed. (upwards or downwards modulation)

It's 3) "significantly increase load" that's most interesting to me. On numerous occasions I've clocked my gas meter before increasing TRV setting(s). In each instance, the boiler input level has increased by the time I get back to the meter and most importantly the increased input level is sustained. Sometimes I think that the 1st cubic foot (2 rotations of the 1/2 cubic foot dial) is faster than the 2nd cubic foot, but after the 2nd cubic foot consumption is consistent over time.

In my fully TRVd system, the magnitude of increased input is directly related to how closely the heating curve maintains the sun dial setting, e.g. the amount of "heat authority". Google "heat authority" for interesting results...

larry

My eventual plan is to implement panel radiators and limited radiant floor heating on the first floor so the AHer will be used mostly for AC. This will obviously require a second heating circuit. The existing AHer may also be setup as second stage heating as I fear going too high with floor temps. A heat loss analysis shows radiant floor may be insufficient without some type of supplement. The second AHer services the second floor. It's just completely impractical to change to something else short of gutting much of the house so the AHer will remain for heating and cooling.

Back to the current situation...

I didn't mean to make it sound so dire. It's probably not ideal, but still works pretty well. It's a 500% improvement over what it replaced. The previous oversized equipment never had sufficent runtime to allow good comfort and humidification via a flow through humidifier. In comparison, now I have such long runtimes that I need to carefully set the humidifier control to prevent too much humidity. Proper humidification is a huge IAQ factor, and we are all healthier as a result.

Unfortunately the AHer control is fixed. Both heating and cooling modulate based on temperture probes and time to satisfy setpoint and it's preset logic. I'd have to instrument the AHer and build-in some bypasses to affect the operation of the AHer. I've made some suggestions for changes to the manufacturer, but I'm not expecting any huge changes anytime soon. Given the lack of any standard for exchanging data between equipment, it does a pretty good job balancing comfort and efficiency. I've toyed with the idea of engineering my own control which could address these issues, but that would be a huge undertaking. For however gentle this AHer is, it is still moving air and that is the reason for the heating curve thats been set. Compared with other AHers, this is operating at a much lower boiler temp than you normally see (160F and higher are common), but it's a comfort issue if you start moving air that isn't perceived as atleast warmish. This is why the curve with it's offset starts out at about 109F with an outdoor temp of 68F. At design temp the boiler temp of 139F seems to be just fine as the AHers haven't gone into "catch-up" mode. With an AHer it seem improbable that you could modulate output well enough over long-enough time periods. I'll try to do some tracking of the gas meter as you suggested. From watching the exhaust coming out of the termination, modulation is happening. It not quantitative, but one AHer vs. two are different, as is warmer vs. cooler weather. DHW production seems higher than anything else. None of this is the same level of fine tuning you've decribed for your own system. I've often wondered if a buffer tank might add value to my system by dampening the wide temp range used by the Vitodens through a heating cycle. I think the AHer would rather see a more constant temp through it's cycle.

I tend to agree with you that the operating principals have similar goals, but conflict in operation. I didn't mention the whole thing with the AHer. It actually tries to operate almost continuously, but in a very different way from the Vitodens. After the AHer meets setpoint, it actually continues to operate for an additional 15 minutes at an even slower speed. It's goal is to have very long runtimes, operate at slow speeds and temper the variations between setpoint and 1 degree below setpoint.

Dave_4

In the past two days I've experienced the coldest weather since I last changed my heating curve (3 months ago). What happened is that the heating curve was calling for a setpoint of 51C but the boiler never came close to reaching that setpoint as it idled at 44C for most of the day. Is this normal "learning" behavior or is it more likely that some other setting (such as poti adjustment) was limiting the output of the boiler preventing it from modulating up to reach the setpoint?

McKern

Martin...

It just sounds oversized actually.A 6-24 might have been too big for just the heating side as well. Do you check your email?

Mike T., Swampeast MO

Any problem with indoor comfort?