Heating Curve- How do I know when it’s right?
hi folks- I just had a brand new Viessmann 100w Propane-fired boiler installed. I’m new to the outdoor reset feature and I’m trying to dial in the heating curve. I’ve studied the manual, watched Viessmann videos and read a bunch of discussions here. My question is this: how do I know when I’ve got it right? Seems simple but i don’t yet know what parameters I should be measuring. I’m using a non programmable t-stat set at 70. The house stays right there (outdoor temps between 3f and 31f). Yes, the house is consistently warm, but should i be looking at something else to get the most efficiency as possible? Am I overthinking it?
Comments
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keep cranking it down until it cannot keep up on a cold design day
It takes some trial and error to tune it to your exact application
Are you able to get into the program to make adjustments?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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yes- I can adjust the curve. Whats the goal? Should it maintain house temperature while calling for heat most of the day?
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Interesting timing on this post! I woke up early with this on my mind. Currently in the process of researching the boiler we will get to replace our coal stoker boiler and the ODR is going to be part of the new setup. I will be going through the same learning process!
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If you want to play with ODR curve on your own boiler, more power to you! As an installer, I cannot set customers curve so close that I get call backs. One reason I like the boost function on boilers is that I can start with a lower curve and if that doesn't keep after, the boiler will up the water temperature. However, the boost function is not a very good option for floor heat because of the slow response and long run times. On a typical floor heat job, the heat does not run much on a sunny winter day. Then about sunset it starts runnin' for hours. That is when the boost function will kick in when it has no need to as floor heat is just that slow.
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the ideal situation is to have the burner change BTU input so it never shuts off. that wya there will never be down time loss and you will only be using the exact amount of fuel needed to maintain the comfort in your home. So I think you got it right on both counts
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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It’s supposed to run all day, even all winter! The goal is to stay comfortable and use the lowest temp you can. But beware diminishing returns.
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You know you have the curve just about perfect when the temperature is maintained in the building WITHOUT the thermostat (you set the 'stat on 80 so it is not part of the equation). The boiler will run continuously in this condition and will not shutdown on limit.
Either it will get too warm in the building or it won't maintain a temperature that is comfortable for you.
This process is not a set once and forget deal. You really cannot accurately set the bottom point of the curve at these low ambients. As it warms up outdoors, you might find the building gets too warm and then you need to tweak the bottom point on the curve (lower it's water temperature).
Most people will give up on this iterative process due to the complaints by the other residents of the building. The biggest problem when the building gets behind the curve is that it takes a bit of time to recover unless you ruin the entire plan and turn the boiler up 25 degrees.
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Wind and wind direction plays a large part. Activity, a few people or a lot. Doors opening and closing. There is no One Point is perfect. Get it as close as you can.
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All you folks saying the mod boiler should run all winter… certainly not with floor heat! Per my earlier comment, with high mass systems it WILL shut off nearly every day. Early morn is the coldest part of day and the system is pumping the heat in the floor to keep up. Sun comes up, day begins to warm and the floor was just being heated for the coldest. Most often there will be no need for additional heat for many hours until the diminishing floor output again matches the loss to outside. At one massive house I remember watching this cycle day after day, where about 4pm the zones would one by one start calling and they'd run for hours.
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Speak for yourself @pecmsg. I have the perfect system in my son's home (formerly my home). Including 3 Nest thermostats and about 20 temperature sensors all around the home and detached garage with 6 Nest cameras. I am all knowing, and seeing, all comfortable, and it's all perfect!
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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doubling down,
damn
known to beat dead horses
1
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