Outdoor Temperature Control for Boiler

Wcurtis

I have a Honeywell Vision Pro 8000 thermostat and it can pair an outdoor temperature sensor with T-stat.

Have read various forums discussing outdoor temperature control for heating however I am not understanding what the benefits are of using on residential steam application.

Does it turn on the boiler based on outdoor ambient to be ahead of the indoor temperature drop?

Something else?

Can see a possible comfort advantage, cost saving?

I am likely ignorant on this matter however any assistance is appreciated,

Thank you

EdTheHeaterMan

There are no benefits for Steam boilers. The outdoor reset feature of a water boiler allows the burner to stop operating when the boiler temperature reaches a lower temperature based on the outdoor temperature. For example when the outdoor temperature is 40°F your radiators do not need to have 180°F water temperature to heat the home. 140°F water will be more than enough. So any enefgy used to make the water hotter than 140°F is wasted energy.

With a steam boiler, you can't heat any radiator with 140° water. The water must get to 212°F (or hotter depending on the boiler pressure) before any radiator gets steam to heat the rooms. so outdoor reset is not n option for a steam boiler.

Jamie Hall

Outdoor reset works very well for hot water heat in general, and particularly radiant floors or ceilings. The idea is to control the temperature of the water going to the radiators or baseboards or floor etc. so that they are just warm enough to provide the required heat, but no more, so that they never turn off. This gives nice even heat.

This is accomplished in one of two ways: if the heat source — the boiler — can adjust its firing rate ("modulate") to produce just the right amount of just right water, that is the best approach, and is the one used for very high efficiency systems. If the boiler can't do that, instead some of the return water is mixed with the source heated water from the boiler to obtain the desired temperature — just like changing the temperature in a shower — and the boiler turns on and off to provide enough hot water to mix with the circulating water.

However. Steam systems operate at one temperature only. Steam (unless they are very sophisticated vacuum systems, which are vanishingly rare). So there is no opportunity for them to modulate either the heat — when the heat is on — or the power output of the boiler. The only way they can adjust to provide "just right" amounts of heat is to turn on and off. Fortunately, they also have very long lag times, so the heat remains remarkably even, even so.

But a brief analysis of that will show you that an outdoor reset will have real benefits on hot water systems — but gains you nothing on residential steam systems.

I should add before someone yells — in a very big building, steam systems are controlled differently, with local control on the radiators and the boiler cycling controlled by outdoor conditions. In theory this type of control could be used in smaller systems — but it doesn't seem to have proven very satisfactory, and there are some interesting obstacles to having it work well.

Wcurtis

Thank you guys for your valuable input, glad I checked here before any purchase which will not happen.

Time and time again I have had a dilemma and each time found a remedy on this site, bar none the best site for heating questions, you guys are awesome.

ratio

The outdoor temp sensor on a Honeywell 8000 series is used for temperature display or to decide whether to use heat pump heating or backup heating. It can't be used for reset.

LRCCBJ

The individuals who make the conclusion that you cannot utilize outdoor reset on a steam system are simply incorrect. A steam system with reset doesn't modulate the TEMPERATURE of the fluid. What is does, extremely well, is to modulate the TIME that the boiler operates. This eliminates the problem of a thermostat which is fundamentally inaccurate with a steam system unless you are devoted to spending hours with the anticipator and then suffer with inaccurate results because the anticipator is only good for a single outdoor temperature.

A proper controlled system with a unit such as the Tekmar 279 measures the outdoor temperature and operates the steam system on a similar curve that is utilized for a HW system. Why run the steam system for 30 minutes per hour when 15 minutes per hour is more than sufficient at, say, an outdoor of 50F.

The 279 gives unlimited capability to move the heating curve up or down and also provides the capability of determining a percentage of full output depending on the size of the boiler relative to the size of the radiation.

Suffice it to say that none of the above is accurate since none of the above have used a Tekmar 279 to actually control a steam boiler. The results are simply fantastic.

ratio

Regardless of how you implement outdoor reset, the Honeywell 800 series doesn't do it. Even if you consider 'cycles-per-hour' as the PWM you're describing (it certainly is), the thermostat simply can't use the outdoor temperature as anything other than a convenience display or a decision point of which heating system to use.

Jamie Hall

Not sure what happened to my comment… a different thread? But… on outdoor reset in general (not just steam). It works well — sometimes remarkably well — if the major determinant of the heating load of the building is outdoor ambient temperature. However, if other factors, such as wind or effective insolation, are significant, it doesn't. On three buildings which I manage, one has outdoor reset and it works splendidly. The building is in a sheltered location and has relatively little sun load. A second building is also in a sheltered location, but has a significant sun load. It also has outdoor reset, and the results are marginal; it overheats badly if the sun load is high, as the reset curve is set for night time and cloudy day conditions. The third is in a location which is anything but sheltered (the wind load is very high) and also with a significant sun load. The heating load on the building can vary from negligible at 10 F exterior temperature with bright sun on snow and no wind to around 100,000 BTUh at 32 F with no sun and moderate (20 mph) wind (those aren't guesses — actual observation).

I have yet to hear of an outdoor reset with could handle the third case.

So my point is — it depends. Sometimes outdoor reset is just wonderful. Sometimes… not so much.

I would also note that there is no reason, with modern sensors and some computer programming, that what might be called augmented reset couldn't be made to work, incorporating ambient temperature, wind speed and direction, and effective insolation. It could. But why? The old faithful mercury T87, which has been there for over five decades, holds the interior temperature within a degree, day in, day out…

JDHW

Controlling heating systems with radiators or underfloor is an interesting problem in control theory because in most systems there is a significant time lag between starting up the heat source and the heat actually warming the air. When the heat source is switched off there is then considerable heat stored in the radiation which then leads to overheating.

Using an old fashioned on/off thermostat you end up with a system that oscillates around the set point temperature with temperature swings of a few degrees. The thermostat has hysteresis which means it switches on below one temperature and then off at a slightly higher temperature. You can make the swings smaller with little fudges like adding a small internal heater to the thermostat and if you get the internal heating level just right the system still oscillates but with much smaller swings (this is the anticipator in the T87). With a high thermal mass system like underfloor heating embedded in a concrete slab this kind of control system does not work because of the amount of heat stored in the thermal mass.

The solution to all this is to vary the amount of heat being put into the system to match the demand of the building and there are two ways to do this. First, control the temperature of water going into the heating system, modulating gas boilers are really good at this, some heat pumps to a lesser extent, or staged heat sources (multiple boilers/heat pumps) and mixers are another. The second approach, is to continuously switch the heat source on and off for controlled amounts of time and allow the thermal mass of the radiation to average out (smooth) the flow of heat into the building. This is pulse width modulation (PWM) and is really useful for sources that are on/off. You can pick the cycle time to suit the kind of heat source you have. If you have electric heaters a frequency of 12 per hour would be fine but not so good for a steam boiler where 1 of 2 cycles per hour would be more appropriate. Longer cycles mean greater temperature swings but less "wear and tear" on the heat source.

The key question is how to determine the amount of heat required at any point in time? Outdoor reset (ODR), it is called compensation in Europe, is one way to do this and is based on the assumption that the temperature difference between the outside and inside of a building determines the heat demand. As @Jamie Hall pointed out above this assumption may or may not be correct for a particular building. ODR is a guesstimate of the heat requirement not the complete answer. ODR can work controlling the temperature of the heating system water or the PWM of a boiler. It has become really popular with modulating gas boilers because running modulating boiler continuously at the lowest possible temperature optimises condensing. Getting the reset curve just right is a bit like voodoo.

Feedback is a much better was of controlling the temperature of a building and that is what an old fashioned on/off thermostat does, in a less than optimal way because it relies on making the system oscillate around the set point. A proportional feedback system measures the difference in temperature between the set point and the actual temperature and uses this to control the heat source. So for example, a thermostat could call for maximum power if the building is 5 degrees too cold and no power if it is 5 degrees too warm (proportional band +- 5 degrees). This alone would not be a great thermostat because it would only set the right temperature when the building demand is 50% of the boiler power. If the thermostat also measures the average error (integrates) and uses this to modify the set point internally you can have a thermostat that always gets to the correct temperature with zero error and continuously controls/modulates the heat source. This is a proportional/integral (PI) control system. A PI thermostat could be used alone to control a heating system and this is load control, or combined to an ODR system to adapt the reset curve. This is common on modern gas boilers with their proprietary controls.

In answer to the original post - get a Honeywell (or equivalent) TPI (time/PWM, proportional/integral) thermostat and setup to 1 or 2 cycles per hour and a minimum on time equal to the time for a warm boiler to start generating steam.

John

AdmiralYoda

Some anecdotal evidence. I have an Ecobee thermostat with a bunch of remote sensors in various rooms of the house. I've configured the Ecobee to maintain a set temperature based on the average of all the sensors, not just the thermostat itself.

In addition, being wi-fi enabled the Ecobee knows the outdoor temperature. If the temperature outside is getting warmer or cooler it alters its heating strategy. Probably by changing the cycles per hour and anticipation settings. It seems to do pretty well and doesn't overshoot.

The Steam Whisperer

Outdoor reset has been used for two pipe steam/vapor systems for over one hundred years here in Chicago. Larger buildings used vacuum reset controls to change the temperature of the steam vapor systems from about 140F at deep vacuum up to 215 for pressure (Vari-Vac systems) while varying the amount of steam introduced into the system using modulating steam valve. In addition a number of naturally induced vacuum systems have been around for even longer that operated the system in a vacuum to modulate output ( the Moline was one). Today. we have been using these same techniques in a number of two pipe systems. The most common technique we use is close to the moline system, using orifices at each radiator and then varying the boiler output based on the outdoor temperature. Sometimes we add naturally included vacuum to the system as a simple improvement to these very same systems. Other systems where we have used orifice plates, but an on/off boiler with naturally induced vacuum. The modulating output boiler systems are usually larger applications due to the cost of the modulating burners. We directly control the burner output based on the outdoor temperature.