how to set curve on Viessmann

J.M.

Happy New Year. It's a bitterly cold day today,so I'd like to set the curve on my Viessmann boiler (the installer told me to do this on the coldest day, but every time he explained how, I didn't understand).
We keep the upstairs warmer than the downstairs, and we have an enclosed porch that has its own thermostat and is much colder than the rest of the house, but we intentionally leave it cooler unless we're in there.
Our instaler said something about cranking up the thermostat in the coldest part of the house and playing with the curve until the heat to that room was flowing continuously without satisfying the thermostat temp. Is that right?
Thanks,
J

Glen

you are in the right place -

but could you add a bit of info? current settings, type of system, Boiler model etc, even location is helpful.

J.M.

Great. We have a Vitotronic 200,Model KW2,with a Vitocell 3000. We're located in Boston. Normal room temp on the Vitotronic is set at 72, slope is 1.5, and current boiler temp is reading 131. We keep upstairs at 70 (too warm for us, but it's the magic temp for child to sleep through night), downstairs at 68, and enclosed porch at 54 except when we go out there. Does that help?

Paul Pollets

Heat Emitters?

I suspect you have radiators or baseboard. Raise the curve to 2.0 and if that doesn't do it, raise it to 2.2.
This will be under heating circuit "A". Heating circuit "B" is for a mixing valve.

You should have the owner's manual, which will show you what buttons to push. Make sure you acknowledge the heating curve change by pressing the "A" button, or it won't change.

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GW

temp

that seems odd, if it's set for 72, and it's (let's say) 12 outside, that's a 60 degree dif., at a 1.5 slope I would think the boiler would be running at 162 or so. Did the 'shift' get moved at all? Is the boiler puffing out a big bunch of flue gas exhaust or just a small amount? What boiler do you have? Gas or oil? Is it running non-stop?

In the mean time, just crank up your 72 to 80 or whatever and that will get you more heat. If your boiler is maxed (??) you will not get any more heat unless you turn down a zone.

Might you want to install more heat out in the porch??



Gary



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J.M.

Thanks. We have gas and some radiators, but the enclosed porch has convectors, which are terrible. We bought the house a few years ago, so we were the ones who installed the Viessann bt the radiators and convectors are from prior owners.
Just checked. Zero shift. 138 boiler temp. 12 degrees F outside. I think we have a lot of flue gas exhaust (from watching it while shoveling over the past week).
We're getting enough heat, but I want to have the system run as efficiently as possible, and my understanding was that the lower the slope, the better. So I'm trying to use today to figure out slope we should use in general. Or do I have this backwards?
Thanks.

Constantin

If the house heats OK on a design day...

...and at least one zone is not calling all the time for heat, then the curve may need to be adjusted down, not up.

The idea being that you lower the supply temperature until at least one zone needs to be active 100% of the time at a particular outdoor temperature. That's because cooler heating system water will transfer less heat to the room than hotter water. Once at least one zone calls for heat all the time, you know that you've hit the lowest supply water temperature possible for that particular outdoor temperature. Yet, that is just one point on the ideal curve for your home.

There is more work ahead: Ideally, you determine optimal heating supply water temperatures at various outdoor temperatures (let's say in 10 degree increments and under similar outdoor wind conditions between 10 PM and 6AM). Then, with the help of your installer (or the somewhat technical owners manual) you can input a curve based on slope and offset that best reflects your home's heating characteristics.

Glen

I agree -

if the slope indicated in the manual to your expected DD is correct - lower may be the route you need. Ultimately it is your comfort that will dictate the final numbers. Another method I use to fine tune a system, regardless of heat emitters, is to use negative shift. It's a quick method of curve adjustment that is easily adjusted.

J.M.

thank you!

I appreciate the guidance and explanation you all provided!

TimS

After sizing you're boiler to the heat load, I would refer to the ASHRAE Fundamentals Handbook and find therein your Winter Design Dry-Bulb temperature 9°F/97.5% and set your heating curve to the greater percentage of the radiation (cast-iron) about 150°F.


I might even size the radiation to get a more exact design temperature which the boiler would produce during design conditions exclusively thereby producing more comfort at a lower operating cost.

Waiting for a "coldes day" is a new one, but have to give him credit for originality. Your contractor should set the curve as part of a proper and professional installation.

MassPatriot

Viessman recently serviced, not heating baseboard above 40C

Viessman 100 with Vitogas 100 control.  12 years old.

Had a section of baseboard removed as part of an addition. Installing firm unable to do the work, so had another local HVAC firm do the work. Post removal, the boiler only erratically ever heats above 40C.  I've tried every possible thing I can think of: main power and power switch off/on, setting the curve and off set all the way up and down (they were set at 2.0 and 70 for 12 years and worked fine), turning thermostats all the way up and down.  Occasionally, when I turn the main power to the boiler off and then on after a minute, the boiler will come on and stay on until the water temp hits 60-70C.  Presently, I can't get it to heat above 40C, even though its in the 20s and 30s outside the past few days, and both zones are several degrees below the set temperature.  The pumps continuously run only barely warm 40C water through the baseboard.  Could there be an issue with water pressure, air in the system, sensor failure (we have an outside temp sensor)?  Thanks for any suggestions.