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As I noted above, short cycling is not a concern with oil burners. They're commonly configured to maintain boiler temperature when idle with zero flow. With a fifteen gallon boiler and a 20F swing, a 100K BTU/hr boiler only needs to run for about 90 seconds, they seem to be able to do this for decades without ill effect. Any heating load is going to give longer run times.

Where you typically run into trouble with small loads — and this is not specific to oil burners — is with the distribution, specifically getting a circulator to work properly both with tiny loads and the full capacity of the boiler when it's needed. The solution is primary/secondary piping or a similar variant.

On buffer tanks, if you will permit me. And boilers. The problem is a practical one: matching the boiler output to the system input requirements. This is a common problem with steam systems, as modulating the temperature of the emitters is, while not impossible, impractical in most cases. It is much less of a problem with hydronic heat, as it is entirely feasible to modulate the emitter temperature to anything needed to match the load, and thus modulate the system power requirement from the boiler.

So now we need to consider how to modulate the boiler power output.

Many modern gas boiler designs can and do modulate their power output over quite wide ranges — as much as 10 to 1 — while under continuous fire, but with varying firing rates and corresponding air flow rates and, at the same time, maintain decent efficiency. With intelligent design it is not particularly difficult to manage boiler input and return temperatures at values which best suit the boiler, while maintaining the circulating temperature as desired.

Thus a modulating gas fired boiler will only be producing more power output than the system requires at very low power requirements — although even there there may be advantages in using a buffer tank.

The real need shows up with oil fired boilers, which are remarkably difficult make with a modulated power output: they are either on or off (note that his commentary is dealing only with residential and smaller commercial boilers, and is not applicable to larger, especially power, boilers, which can and do modulate).

If you cannot reduce the steady firing rate and hence power output of the boiler, the only other way to match the power output to the load is to turn the boiler on and off, with the duty cycle such as to match the average power output over time to the demand. Thus, for example, if the power output of the boiler is 100,000 BTUh, but the demand is only let's say 75,000 BTUh, you would run a 75% on duty cycle — which could mean anything from 90 seconds on in 120 minute cycle to 45 minutes on in a one hour cycle.

The function of a buffer tank is simply to provide water hot enough for the system use during the times when the boiler is off. The tank size will determine two things: first, the variation in water temperature available to the system as the boiler cycles on and off (which, using a thermal mixing valve, can be reduced to zero or nearly so provided the buffer tank water is always hotter than the desired system temperature) and second, relatedly, the total length of one cycle — with a larger buffer tank allowing both longer cycles and more even temperatures.

Now longer cycles are desirable (or very very short cycles, but that's another and more complex topic) as they allow the boiler to operate more efficiently, as well as causing somewhat less wear on the boiler components.

It must be added that the thermal inertia of the emitters is also a factor which enters into the judgement as to how big a buffer tank is optimal for the job.

Unfortunately, I do not know of any "rules of thumb" which are generally applicable, although Caleffi through its Idronics research and publications, has done a great deal to fill this gap; there are many variables which must be considered, some of which are related to the overall system, some related to the variation in power required, and some simply related to economics.

As a general thing, however, it is my opinion that for an oil fired hot water system, or a non-modulating gas fired system, a buffer tank is a required part of a competent design.

Stay safe out there everyone.

this is the instlal sheet that would come with a replacement lgm29

https://www.viessmann-us.com/content/dam/public-brands/ca/pdfs/doc/wb2_rep/vitodens_200_lgm29.pdf/_jcr_content/renditions/original./vitodens_200_lgm29.pdf

I am pretty sure they still had an older form of what they call lambda pro today, on those boilers. so the gas valve is most likely non-adjustable. but you would want to make sure both units were set to the same fuel type. Also there is a water pressure sensor inside the unit which can shut it down if water pressure is too low.

The donor control board may have been configured differently (DIP Switches and electronically stored parameters).