Programable thermostats

Dan M

I have one of those honeywell programable thermostats with multiple set points for different times of day. It is running my single zone( one Taco 007 no zone valves , flow valves at radiators)two pipe hot water system. The boiler is a two year old weil-MClain GV5. I have friends with forced air that swing their room temps all over the place Low at night and when their at work and then jump it up before they get up or come home etc. They feel that they are saving money. My question is does that make good sense with a different animal like a boiler ? Does it have a heating cost bennifit or is it hard on the boiler ? Is it better with my single zone to maintain one even temp. If you do beleive in swinging the temps do you range them very much ? (My system drops to 66 at night and while I am at work and then 70 when I am home)As professionals what is your opinion ? I know their are probaly many facters envolved in whether this helps in heating cost or not .

Thanks
Dan

BillW@honeywell

Saving energy

If you have one of our Chornotherm 4 programmables, T8600 family, you have "Adaptive Intelligent Recovery". To make a long story short, the stat starts the recovery period from setback temp so that the set point is reached at the beginning of the "wake" or "return" period. This feature is not available on the retail model thermostats, and I'm not sure which model you have.

A reset system works very well for hydronic heat, as others on this site have termed it, "cruise control for your house". Basically the colder the air gets outside, the warmer the water gets in your system. It's all handled automatically. We make several controls that do this, so do others, and some may require some piping modifications. Ours are the AQ475, AQ675 and AQ775. Go to http://hbctechlit. honeywell.com and check them out. For most standard hydronic installations, either the AQ475 or the AQ675 work best.

bigugh_4

Comfort verses. Efficientcies

a teeter totter trade off. ya canna have it both ways. 1st and formost, insulation is the most efficient way to stay warm. (teeter totter very close to the ground!,low fulcrum) 2nd. repacinhg lost heat is always costly. 3rd. Comfort means that you have to replace heat.. The swing you refer to is a savings only as you can accept the heat loss your building has. You'll have to look at your fuel bills and access your savings to your comfort. Compareing hot air to hot water heat, is just not possible, first the whole comparasion is off by a floor to cieling temperture difference. and a draft factor and therefore a comfort factor, and a clean factor. jmho

Mike T., Swampeast MO

Setback

Let's start with the one statement with which nobody will disagree: At a given outdoor temperature it requires less energy to maintain a lower indoor temperature. General setback (keeping the thermostat lower at ALL times) will ALWAYS save energy.

After that, things get a bit murky and quite complex.

Imagine a real-world room but with imaginary perfect control. Type of heating system doesn't matter at this point, be it radiant panel, radiators, b/b, even (shudder) forced air. The perfect control has allowed just as many BTUs to enter the room as are leaving the room and this has been maintained for a period of time.

EVERY object, including the air, in that room is at perfect thermal balance with the outdoor temperature. Make a change in any object and the temperature of another object must change as well to maintain that perfect state.

Now set back the thermostat significantly. What happens? You are no longer adding heat being lost. The air begins to cool. Air actually holds very little heat considering its volume so you would think the air temperature would drop very rapidly. Why doesn't it? Because all of the rest of the objects in the room that were at their maintenance levels hold significantly more heat per unit of volume than the air. The other objects must cool as well.

When the air reaches the new desired temperature, heat is again added. Remember this is a perfectly controlled system so it is added in just the right amount and just when needed. The air had reached a new maintenance level with all the rest of the objects in the room at a lower temperature as well. Less heat is now used to maintain this lower temperature.

This setback process took time to achieve maintenance because it was impossible to instantly achieve a new maintenance level.

Now end the setback period and return to the higher air temperature. What happens? Our "perfect" control system is at a loss of what to do. It must add BTUs faster than they are being lost. Because EVERY form of heating system heats the air to some degree it must add BTUs MUCH faster than needed. Why? Not only must the air be increased in temperature but all of the other objects as well. Since the other objects heat at a much slower rate than the air even this perfect control system MUST overshoot the air temperature to some degree to achieve a higher maintenance state in any reasonable length of time. Heat loss during the recovery period will actually increase a bit as some other objects (besides the air) will be heated to a higher temperature than required at perfect maintenance.

Has there been any energy saving in this setback cycle? ONLY if the space was MAINTAINED at the lower temperature for enough time to offset the "penalty" incurred during the recovery period.

If the space never achieves maintenance at the lower level the setback has actually consumed MORE energy that were it perfectly maintained at a constant temperature.

How long does it take to actually reduce energy consumption? It all depends--both on the structure and the method of heating.

A poorly insulated room on a windy night will achieve maintenance at the lower level much MORE rapidly than a well insulated room on a still night.

A radiant slab on grade room will achieve maintenance at the lower level much LESS rapidly than a room heated by forced air.

---------------------------------

My conclusions:

1) ALWAYS set back unused spaces to the highest degree possible.

2) Use no or VERY little setback with extremely massive systems like radiant panels. Maintain such a space at the lowest air temperature for comfort. We all know that such will be lower than with forced air anyway

3) If using automated setback, do so sparingly and with HIGH consideration for the length of the setback period. The shorter the period, the less setback, the longer the more.

4) If setting back manually, give consideration to the conditions outdoors. Set back more when the temperature is expected to fall rapidly; less when it is expected to drop slowly or stay steady.

Uni R

Wild temperature swings

Dan,

Two things you should keep in mind. Building mass and thermal expansion.

If your house is tight and has high mass, it won't significantly cool down during the setback phase and it will take a long time (and use a lot of energy) to get back up to temperature. The higher the mass, the less the energy savings benefit of using a setback thermostat.

With thermal expansion you need to keep in mind the effect of the temperature swings on the interior of your house. The wider the swings the more things (like your hardwood floors) within your house will be expanding and contracting each heating cycle. The money your neighbour saves on fuel may well go into the pocket of the drywaller who gets called in because the thermal expansion caused his drywall screws to pop.

The best thing is to find the lowest temperature you are comfortable at and then use a minimal setback. This sounds basically like what you are doing now.

Uni R

Envying those who disagree!

"Let's start with the one statement with which nobody will disagree: At a given outdoor temperature it requires less energy to maintain a lower indoor temperature. General setback (keeping the thermostat lower at ALL times) will ALWAYS save energy."

I'm sure that everyone living in a climate where they are fortunate enough to still have their AC on would completely disagree with that statement Mike. Curse them!!! ;-)

Mike T., Swampeast MO

Awww...

Cooling "setback" gets even more complicated. A/C systems remove humidity as well as heat and it is the humidity that holds much/most of the heat in the air...

HP80

Sorry to cloud the water hear Bill but what about the Honeywell T775 controller for reset.
As far as the original question, I live in a climate where it makes no sense to use set back for hydronic applications, so I believe that climate may play a role in determining whether or not set back should even be considered. The only other real use for set back use with hydronics would be extended terms of un-occupancy.

BillW@honeywell

T775J

You're not clouding the water, the T775J is a great reset system, but it's usually overkill for a residence, unless you are working in a very large custom home. It's usually used in commercial applications, but teamed up with a 3-way mixing valve and the appropriate actuator & piping, it will work fine.

Dan M

Thanks !

Thanks for the Education!

Dan