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Joined on November 14, 2009

Last Post on April 7, 2014

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Outdoor Reset Modeling with BMC

@ November 20, 2012 3:42 PM in A Steam Enthusiast's Outdoor Reset Control

BMC supports two outdoor reset mathematical models; time-proportional and heat loss. Each model calculates the number of minutes that the boiler needs to be ON during the heating cycle. The model is calculated when the boiler transitions from the Preheat state to the Heat Cycle ON state. The “active” model that is used for control is selectable from the Outdoor Reset Model display.

Time Proportional Model

The time-proportional model uses the relationship between the current outdoor temperature and the configured indoor and outdoor design temperatures to calculate the boiler’s “On” time during a heating cycle. This method is commonly used in the commercially available outdoor reset control systems.

Heat Loss Model

The heat loss model uses the building’s calculated heat loss (in MBtu per hour) and the boiler’s net MBtuH rating, along with the indoor and outdoor design temperatures, to calculate the boiler’s “On” time during a heating cycle. The heat loss model also employs a setback recovery feature that adds a setback “boost” to the calculated “On” time when the current temperature is below the comfort setpoint by a configurable amount.

Model Equations and Boiler Operating States

The forum software doesn't support tables, so I'm providing the Outdoor Reset Model equations and boiler operating states as image attachements.

Temperature Setback and DHW Scheduling with BMC

@ November 20, 2012 3:40 PM in A Steam Enthusiast's Outdoor Reset Control

Temperature Setback

BMC includes 7-day 4-period-per-day temperature setback scheduling, as is typical on many thermostats.

Domestic Hot Water Scheduling

The boiler supplies domestic hot water through a tankless coil. The old boiler and aquastat would keep the DHW temperature between 110 and 140 degF during the warm weather season. It burned on average 150 gallons of oil each season to do so. DHW scheduling was added to BMC so that water is only heated during scheduled demand periods – in the morning for bathing and evening for dishwashing, for instance.
Domestic Hot Water production can be configured to run in several modes:
  1. On a programmable on a 7-day, 4-period-per-day time schedule,
  2. Delayed On-demand (see discussion below)
  3. Using hi/lo setpoint control based either on temperature sensor or the aquastat
  4. It can be turned off entirely for extended away periods
A “delayed on-demand” feature detects hot water flow during off-schedule periods, and if there is flow for a predetermined time (10 seconds), BMC will run a DHW heating cycle. DHW can be available in as little as three minutes from a cold start.

Two means are provided for on-demand hot water. The first is a McDonnell-Miller FS-1 flow switch installed in the DHW supply line on the inlet side of the tankless coil. In addition to the flow switch, Insteon lighting control keypads in the kitchen and bathroom have a dedicated button that wirelessly activates a “Call for DHW” contact closure.

BMC’s DHW scheduling was active for the warm weather season last year, and oil consumption was reduced almost 50%, from 150 gallons to 78 gallons from April through September 2011.

BMC Features (continued)

@ November 20, 2012 3:40 PM in A Steam Enthusiast's Outdoor Reset Control

Data Acquisition

BMC’s data comes from nine analog and four digital input signals:
  1. (2) indoor temperature sensors
  2. (2) outdoor temperature sensors
  3. (1) un-tempered and (1) tempered DHW temperature sensor
  4. (1) condensate return temperature sensor
  5. (1) 0-3 psi pressure transmitter
  6. (1) K-type thermocouple for vent stack temperature
  7. (1) thermal switch on the boiler header
  8. (1) thermal switch on the steam main
  9. (1) flow switch in the domestic hot water supply line
  10. (1) Insteon I/O Linc wireless relay
Dual Temperature Sensor Selection

Redundant temperature sensors are provided for indoor and outdoor temperatures. Sensor selection for each pair of indoor and outdoor temperatures can be configured as Sensor 1, Sensor 2, First Good, Lowest, Highest, or Average. If one of the sensors goes bad (either high or low scale), the system will automatically switch to First Good. If both sensors are bad, the system will automatically go off-line and return control to the thermostat.

Eliminating Pressure Short Cycling

Steam pressure is measured with a 0-3 psi, 4-20 ma pressure transmitter at the boiler. (Point of interest: the pressure transmitter was less than half the cost of a Vaporstat) Normal operating pressure of this system is 1.5 ounces per square inch, with the high pressure trip point set at 2.5 ounces. If the trip point is reached, the burner will shut off and the system will enter a “Pressure Lockout” mode until the condensate return temperature drops below a preset value. This allows the pressure to subside, and eliminates short cycling while allowing the heat already in the system to radiate into the living space. If there is still a call for heat after the lockout temperature is reached, the burner will restart. In practice, high pressure is usually seen towards the end of the heating cycle, and the comfort setpoint is often achieved during the lockout period.

I've attached a photo of the pressure controls on the boiler. I teed two pigtails to the single port on the boiler; one for the pressuretrol and 0-30 psi gauge, and another for the vaporstat, 0-3 psi gauge, and 0-3 psi pressure transmitter. The latter pigtail has a shutoff valve for blowdowns, and a calibration port to allow the vaporstat or pressure transmitter to be calibrated in place. The vaporstat is currently out of service; it was used along with a time-delay relay in an earlier pressure lockout control scheme.

Use of Wind Chill as Outdoor Temperature

BMC can optionally fetch the current wind-chill (or “feels-like”) temperature over the Internet, from a nearby weather station. If the wind chill temperature is less than the temperature from the outdoor sensors, BMC will use the wind chill value in the outdoor reset calculations.

Web-Accessible Thermostat Display

A simplified thermostat-like display can be accessed from a web browser.

BMC Environment & Features

@ November 20, 2012 3:39 PM in A Steam Enthusiast's Outdoor Reset Control

The house is of four course brick construction, and has approximately 3100 sq ft of living space plus an attic and an unfinished basement. All of the windows on the first and second floors were recently upgraded with low-e glass replacement windows. The remaining windows are double-pane glass and were replaced by the previous owner about 20 years ago. The majority of the walls are uninsulated save for a 2” air gap between the brick and wall. The third floor and attic spaces are fully insulated. Using the Slant-Fin HeatLoss Explorer software, I calculated the building’s heat loss at 133 MBtuH at an outdoor design temperature of 10 degF.

The boiler is a Utica Starfire SFE-5175S oil-fired unit, rated at 152 MBtu/hr of steam, or 633 sq. ft. EDR, with a tankless coil for domestic hot water. The EDR of the connected radiation is 607 square feet, making the boiler about 4% over-sized. After fine-tuning the system, it heats the building with only 1.5 ounces of steam pressure.

Boiler Monitoring & Control Features

BMC’s features include:
  1. Outdoor Reset Control using either time-proportional or heat loss mathematical models.
  2. Redundant indoor and outdoor temperature sensors, with configurable sensor selection (High/Low/Average/First Good).
  3. Automatic switchover to a good sensor if one of the indoor or outdoor sensors fails.
  4. Optionally retrieves current local wind-chill temperature over the Internet, and uses it as the outdoor temperature in outdoor reset calculations.
  5. Eliminates short-cycling on pressure by entering a pressure lockout mode when the adjustable high pressure limit is reached.
  6. 7 day, 4 period-per-day temperature setback scheduling.
  7. 7 day, 4 period-per-day Domestic Hot Water heating schedule.
  8. A web-accessible thermostat-style display
  9. On-demand DHW by pushbutton or when hot water flow is detected.
  10. Highly configurable – many parameters can be adjusted to tune the system to its environment.
  11. Logging of continuous analog data and heating cycle events, with trend charting for analog data.
  12. Automatically reverts to wall thermostat control in the event of a critical failure.

A Steam Enthusiast's Outdoor Reset Control

@ November 20, 2012 3:37 PM in A Steam Enthusiast's Outdoor Reset Control

I live in an 1890’s three-story brick home in south-eastern Pennsylvania with a one-pipe steam heating system. The boiler was replaced in 2009, and after fine-tuning and balancing the rest of the heating system (insulating the mains, re-sizing and replacing vents on the steam mains and radiators using the Gill & Pajek vent capacity charts), I began looking at other ways to reduce fuel consumption. An outdoor reset control was at the top of the list. The commercially available outdoor reset controls all lacked specific features I wanted, so with a background in industrial process control I decided to develop my own control, which I’ve dubbed "BMC" for Boiler Monitoring & Control.

This has been a work-in-progress since 2010. The first iteration was a time delay relay to control end-of-cycle short cycling on pressure. This was followed by a clock timer/relay control to schedule production of domestic hot water. For the third iteration, I purchased sensors and wrote some software to do data acquisition and charting of temperatures and pressure at the tail end of the 2010 heating season. The current iteration incorporates the mechanical controls into the software and adds outdoor reset modeling. I've had the software in control of the boiler since January 2012, with some fine-tuning along the way.

Before I get started, I'd like to take this opportunity to thank Dan Holohan and all the contributors here at for sharing their knowledge and expertise over the years. Thanks guys!


Looks interesting

@ September 20, 2012 11:32 AM in Midco Radial Burner

I've already prepaid my oil for this year (ouch!), but come spring I'll be taking a closer look at this unit.

I wonder if this burner could be modulated using steam pressure. Seems to me that given an otherwise well-tuned steam system, the objective would be to first fill the mains on high fire, then turn down to maintain an optimal system pressure. You'd need some way to de-bounce the pressure signal, especially if the operating pressure is very low. I have a 4-20ma, 0-3 PSI pressure transmitter on the gauge pigtail (which I'm currently using with my home-grown outdoor reset control) that would be perfect for this application.

1st-generation Starfire?

@ August 19, 2012 7:26 AM in Utica Starfire 3 steam boiler- oil burner conversion to gas burner

Bill, how do you distinguish a first-generation Starfire, and how is it better than a later model? Mine was installed in 2009 and, like Helm, I'm thinking about a gas power burner conversion.

I thought the worst part

@ May 19, 2012 5:47 PM in Is this why we make fun of engineers

was when the author wrote, "Steam heat is wasteful". Steam heat is wonderful, poor maintenance is wasteful.

No gas

@ March 31, 2012 6:31 PM in Tankless coil into water heater tank?

My friend doesn't have gas, so Icesailor's side-arm solution just might work for him.

Icesailor, do you have any data on fuel savings in the off-season between a regular tankless coil and your system?

my outdoor reset control

@ March 31, 2012 5:58 PM in energy usage this winter .. (it is over now right??)

is a PC-based application program that I've been working on since 2010. It runs on an repurposed laptop that's been dedicated to home automation - it runs the boiler and lighting controls for the house. Sensors are connected to the PC using Phidgets USB-based I/O (

I put in a couple of features that are lacking in the commercial systems. The outdoor reset model uses the heat loss of the building and the net firing rate of the boiler in addition to the outdoor temperature to calculate how long the burner should run. It also has the option to fetch wind chill temperature over the Internet from a local weather station and use that as the outdoor temperature.

I'm working on a more detailed writeup to post in the near future. Stay tuned.


@ March 29, 2012 10:26 PM in Tankless coil into water heater tank?

In this configuration, if it were me I think I'd disconnect the aquastat on the boiler altogether.

another boiler heard from

@ March 29, 2012 10:20 PM in energy usage this winter .. (it is over now right??)

This year: 12/1/11 to 3/6/12
Consumption: 635 gals
HDD: 3390
Therms: 881
Gal/HDD: 0.187

Last year: 12/15/10 to 4/6/11
Consumption: 991 gals
HDD: 4368
Therms: 1375
Gal/HDD: 0.226

I put an outdoor reset + heat loss mathematical model of my own design in control of the boiler in January, and while the early results are promising - 0.226 vs 0.187 Gal/HDD is a 17% reduction in fuel usage - whether it holds up under a "normal" winter remains to be seen.

Tankless coil into water heater tank?

@ March 29, 2012 4:31 PM in Tankless coil into water heater tank?

I'm posting on behalf of a friend who has an oil-fired steam boiler with a tankless coil for DHW, and he's wondering if it would be practical to pipe the outlet of the tankless coil into an electric or gas fired water heater tank. His thinking is that in the summer he could shut down the boiler and let the water heater do all the work, while in the winter with the boiler running the tankless coil would preheat the water going into the water heater.

Seems simple enough, so there must be a catch somewhere. What are the pros and cons of such a setup?


@ March 8, 2012 11:12 PM in Repipe for water hammer

for the feedback, Jamie. Yeah, it is amazing the difference an inch or two can make. The drawing isn't quite to scale, where the sloping return ties in is only 15" or so above the floor so it's definitely wet. I used a mechanic's stethoscope to listen for the hammer (more of a tapping, actually) along that sloping return, and it was most pronounced around the tee at the water line.

I figure when the Dead Men piped this back in 1890, the water line on the original coal-fired boiler was a lot higher than it is today. It's probably just coincidence that the current boiler's water line just happens to be at that tee. I wonder if that whole sloping return used to be wet, since the initial vertical drop on the south wall is more than the 28" 'A' dimension.

Repipe for water hammer

@ March 8, 2012 6:36 PM in Repipe for water hammer

I have a water hammer problem that I'm pretty sure I know how to fix, but would like to get a second opinion from the folks here. Here's the situation (sketch attached):

The return line off of the main on the south wall of the house initially drops 34" from the main, then traverses the full length of the west wall with about a 1/2" per foot slope, then again about 10 feet on the north wall before it ties into a vertical return pipe midway down the main on the north wall and returns to the boiler.

The return line off of the end of the north wall main ties into the sloping return pipe on the east wall about 32" from the floor, which coincidentally happens to be where the water line is on the new boiler. I'm getting water hammer in this line midway thru the cycle, and I'm guessing that the water line isn't high enough at that point to flood the tee, so there are two steam paths into a what's become a partially dry return. If I raise the water level in the boiler an inch or two the problem subsides.

My plan is to repipe the return on the north wall main so that it drops to floor level before tying into the wet return. Am I on the right track here, or should I do something else?


If you don't mind a little math

@ February 19, 2012 8:51 PM in Single Steam Pipe Venting - Sizing

spend ten bucks at the shop here and get a copy of this:

Balancing Steam Systems Using A Vent Capacity Chart

Oops, it's the other way

@ February 19, 2012 10:46 AM in Cycleguard on Smith boiler preventing radiators from heating

Sorry about that, I mixed up the product names. Should have read replaced CycleGuard with SafeGuard, for the same reasons you're replacing yours. I found that every time it cut off the burner for 90 seconds it took a couple of minutes to rebuild the head of steam.

The existing probe

@ February 18, 2012 11:08 AM in Cycleguard on Smith boiler preventing radiators from heating

can be used with the Cycleguard. I replaced my SafeGuard with a CycleGuard more than two years ago. Removed the SafeGuard probe to compare with the CycleGuard , found the probes were identical, so I put the SafeGuard probe back in and put the new CycleGuard probe away as a spare. Works fine.

Radiator enclosures

@ October 21, 2011 7:29 AM in Radiator enclosures

After reading the Radiator Enclosures article in the Library here, I'm interested in constructing a few enclosures for our 1-pipe steam radiators to increase their efficiency.  In the first example,marked "Deduct 10%", the article is unclear on a couple of points and I'm hoping someone here can  answer a few questions: 
1. What is the "B" dimension for a 36-39" tall radiator?
2. How far above the top of the radiator should the board extend? 
3. Is the board supposed to enclose the radiator on the sides as well as across the front?
4. Will it help, hurt, or make no difference if I put reflective foil behind the radiator?

If someone has built covers in this style, I'd love to see some photos.


Firing rate - ratings versus actual

@ April 21, 2011 1:42 PM in Firing rate - ratings versus actual

I have a Utica oil-fired steamer with a 1.50 GPH @ 140 PSI nozzle. The service tech confirmed the actual nozzle size and operating pressure. I have an hour meter installed on the boiler, and track usage when the tank is filled. In 9 fills over the last two seasons, the calculated actual flow rate has been between 1.67 and 1.96 GPH, with the majority in the 1.75 GPH range.

Are these normal deviations? I don't expect it to hit 1.50 all the time, but if it's consistently 0.25 higher than that, does that indicate a problem or something to be checked?


Suspension boiler

@ March 5, 2011 9:49 AM in Suspension boiler

A friend is having a new basement slab poured. Here's a pic of how they got the boiler and hot water heaters out of the way.


@ February 11, 2011 6:49 AM in How low can you go?

On the L408J1009 vaporstat, R-B are normally-closed switch contacts, R-W are normally-open. So on pressure rise, the switch action will break R-B and make R-W. R-B are the contacts used in a typical line voltage installation where the vaporstat interrupts the burner power circuit.
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