Vitodens Reset Curve

rob_26

) Set your slope to the minimum recommended for your type of system: 0.2 for high mass radiant floors; 0.8 for standing rads and "staple-up" radiant; 1.4 for baseboard & hydro-air.
what do you mean by slope is that the setting for the odr control say for a c.i. high at 190 low at 130?

Mike T., Swampeast MO

Viessmann told me that Americans typically operate their Vitodens with a curve that is significantly too high.

I'm really starting to agree...

Coldest air so far blew in yesterday afternoon. Freezing around sunset, gusty NNW winds and down to 21° (sustained low 20s for 7 hours).

Boiler temp reported by Vitodens nailed on target.

At 21° outside:

105° boiler temp and boiler target.

95° supply temp measured 3' from boiler.

95° flue temp (flue temp stayed utterly nailed at the measured supply temp through the period).

78° return temp.

Calculated heat loss: aprox. 45 MBH/hr

Boiler input: 25 MBH (clocked twice)

Gas consumed in 24 hours (8:30 am - 8:30 am): approximately 2.5 therms for a 3,400 sq.ft. 1903 home.

This is the highest sustained differential between flue gas and boiler temperature that I've measured; with flue gas right at 10° less than boiler temp and essentially identical to the "buffered" (gravity mains) supply temp.

Granted this is fall and I certainly believe that it takes less heat input when average outside temps are on the way down compared to when average outside temps are on the way up at the end of winter, but 45% less than the calculated loss???

Boiler kicked out of modulation at about 8:30 a.m. with 31° outside temp (at the N) and 34° outside temp (on the sunny S). Return and supply are again stabilizing near 69° and 75°...

Inside temp is being maintained about 5° cooler than last year, but for similar weather, gas consumption has plummeted.

For those with a Vitodens who want to extract the maximum fuel savings I HIGHLY suggest that you find just how low (and flat) you can go with your reset curve.

1) Set your slope to the minimum recommended for your type of system: 0.2 for high mass radiant floors; 0.8 for standing rads and "staple-up" radiant; 1.4 for baseboard & hydro-air.

2) Gradually (like every two days) reduce your shift (you can go negative) by 1°C.

3) If you're using timed setback, reduce the amount of setback by increasing the moon dial setting by 1°C each time you reduce the shift.

4) When you can no longer maintain your desired TRV or thermostat settings (or you feel truly too cool at ALL times) increase the shift by 1°C.

5) If the only time when you cannot maintain your desired TRV or thermostat settings is when the weather gets cooler, increase your slope by 0.1; wait; increase more if necessary then attempt to decrease the shift.

Mike T., Swampeast MO

Slope only determines how much the supply target changes with changing outdoor temp: with a slope of 1.0 supply target increases 1° for each 1° drop in outdoor temp; with a slope of 1.2 supply target increases 1.2° for each 1° drop in outdoor temp; with a slope of 0.8 supply target increases 0.8° for each 1° drop in outdoor temp.

The shift of the curve is a fixed amount of temperature change at ALL outdoor temps; 5° shift means the target is always increased by 5°; -5° shift means the target is always reduced by 5°.

The origin of the reset curve is the sun (or moon) dial setting. Moving the sun (or moon) dial is the same as changing shift via the user-accessible programming.

In theory it's possible to have the "perfect" program curve where the boiler nearly exactly maintains the sun/moon dial settings even though the Vitodens ordinarily has no indoor temperature input.

tom_49

mike, would you say the same applies for those of us w/ a vitola?

rob_26

what would be the best reset curve for a c.i. with a honeywell aq475 based the design low of 10 f?

tom_49

rob, try what mike says and keep dropping your high limit ( 1o C every 2 days ) temp. until your house is not heating up. At that point, raise it very slightly. As far as low temp, make sure its not below 130o.

Constantin

Why not?

I have been gradually reducing the slope on Ms. Vitola... as soon as I discover that all the circs stay on most of the time, I know that I have the slope at the hairy edge. Meanwhile, my RFH system is happy at 0.6, while 0.5 was calculated, and 0.4 is probable (taking the ACCA padding into account).

I recently discovered that the only thing the Vitola won't stand for is sustained return water temperatures of 50°F or below. In a residential installation, that should not be a problem.

The other interesting observation I have made is that as long as Ms. Vitola is making hot water that her boiler temperatures do not drop much below 100°F. Her insulation is so good that the boiler temps have no opportunity to drop much, even if the circs are running. That may change as the weather gets colder. Time will tell.

Mike T., Swampeast MO

Since the Vitola has no low-temperature limit, this would [seem] to apply.

Mike T., Swampeast MO

With a cast iron boiler (think that's what you're saying) you're likely to dip well below the recommended return temps way before your curve is low enough.

The only way this could apply to a cast iron boiler system is if you have primary/secondary and can accurately adjust the secondary (radiation) side temps via reset (ourdoor and/or indoor) while still maintaining the required return temperature in the primary (boiler) side.

Remember: mine is a Vitodens under "direct drive". Fully TRVd and no low-loss header. ALL flow is passing through the boiler. Am not certain if you could get the curve so low with the low-loss header and wall thermostats. Am rather certain that other condensing/modulating boilers using primary/secondary piping would behave quite differently (even though you could use nearly the same reset curve).

The beauty of the wall-hung Buderus is that the indoor reset and vector driven indoor temperature controller are attempting to automatically optimize the reset curve.

rob_26

so having the high at 170 high & 130 low do you think that the baseboard would emit enough heat for convection & having a bypass with temps running at 154 supply @158 & return at 143 based on 180 high low 130 so by lowering the high & mainting the 130 low would you think the boiler would short cycle based on a shorter span?

S Ebels

BB temps

I have several BB systems, two of them in apartment buildings that never see anything above 155* supply temp. Even on design days. I was fortunate enough to be asked to design both of these systems and recommended/installed about 20% more BB than you would usually find in this type of construction.

(additional info) Both of these apt systems are tied to Buderus Iron, G334 series with Buderus' 2107 control. This combo enables the system to run down into the 100* range before bringing on the burner and temporarily interuppting circulation while the boiler heats back up. At about 125* the circ will kick on while the boiler continues to the calculated setpoint on the 2107. I designed these systems figuring a max output from the BB of 350 btu's per lineal foot. It is of course capable of much greater output given traditional water temps but lower temps maximize system efficiency as well as comfort and boiler efficiency.

In other words, if you have enough BB you can certainly play with your reset control to obtain optimum system performance with lower than usual water temps. If your system is shy on lineal feet of fin you will be locked into high water temps and have very little room to move your heating curve around.

If the boiler you have is a traditional type Cast Iron job, you have to stay above 140* boiler temp to be condensation free.

rob_26

i have enough bb so based on that i think lower supply temps. would benifit me the only thing is the shorter cycle times based on the termistat on a one degree differatial so say low at140 high at 160 20 defferatial that would seem to have a short cycle

Mike T., Swampeast MO

Rob,

No matter how you adjust the reset curve, you won't be able to achieve anywhere near these apparent whole system efficiencies with traditional a cast iron boiler.

Condensing and especially condensing/modulating boilers operate with the highest efficiency when the supply/return temperatures are at the lowest possible level. As supply/return temperatures increase, efficiency drops. This is the opposite of a traditional boiler whose efficiency increases as both supply/return temperatures and especially load (e.g. boiler burn time) increase.

Again, I'm sacrificing some comfort for fuel efficiency. When I compare my measurements to Vitodens ratings, I'm getting the maximum efficiency possible for the boiler, at or even slightly above 98% for outdoor temps/conditions quite close to "average" for the heating season as a whole.

Not too shabby for a heating system (iron radiators) that many in the US view as hopelessly antiquated...

rob_26

i total realize that i would not achive that with a c.i.boiler but what would you think would opimize my system with what i have in regards to the reset control i.e. 180 high for fall & spring & 190 for winter with a 50 differatial on the low side again longer cycles instead of 20 differatial with shorter cycles

Mike T., Swampeast MO

I'd have to lean towards higher differential with longer cycles.

rob_26

like i said i have supply 156 return 149 and bypass at 154 based on high set to 180 low 130 on the o.d.r with the outside prob. set to 10f with temps around 20 so if i set it right this matchs you reading aswell

Glen

I think you've got it!

I set both the little wall hung gear and the larger vertomats with almost flat curves. 0.8 or less, and like your experience - adjust by shift. Yours is the most simple and effective control strategy - constant circ on TRV's. The true test though is your comfort - are you warm and happy?

RadPro

Thanks Mike ,I like your attention to the details on the Vito,s... How do you come up with a final number of heating to 20 deg. f. with about 10,000 btu total when you state the heat loss at the time is in fact 45KBTU Paul

Mike T., Swampeast MO

Glen,

Not exactly "warm", but am dealing with the situation--sweater sleeves aren't pushed up. Should be relatively happy when I write the utility check. Have to get materials for my main stair ballustrade and turned spindles with graduated turned area lengths don't come cheap...

Same optimization will work for more normal room temps. The "headroom" I had last year that allowed setback spaces to be raised in temp very rapidly seems to have come at a rather high price in efficiency. Once house is done and fully occupied will probably remote mount the Comfortrol™ on the 2nd floor so it will be easy to make temporary adjustments.

Mike T., Swampeast MO

HVAC-Calc tells me I should have been loosing around 45mbh with those indoor/outdoor conditions, but boiler was running at minimum modulation of 25 mbh input. Fall, so average temps are on the way down and this was the coldest sustained period so far--suspect that has much to do with it.

For similar period last year however, the boiler input during sustained modulation [seemed] to be much closer to the prediction (85% of calculated loss). Nearly everyone agrees that radiant floor systems (particularly high mass) have MUCH less loss than Manual J predicts. Am honestly operating with a heating curve appropriate to such a system and there's plenty of mass in a gravity conversion with oversized standing iron.

Am getting a significant and seeming undeniable buffer effect in the mains. Save the boiler itself the system temps are similar to the lowest I've heard reported here for any type of system in super-insulated structures.