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Joined on November 14, 2009

Last Post on April 7, 2014

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@ February 1, 2013 7:41 PM in New Boiler at the Best Mansion

Thanks, Dave. I appreciate your taking the time to do this. I'm always interested in getting empirical data from other systems to supplement what I'm learning from my own. This will no doubt keep me busy for a while; I'm not up on vapor vacuum systems so it's time to crack open the Lost Art book again.

I had a look at your website for the mansion. What a great building and history! Fantastic that the 1928 conversion was done with such care, and how well it's been maintained over the years.

Cycle times

@ January 30, 2013 8:29 PM in New Boiler at the Best Mansion

Thanks Dave, that's the information I was interested in. I take it you're not running the setback/recovery program on the Tekmar, those would be affected too. Sounds like you've got it dialed in just about perfect. I'm curious what sort of cycle times you're running for preheat, heat on, heat off.

Overrides & Tekmar

@ January 30, 2013 8:44 AM in New Boiler at the Best Mansion

Does running in either of the override positions have any effect on the adaptive algorithms in the Tekmar control?

What's the purpose

@ January 30, 2013 7:07 AM in New Boiler at the Best Mansion

of the High / Low / Auto switch?

Dew Point

@ January 28, 2013 2:06 PM in Calculating flue gas condensing temperature

Thanks SWEI. I found a dew point chart on that indicates dew points for natural gas between 140 and 128 degF for excess air between 0% and 50%.

Are these absolute temperatures, or are they relative to the ambient temperature?

Calculating flue gas condensing temperature

@ January 28, 2013 1:03 PM in Calculating flue gas condensing temperature

In the spring I'm having JStar replace the oil burner on my steamer with a Midco Low-NOX modulating gas burner. Charlie from Midco informs me that they haven't yet determined the actual low fire DC signal required to avoid condensing in the flue.

My boiler control measures stack temperature via a thermocouple in the flue about 3 feet from the boiler. Current operating temperature is about 470 degF.

Is there a way to calculate the condensing temperature? If so, I can design the modulating control with a low limit during turndowns based on the stack temperature.

I wrote to Nest

@ January 15, 2013 1:37 PM in You Can't Set Cycles per Hour on a Nest Thermostat

a while back asking them to compare and contrast their True Radiant controls with an outdoor reset control. The Nest Support person who replied basically said they didn't know anything about outdoor reset systems and then launched into a marketing blurb about how awesome True Radiant is. Really? They couldn't go ask someone in engineering or product development? As much as I like Apple products (the prez of Nest used to work for Apple) the thing that bugs me most is the "you don't need to know what's under the hood" attitude.

This sounds similar

@ December 31, 2012 3:21 PM in New steam control principle

to what I've been working on with my own control. I fetch the wind chill temperature over the internet from a local WeatherBug station, and if it's lower than either of the two outdoor temperature sensors, I use the WeatherBug temperature in the outdoor reset calculations. I've been running a heat loss replacement model for a while now, and find it more accurate than the time-proportional model in calculating the run time. I've recently implemented an anticipator, and while it's not yet smart enough to react to outdoor temperature in the way Dave describes, it's helped keep the indoor temperature swings to 1 degF or less.


@ December 6, 2012 9:47 AM in Aquasmart, ODR with steam?.

Hi Colleen,

Let me start by saying that I'm a homeowner, not a pro. From what I can tell, the AquaSmart control is only for hot water boilers, not steam. The AquaSmart works by adjusting the boiler water temperature, and we can't do that with a steam system because if the water isn't boiling, it isn't making steam.

The Tekmar 279 control works by adjusting the length of time that the boiler runs based on the outdoor temperature. Unfortunately, it is only a single-stage (on/off) control and won't directly control a modulating burner. Honeywell makes a "modulating pressuretrol" that might work with the Tekmar, but I don't have any experience with it and will defer to a pro's recommendation. There's a thread here about how it works.

Something else to consider is the Pajek Pressure Controller, which is a high/low fire system and was originally developed for a vapor system. Contact Gerry Gill for more details.

The ODR system I'm using is of my own design (I do industrial process controls for a living) but it's very experimental and not something I'd be comfortable installing in someone else's house at this point. There's way more information than you wanted to know at the link in my signature line. I'm looking at converting from oil to a modulating gas burner in the spring, so I'd be very interested to hear what unit the Midco folks recommend to you.



@ December 5, 2012 9:12 PM in A Steam Enthusiast's Outdoor Reset Control

Thanks, Dave. I spent a lot of time studying the commercial units before deciding to embark on this project. The two big shortcomings for me were the lack of pressure short-cycle management and historical data logging capability.

On the Tekmar, if you increased the cycle time wouldn't the boiler ON time also increase for the same spot on the heating curve? I see what you're getting at though. If you also adjusted another parameter, maybe reducing the boiler design percent, you could raise the cycle time without increasing the ON time for that point on the curve.

Like you, I saw some pretty short calculated cycle times on my own system. That really bugged me because from a completely cold start my steam established time is around 15 minutes, 5 to 10 if the boiler is still warm, and that's an awful lot of fuel for preheating if the cycle is less than 5 minutes. I got around it some by lowering the WWSD temperature. I like your idea of checking the calculated cycle time before firing the boiler; mind if I borrow it?

Which Midco?

@ December 5, 2012 4:26 PM in Aquasmart, ODR with steam?.

On a non-modulating steam boiler, ODR works by changing the length of time the burner runs (and thus the amount of steam produced) based on the outside temperature. The Heat-Timer site has a general overview, while the Sequence of Operations section of the Tekmar 279 manual goes into more detail.


Which Midco burner are you looking at for your conversion?

Pressure transmitter

@ December 4, 2012 10:36 PM in A Steam Enthusiast's Outdoor Reset Control

Hi Tim,

I used an SSI Technologies model P51-3-G-UC-I36-20MA-R. You can get one from DigiKey here.

As a lower cost alternative, Phidgets now has a series of differential gas pressure sensors, including a 3.6 PSI model (1126_0) that plugs right into the Phidget Interface Kit. No need for a separate 4-20ma converter board. If they'd offered this two years ago I would have bought one instead of the pressure transmitter/converter combination. The only concern I'd have is the max operating temperature is only 185 degF. While you shouldn't see anywhere near that temperature as long as there's water in the pigtail, direct contact with steam would probably fry it.

The code generates the dashboard, faceplates, and all other displays. The gauges on the dashboard are an open source package called SpeedyHMI that's available for free download on

Phidgets is a great platform for experimenting, and it supports many, many languages inlcuding C++. The first pass of my code was for data acquisition and recording; once that and the user interface were in place it wasn't that much more work to do the actual boiler control (except for the testing part. Lots of testing).

Good luck!

Thermal calcs

@ December 1, 2012 11:08 AM in A Steam Enthusiast's Outdoor Reset Control

I didn't do the thermodynamic math to quite that level, George.

The heat capacity of the radiators is calculated using Equivalent Direct Radiation. We know that one square foot of EDR can liberate 240 btu per hour, so that value multiplied by the total EDR of the radiators (which you can calculate from the chart referenced in this thread) gives you the total heat capacity in btu/H.

In my case, the total EDR is 607 (from the system diagram in my second post above) which yields a total radiator heat capacity of 145.6 MBtu/H

We get the net I=B=R heat capacity rating of the boiler from the manufacturer's specs. Mine's 152 MBtu/H, so the boiler is reasonably well matched to the load.

I used the Slant/Fin HeatLoss Explorer 2 software (available here for Windows and here for iPhone/iPad) to calculate the building heat loss. It takes into account the heat capacity and thermal conductivity of the components (walls, doors, windows, roof, etc) of the building envelope, and returns the heat loss of the building in MBth/H at the outdoor design temperature.

The heat loss coefficient of the building is then calculated as Q = UA / (Tin - Tout), where:
  1. Q = heat loss coefficient in Mbtu/H/degF
  2. UA = heat loss in MBtu/H
  3. Tin = indoor design temperature
  4. Tout = outdoor design temperature
Mine is UA=133 Tin=67 Tout=10, resulting in a coefficient of 1.51 MBtu/H/degF.

There's a very good article here by John Sigenthaler where he discusses the mathematics of reset control. While it specifically addresses hot water heating, most of the principles are applicable to steam.

Good news

@ November 24, 2012 4:53 PM in Columbia Joins the Wet-Base Power-Gas Steamer Ranks

Good to know that the Utica Starfire can take a power gas burner. If it turns out the Midco radial burner isn't a fit for my own Starfire, I'll at least be able to have a Carlin installed in the spring.

I'd be looking at a 275 MBtuH top end firing rate. What else do you take into consideration when selecting one of these units? Is there any particular advantage of the 50-275 MBtuH unit over the 150-275 MBthH?

duplicate post

@ November 22, 2012 6:57 PM in A Steam Enthusiast's Outdoor Reset Control

duplicate post deleted

Making the leap

@ November 22, 2012 6:57 PM in A Steam Enthusiast's Outdoor Reset Control

If that's the only thing holding you back from going to a Tekmar, Rod, all you may need is maybe $10 worth of parts and some spare time.

You could get yourself a DPDT (double-pole, double-throw) switch and a mounting box, and find a place to mount it where your wife can find it. Find the terminals on the boiler where the thermostat wires attach. Disconnect the thermostat wires, and run a new pair of wires from the boiler terminals to the center terminals on the switch. Connect the thermostat wires to one of the end pair of terminals on the switch, and wire your new control to the remaining pair. Finally, label the switch positions, and you'll have a quick and easy way to switch between your new advanced control and the thermostat backup.

I never expose http ports

@ November 21, 2012 2:02 PM in A Steam Enthusiast's Outdoor Reset Control

for just that reason. Everything sits behind a VPN. And because this is still in the experimental stages, I have safeguards in place outside of the software to prevent a runaway.

It's written in

@ November 21, 2012 10:42 AM in A Steam Enthusiast's Outdoor Reset Control

VB.NET. The code runs on a laptop that's dedicated to home automation (it also runs the lighting controls). The Phidgets I/O boards take care of the low-level sensor management and data acquisition.

Not ready for prime time

@ November 21, 2012 10:41 AM in A Steam Enthusiast's Outdoor Reset Control

It's nowhere near ready for prime time, Frank. Think of it more as a prototype. To make it marketable, it really needs to run in firmware instead of on a laptop.


@ November 21, 2012 8:24 AM in A Steam Enthusiast's Outdoor Reset Control

Insteon is similar. Crestron sells through a dealer network, so you need to hire an installation firm to set up the system. Insteon is geared more towards the do-it-yourself market.

I use a software package called Home Control Assistant to coordinate the lighting controls (


BMC Hardware and Software Platform

@ November 20, 2012 3:43 PM in A Steam Enthusiast's Outdoor Reset Control

Computer Hardware and Software

BMC was written in Visual Basic .NET and runs on a repurposed Dell laptop under Windows XP. This computer is dedicated solely for home automation – it runs the boiler control as well as the INSTEON lighting automation system for the house. Sensors on the boiler and elsewhere are connected to the PC using Phidgets USB-based I/O. Phidgets (“physical widgets”) are a set of ‘plug-and-play’building blocks for low cost USB sensing and control from a PC. For more info on Phidgets visit their website at

USB I/O Interfaces

BMC uses two Phidgets USB I/O interfaces:
  1. Model 1018 Interface Kit 8/8/8. The interface kit allows you to connect devices to any of 8 analog inputs (0-5 volts), 8 dry-contact digital inputs and 8 digital outputs. It provides a generic way to interface a PC with various devices.
  2. Model 1051 single-input temperature sensor. This module supports one type J, K, E, or T thermocouple.
Analog Sensors

There are nine analog input sensors:
  1. (2) indoor temperature sensors
  2. (2) outdoor temperature sensors
  3. (1) un-tempered DHW and (1) tempered DHW temperature sensor
  4. (1) condensate return temperature sensor
  5. (1) low pressure sensor
  6. (1) K-type thermocouple for stack temperature
For the indoor and outdoor temperature sensors, one is a Phidgets 1124 Precision Temperature Sensor and the other is a 10K ohm thermistor. Sensor selection for each pair can be configured as Sensor 1, Sensor 2, First Good, Lowest, Highest, or Average.

The boiler provides domestic hot water through a tankless coil. Two 10K thermistors provide readings for un-tempered and tempered DHW. The un-tempered DHW thermistor shares the thermowell with the aquastat probe. The Tempered DHW thermistor is mounted upstream of the cold water mixing valve.

Another 10K thermistor is secured to the base of the riser at the furthest radiator. This is the Condensate Return sensor and is used to determine when the mains have filled with steam.
Steam pressure is measured with a 4-20ma, 0-3 psi pressure transmitter. It is attached to the Phidgets interface via a Phidgets 1132 4-20ma converter board.

Stack temperature is measured with a type-K thermocouple, inserted in the stovepipe about four feet from the boiler. Operating temperatures are typically around 460 degF.

Digital Inputs

Thermal switches that trip at 180 degF are installed on the boiler header and at the entrance to the steam main. These were used for time trials to see how long it took for the header and main to get hot after the burner went on, and to cool when off.

A McDonnell-Miller flow switch is installed in the DHW supply line. This allows on-demand (with a 3 to 4 minute heatup delay) hot water during periods when the DHW schedule is in OFF mode., by detecting hot water flow.

In 2011 a major remodel was done on the house, part of which included replacing all of the old knob-and-tube wiring. INSTEON lighting controls were installed throughout the house. INSTEON keypad light switches have On and Off buttons for the lighting load, plus four additional buttons that can be programmed to talk to other INSTEON devices in other rooms. One of the keypad buttons in the kitchen and bathroom was assigned as a “Call for DHW”; when pressed these buttons activate an INSTEON IOLinc module, which closes a dry contact wired to a Phidgets digital input in parallel with the flow switch.

Digital Outputs

All of this controls two relay outputs. The first is a “mode” relay. When BMC is off, the mode relay is de-energized and is wired to allow the wall thermostat and aquastat to run the boiler. When BMC is active and in Outdoor Reset mode, the relay is energized, removing control from the wall thermostat. A second relay then controls the burner.
An “Emergency Stop” button on the main display will immediately take BMC off control by de-energizing the burner relay output and keeping it off until the E-Stop button is deactivated.

Faceplate and Detail Displays

Clicking on a gauge or discrete tag on the main display will call up a faceplate for the associated sensor. The faceplate and detail displays show the sensor values, alarm status, control limit/setpoint configuration and raw Phidgets sensor data.

Data Logging with BMC

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

Event Log

BMC saves data to an event log whenever a system event occurs, such as transitioning from one state to another (Idle to Preheat, for instance). Depending on the event, BMC may store several lines of event data. For instance, when transitioning from Idle to Preheat, it logs the event change, current outdoor/indoor/comfort setpoint temperatures, burner status, and thelength of time the boiler has been off since the last heating cycle.

Continuous History Log

BMC saves analog and discrete data values to a continuous history log whenever one of the analog inputs or digital input values changes beyond a configured change threshold. This data is stored in .csv format, and is used by the graphing application to produce a trend chart.

I've used the open source package “LiveGraph” to display trend charts from the .csv file data. LiveGraph is available for free download at:

The attached graph shows trends for the following analog inputs:
- Outdoor temperatures are blue
- Indoor temperatures are green
- Condensate temperature is yellow
- Tempered and untempered DHW are red
- Steam Pressure is purple, and is scaled by a factor of 100 for display
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