Setting the reset "Curve" for a modulating boiler (modcon)
Setting the reset "curve" is clearly an exercise in trial-and-error to find the settings (or two points that make a line) that will make the heating system run nearly constantly at the minimum temperature possible to just make up the heat loss.
Since there will always be variables that can't easily be measured the way an outdoor reset can measure the exterior temperature, I understand that the "curve" will probably need to be set to provide heating water just a little higher than needed for standard heat loss. Variables causing loss might include: wind, open doors, etc. Variables causing gain might include: Solar Gain, cooking in the kitchen, burning a fire in a sealed wood stove, etc. So I understand that nothing is perfect. However, I want to dial-in the curve as much as possible.
I've decided to focus on the cold outside temps & boiler high set point for my system. This seems to make sense because it's been cold recently and I figure the cold end of the spectrum is where most savings are made. If the goal is to reduce cycling for more constant running, we need to understand that cycling in the shoulder seasons is inevitable. The boiler has to turn off when it's warm outside. So I've decided to keep the factory defaults for my Triangle Tube Prestige Solo 110 NG. 86 Degree low point at 64 degrees outside. I noticed that on the 40 degree day we had I was running 100-110 degree temperatures and the system ran most of the day with very little cycling. Good! So I'm pretty happy with the low end of the curve, and realize that there's less to gain there. Lower temperatures will lead to more condensation (more efficiency), but I'm already under the 130 degree mark that Triangle Tube says is the condensing limit. I have however noticed a lot of cycling at the colder outdoor temperatures (hotter boiler temperatures). I assume this is bad because not only am I loosing efficiency by cycling, I'm also running temperatures that are higher with less condensation (less efficiency). This is the stuff I really want to avoid. So I've been dropping the Max boiler temp.
I don't think my installer shared my logic because he'd initially set the max cold to 190 at 20 degrees F outside. That was way too warm for 190 degrees and the boiler was cycling all over. I immediately changed the 20 degree coldest day back to the factory 0 degrees. But still have been getting cycling around and under 32 degrees F outside. I figured that since my old cold start boiler had a max temp of 180, I could drop my TT to 170 without any problems. Still getting cycling. Dropped it to 166, still cycling. I'm afraid to drop it too much because I though the initial curve 190-0, 86-64 was running pretty well today with 138 boiler temps and 32 degree outside temps. But not that it's 28 degrees out at 11 PM, the boiler is cycling even at 28 degrees and the new lower 128 degree boiler temperature. I guess the 138 at 32 degrees might have been causing some cycling that I didn't notice because I was upstairs. So more dropping the high boiler temp limit I guess.
Our house has been a cozy 69 degrees, warmer than we kept it with out old boiler. We let it get down to 65 a lot and 68 was probably our top temperature. We moved it around as we wanted.
So this seems like it could be time consuming. I'm considering a new approach to setting the boiler temps/reset curve. I've read about people having there thermostat set about 0.5 degrees above the room temperature and had constant running of the system, even with multiple zones (and I only have one). This method of having the thermostat always calling for heat seems like a good idea. Maybe I could set the room temperature to 75 degrees, and keep dropping my curve until I reach the desired room temperature. I don't believe I have anything configured for "Boost" in my system. So I'd think this approach would work. Has anyone tried this approach?
Once I find the ideal curve, I guess I could bump it back up a little so we can deal with the anomalies like doors left open or windy days.
If you'd like to know about my config:
- No DHW hooked up (not yet anyway)
- Single Zone
- 2" Cast iron pipes in the basement
- Three floors with cast 13 radiators
- Primary Secondary loop (primary pumped with internal Grundfoss in the TTPS110, Only Zone pumped with Taco circulator)
- TTPS110 came with wired outdoor reset temp sensor
- Honeywell Wireless Thermostat with: 4 programmable times, Additional Wireless handheld touchscreen thermostat, wireless outdoor thermometer with humidistat. (Even with all this, I'm not sure I see any big benefit to this except the ability to read temp from different locations than the main thermostat wall.)
Thanks in advance for any help!
What was you heat loss when the boiler was installed?
How many sq ft of radiation do you have.
Getting close on settings is easy.
Divide the heat loss at design temperature by the square foot of radiation. Next do the same for the heat loss at 60f. This gives you the btu's required per sq ft of radiation. There are charts to determine water temperature beyond that point. Doing this will give you the min and max water temperature required. Adjusting the fan spied will match the boiler input to the heat loss so the boiler input is limited to the required maximum heat loss and can modulate down from there. Why have the boiler fire at 110k when the heat loss is 60k? Make sure you use the OD design temperature for your area.
T-stat reading 69. Got tired of hearing the cycling, and decided to drop the max temp again. This time to 50. Set the Thremostat to "hold" at 72 degrees. An hour later, the TT Solo 110 has modulated down to what I assume is min fire and I've figured out how to read the modulating fan speed. I watched it drop from the 3200 range down to 1900 which is the rate for min fire. My Boiler Setpoint for the 28 degree exterior temp is 122 and my input seems to be coming up, now at 118 with an output of 124 (but still firing). House temp still 69. I guess I'll het the sack and see in the morning if I have a reset-based setback.
(After an hour I think I've confirmed that I don't have any "boost" turned on in my system.)
Oh. I found a good resource for Triangle Tube installers/owners that doesn't come with the manuals shipped with the unit: http://www.triangletube.com/document...20Type%205.pdf
I never did the heat load calculation. This is the DIY forum, right? :-) I know the heat loss is supposed to be easy, but somehow I figure that there are too many variables for my house:
- 80 year old house
- Connected on one wall
- Essentially three exposed sides for about 80 linear feet. Two stories of brick for about 20 feet high... so 1600 square feet of brick walls exposed.
- Roof on top of the 25x30 box sloping from 30 feet front to back to nothing over two more stories (3rd floor plus unfinished 10 foot high attic)
- 18 windows
- 3 doors (one is basement door below grade)
- 4 more basement window well below grade windows
- Single pane windows in steel casements with interior storms on most windows
- one fireplace
- Exterior absorbs sun with dark brick and peachbottom (dark gray) slate. (Our backyard with a brick patio is about 10 degrees warmer than everywhere else with all the warm brick.)
- There's basically no insulation on any of the first three floors, only insulation between the 3rd floor and attic, and in the attic rafters (making the attic quite cozy).
I'm near Philly.
But with all that, I believe that I've read that setting the Reset curve is basically trial-and-error (for a homeowner). With all the cycling it seemed impossible. However, with the approach of creating a demand that can't be met... it seems easy.
I've got my old engineering graph paper from college and I've been potting temperatures that I believe do, and don't work.
I'll try the heat load calc if you think it makes sense at this point. Where is the best/easiest tool?
Hanging in there
It's 24 degrees and sunny out at noon. Since my outdoor reset is in partial sun and on the warm red brick it's been reading around 38 (much better than the wireless outdoor temp/humidity sensor for my thermostat which reads 55). It's still running and hanging in there at 69 after dropping to 68 by morning. It got back up to 69 around 9:30 AM. Thermostat has been set at 72 all along. Boiler temps have gotten down to about 112-114 with a 6 degree delta T across the HX with the lowest pump speed on the internal Grundfoss. I've got a constant light steam out of the flue which seems appropriate for the low fire I seem to be constantly requiring (giving that 6 degree delta T). The fan speed seems to be holding around 1900 so I must assume I'm a low fire as I've seen the fan modulate down slowly from 3200 to 1900 (I think) the last time I made a change to the reset curve (lowering from max temp 166 to 150).
If the house gets cold, I will increase the max temp, or lower the outdoor temperature at which it should occur, or consider moving the outdoor reset to a colder position (right now it's sitting on my brick windowsill, probably gaining some heat from the house). On the brick in a shady spot might be better, nowhere near the fireplace.
I think I like what I've found because my thermostat is basically providing a max temperature for the house. If I run a fire in my fireplace insert, I can change the max temperature I'd like in the house (lower it to shut off the heat, or increase it to assure the heat keeps running regardless of how much heat we get out of the fire.)
I feel lucky that 150 max temp seems to be working so well. It was the first temp I tried with a high thermostat setting.
There's no real reason to this post, but I have to share it with someone who might care (and maybe you do if you are reading this). I was thinking more about the max temp of 150, and was wondering if it would really cut the mustard if we ended up getting below 0. It's probably OK to design for 0 degrees because the coldest day on record is over a 100 years ago and was only about -15 F or so. Nonetheless, I decided that I should determine the equation for my boiler temp line, which would allow me to easily keep the same line but lower the outdoor temperature and increase the effective boiler max temp. So in the event of some really cold weather, my boiler won't quit at 150 F. So I looked up the old linear equation for a line:
y = ax + b
y = Boiler Temp, x= outdoor temp. I need to find "a" and "b" for my line equation. With the points 150 F @ 0F --> b= 150. That's no coincidence. B is the "y intercept". But what I thought was funny was the value for "a". When I plugged in the next two numbers: 86 = a(64)+150 --> a = (86-150)/64 --> a = -1 What the...?
I hope I can keep this curve. That will be hard to forget. (Boiler Setpoint) = -1*(outdoor temp) +150. or (Boiler Setpoint) = 150 -(outdoor temp) or more simply:
(Boiler Setpoint) + (outdoor temp)= 150
Too funny (to me anyway). I'll guess I'll try to mess with (placement of) my outdoor reset more than my curve.
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