Single Pipe Steam Temperature Control

redplumber

I've been doing research on boiler controls because I want to add a smarter control to my system.

I've been looking at the following controls with outdoor reset (based on searches performed on this site):

• Honeywell T775
• Tekmar 256
• Tekmar 279
• HBX Controls CPU-0550 (or similar)

Here's what I have: single pipe steam Peerless 211A-08-S-I (1,470,000 BTU input). I have an outdoor thermostat that is designed to turn off the system when the outdoor temp is too high (55*F or so). On the condensate return, I have a surface mounted thermostat (something like the White-Rodgers 1127-2 Surface-Mount Hydronic Temperature Controller).

Every week or two when the outside temperature changes, I adjust the surface mounted temp control up or down to keep the indoor temperature comfortable. When the temperature outside is below 55*F, the condensate should stay between 78-98, when it's below 30*F, the condensate should be between 100-110, etc.

I would like something that would eliminate the need for me to manually adjust the surface mounted temp controller. I would like something reliable and cheap (if possible).

I want the condensate temperature to be directly correlated to the outdoor temperature and I want it to be turned off when the outdoor is warmer than 55*F.

• Will all of these basically handle my use case? Or are there other controls that I should look at?
• Is there a resource that I missed on this website that would allow me to research the recommended options available?
• Is there any reason for my scenario why the Tekmar 279 would be better than the 256? I notice that it's a lot more expensive, but it is marketed for "steam".
• What suggestions do you have for settings? For example, I assume there should be a minimum off time between cycles.

Jamie Hall

Hmm…

Well, of course the simplest control to keep the space temperature comfortable is a space thermostat equipped with a good response and the ability to anticipate the behaviour of the radiation. Such thermostats did exist — the old mercury switch T87 did (and does) a splendid job if it is properly adjusted, but there were other mercury swtiched bimetal thermostats with anticipators which worked as well. Most folks won't take the time and patience to adjust them properly, but that's another problem.

Now we will assume that for some — probably very good — reason you don't want to depend on such antique technology. So… let's look at aspects here.

First, the simplest part of your comments: minimum off time for the burner and boiler. If you are NOT using thermostatically controlled vents, there is no inherent minimum off time. That said, most burner controllers enforce a post purge and pre purge delay which may total as much as a minute. Otherwise the off time is limited by how long the pressure in the system takes to decay to zero or near zero — which is measured in fractions of a minute — while the overall duty cycle timing of the boiler is determined by the match (or not) of the boiler power to the ability of the system to absorb that power.

Now the duty cycle of the system as a whole is determined by the power output of the system as a whole (including the properties of the radiation) in relation to the power requirement of the space (the rate of heat loss to the outside). In any heating system — the medium is quite irrelevant when working on first principles — the game is to adjust the power output of the system to match the power requirement. In other words, to modulate the system output as a whole.

Thus the question becomes first, how to determine the required system power; how to control (modulate) the system power, and how to trim that modulation for variables not included in the initial system power determination.

The approach used will vary, of course, with the type of system. There are two essential parts of the system to be considered, and they have to be looked at together. The first is, of course, the power source — a boiler, electric resistance grids, a heat pump loop. The second is the power delivery mechanism — radiators, baseboards, fin tube, radiant floors, fixed flow hot air, variable velocity air, mixed flow air, etc.

The following will drastically simplify what is actually a very complex system.

Both essential parts can modulate their power output or ability to absorb power, but the way in which they can do that and the characteristics of that modulation are vastly different.

Broadly speaking, the modulation can take one of two forms: amplitude and pulse width. Let's consider two example systems: a typical residential steam system (one pipe or two pipe doesn't matter) vs. a typical residental hot water system with fin tube radiation and a modulating (usually a "mod/con" but the condensing part is irrelevant here) boiler powering it. The former system, the steam system, is a classic pulse width modulated system (it's actually two related): the radiators are either receiving and condensing steam, or they are not (the other part is the boiler is running or not, but that's a different part of the control). Since the condensing temperature of the steam is constant, the radiators will gradually warm to full output when the boiler starts producing steam, run at full output while the boiler is steaming, and gradually drop to near space temperature and zero output (given enough time) when the boiler stops. Assuming that the boiler has enough power to power all the radiation, the power output of the system into the space will thus take the shape of pulses with a slow ramp up from low or no output to full output, a variable length of time at full output, and a slow ramp back down to low or no output (note that actually, if the control thermostat is properly calibrated, steam supply may be shut off before reaching full output). The overall power output over time will be the integral of the instantaneous power outputs.

In the modulating hot water system, by contrast, the temperature and thus the power output of the radiation can be smoothly varied by varying the instantaneous power output of the boiler (think of the throttle pedal on a car). Thus ideally the system is never off, but the instantaneous power is adjusted in response to a control parameter to match the load.

Jamie Hall

Part two

Obviously both systems can modulate — they just do it differently. It is advantageous for the hot water system to have a basic modulation based on estimated load — usually the outside air temperature — although it should also have a trim based on space temperature, since outside air temperature is only a rough approximation of the actual load. There is no particular reason why a steam system can't also have a basic modulation based on estimated load, with a trim. They usually aren't done that way, since the boiler itself can't modulate and there is simply no advantage to it provided that the space thermostat is adequate and the additional control complexity delivers little.

Where the two types of systems vary rather dramatically is in the inherent lags. A modulating hot water system using fin tube radiation can change its power output into the space very rapidly — both the boiler and the instantaneous radiation output can go from a very low output to full output within a matter of a few minutes at most. Thus the system can track a varying input control parameter, such as outside air temperature, very closely (whether this results in a constant space temperature is another matter — in general, it will not — since the outside air temperature is only one factor determining building load). A steam system, however, can respond only slowly to a change in such a control parameter, since the time lags in the radiation (and to an extent the boiler on startup) are quite long.

This leads — or should lead — to different control strategies. There is a very real advantage to using a base load estimate — outside air temperature — to control a hot water system, provided that the system also has a trim function based on space temperature error to boost output when needed. Provided that the control based on space temperature can take into account the lags in response in a steam system, however, the simplest and most reliable control will be the space temperature alone, with no need for other inputs.

To go back several pages here, the best space temperature controls for a steam system are those with a simple temperature measurement, but biased to take into account the response character — in other words, a bimetal thermostat with an anticipator. There is no particular reason why such devices can't be created which are all digital, but they become rather complex and require a surprisingly sophisticated computer program and a good deal of computational power (and memory) to match the older systems. Thus they are simply not available; there is no market for them.

Now back to the top.

As I understand it, you are essentially intending to use outdoor air temperature to determine the modulation of your steam system, and you wish to sense the modulation using the condensate return temperature. The condensate return temperature is actually not a bad measure of system modulation — except for one thing: it has an extremely long lag time in relation to the rest of the system. Co9mpensating for this in your control logic, to avoid serious overshoots in response to rapid changes, is going to be difficult, whatever sensor mechanism you use.

I do not know of any off the shelf control system which does what you are wanting, at least as I understand it. However, using a thermistor temperature measurement for both condensate return temperature and outside air temperature will, converted to digital of course, give you the two control parameters you are looking for. The program which you use for control, however, will have to be rather sophisticated to avoid either serious overshoots or undershoots or even, given the length of the lags, serious oscillation problems in the outputs.

Steamhead

@redplumber , either the Tekmar 279:

https://www.supplyhouse.com/Tekmar-279-Steam-Control-One-Stage-7959000-p?utm_source=google_ad&utm_medium=pmax&utm_campaign=pmax_best_sellers&gad_source=1&gclid=CjwKCAjwjqWzBhAqEiwAQmtgTxiNoCFT5Oe7xHCxKVI7I9AhpUSGVSP_iedtPEak2lNTPPwbQkbGhhoCWiUQAvD_BwE

or the 289:

https://www.supplyhouse.com/Tekmar-289-TEKMAR-Smart-Steam-Control-289?utm_source=google_ad&utm_medium=search_sku&utm_campaign=Search_SKU_Heating_X_DSA_X&gad_source=1&gclid=CjwKCAjwjqWzBhAqEiwAQmtgTxZVqk2116dj5RU4vKZ781UIWjb65kJCqKZJeUAZiMuT4xR_ugPWgRoCofwQAvD_BwE

DCContrarian

Why would you want to base your modulation on the outside temperature rather than the inside? The purpose of heat is to deliver comfort. I could see doing gross adjustment based on outside temperature for efficiency, then fine adjustment based on inside temperature for comfort.

LRCCBJ

The Tekmar 279 is the way to go. Yes, it's costly and you can certainly spend a hundred hours trying to duplicate it with other equipment, however as a plug and play device you won't beat it.

It, of course, manages the cycle time by using the outdoor temperature. You decide the maximum percentage for the cycle based upon your requirements for the design day. I have one at 66%. So, the boiler will run for 66% of the designed cycle time on the design day.. You also decide that time. I had previously run 60 minute cycles but reduced that to 45 minutes due to the benefit of maintaining temperature in the mains in the cold basement.

If you want to get more sophisticated, the unit will function with indoor feedback using one or two sensors that monitor the room temperatures that have been set and the unit will modify the algorithm to maintain those temperatures. Unfortunately, you cannot find out any info from Tekmar on how the parameters are adjusted and you cannot adjust anything regarding them. I have not utilized them.

If you really have patience, it will also handle setback and recovery, but you had better live on the premises to get that dialed in.

ARobertson13

DcContrarian,

I believe redplumber has a one pipe steam system. I agree that boiler control should as with air conditioning [cooling] units, based on maintaining an acceptable inside temperature. I put together a system of my own design [with UL approved equipment as told by NYC DOB boiler division] that worked well and I intend to improve it next season with additional internal sensors. The only reason why I have an external sensor to measure outside temperature is to verify compliance with New York City regulations that state at what temperature the system is to be on. I have noticed that the more a one pipe steam system relies on outdoor reset and less on indoor steered temperature controls, the more over heating occurs. More so with high mass cast iron radiators and with buildings over 3 stories.

I intend on addressing this next Fall in a new discussion.

Jamie Hall

"I have noticed that the more a one pipe steam system relies on outdoor reset and less on indoor steered temperature controls, the more over heating occurs. "

This surprises you? Read the rest of my rather long pair of comments. Yes, I know they are long.

A one sentence summary: the best possible control for a high mass, slow response system is one in which the primary control is the target parameter — indoor temperature — with appropriate compensation for lags. That also happens to be the simplest and cheapest.

Don_175

I'm not familiar with the Tekmar controllers, but I have been very happy with our Ecobee thermostat for our one pipe steam. It somehow learns how long the house takes to come up to temp and seems to act like the old anticipators, shutting down the boiler a little early so we do not get overheating. Also, with the extra sensors and air vent adjustments, I find our system is pretty well balanced.

Jamie Hall

My point exactly. Thanks @Don_175

Steamhead

https://forum.heatinghelp.com/discussion/comment/1803978#Comment_1803978

Unfortunately, in larger buildings it is not possible to control the system properly with just a room thermostat. That's why outdoor controls like these Tekmars are used.

Jamie Hall

Understood, @Steamhead . It does highlight an interesting point from the standpoint of control choice, though. In a small or at least not huge residence, single point control with careful balancing will do the job most of the time. In a large building with a number of loads, you have statistics going for you — and a timer based control strategy for the boiler should work well, particularly if combined with appropriate local control (TR vents or valves) and, if single pipe, arranging the duty cycle to ensure that the TR vents can gain control. A reasonably large building will have a high probability that at least some loads are calling at any given time — and that the fraction of calling loads will be nicely related to the overall building load, and thus the duty cycle of the boiler can use that (outdoor reset) quite successfully.

It's the intermediate size which is the problem…

And no. I don't have a good answer for the intermediate size, nor do I have a good answer for what constitutes smaller, intermediate, and reasonably large!

AdmiralYoda

I'll second what @Don_175 said about Ecobee. We have an Ecobee thermostat as well as their remote sensors in all the rooms. It takes an "average house temp" as opposed to the thermostat temp only.

It has done great with our one-pipe steam even though our boiler is 50% oversized. Even in the dead of winter it doesn't fire the boiler more than once per hour and it doesn't overshoot. Being connected to wi-fi it knows the outdoor temp kind of acts like an outdoor reset by figuring out when to cut the boiler so as not to overshoot.

@redplumber Is this for a residential dwelling or an apartment complex? Your boiler is pretty large so the Tekmar 279 may make more sense if it is an apartment complex where you need more control. If this is a large residential dwelling that you live in I'd recommend the Ecobee.

dabrakeman

It does seem though that some of todays (vs those from a few decades ago) thermostats utilize multiple sensor location inputs rather than just one and also can incorporate outside temperature in the anticipation. The outside temperature may just be a weather service provided temperature for that general location vs specific to the outside of that building but it is beneficial input nonetheless.

Don_175

The outdoor temp is from a weather service/station, but it seems pretty accurate at our house. I currently have 3 remote sensors. Master bedroom, master bath, and family room. You can choose which times of day that each sensor contributes to the overall heating system. I have master bedroom and bath exclusively in the morning and then the family room during the day and evening.

redplumber

Thank you everyone for the recommendations! It's very insightful!

@AdmiralYoda This is for an apartment complex.

Having a temp sensor in each unit isn't practical or even possible at this point.

Considering the current implementation, almost anything is a better idea in my opinion. But, I want to do my homework and understand the pros/cons of the solutions proposed.

Ideally I would like something simple and serviceable. Ideally it would be inexpensive as well. If it's expensive and complicated, it will be a nightmare for us.

Just an idea that I'd like to throw out there if anyone has done something like this: I was thinking of creating something custom for this system. With a handful of relays and some inexpensive thermostats, I would be able to create a system that would work at keeping system heat balanced as the outdoor temperature changes. Anyone done something like this that would like sharing tips or even pics/schematics? I'll post my ideas soon.