Outdoor Reset Savings

Fred Carpenter

Great Site! I visit often. This is my first post so here goes...I care for a turn of the century 6500 sq. ft. multi-unit house with oil/FHW and C.I. Rads. A new Weil Mclain 68 series boiler (2.05 GPH) was installed in 1995. There are 7 zones with double setback thermostats. Ocasionally, a few of the zones will overshoot setpoint by two degrees but, the house is basically comfortable through out the heating season. Oil bill is about $2,600-$2,800 per year. I am considering upgrading the controls to include outdoor reset. Does anyone have any real world experience with the payback of such a conversion? Is it 2 years? 5 years? I will need to select a manufacturer and an experienced contractor as well. Any input is greatly appreciated.

Chris Maderia

Out-Door Reset

I would really take a look a primary/secondary piping (with bypass) on constant circulation. I would be more inclined to reset my system vs reseting my boiler. I would use a modulating 3 or 4-way mixing vlv (which ever you prefer)that has out-door reset capability. My mixing valve would be on my primary loop that way I'm not robbing all the btu out of the boiler. This will give you longer runs and less cycles. The boiler sensor on the mixing vlv control will shut the valve when it senses low return water temp allowing the boiler to fire and provide warm temp water back to the boiler preventing you from shocking it.

Jim Davis

A 3-way or 4-way valve cycle hot water back into the boiler, which will cause it to reach limit faster. This causes shorter run cycles and more cycling. Heating a boiler to 200 degrees on a 40 degree day and then only supplying 130 degrees to the loop is a waste of heat. If their isn't another call for heat that 200 degrees will cool down and be wasted.

Jim Davis

Outdoor reset will save usually a minimum of 15% or that is my experience for the past 24 years. For it to work properly the equipment that is it controlling must be in proper operating condition.

Fred Carpenter

outdoor reset savings

Thanks, sounds like it is worth persuing. The boiler does have a 3/4" thermal bypass. How does a tech verify if the the boiler is condensing excessivly?

Chris Maderia

A waste of Heat

Now on that so called 40 degree day when we run (in CT 180 degree water) wouldn't you call that a waste of heat? It's not needed. And what would you be wasting anyway? IF your heat loss is accurate and you've done your calcs correctly, you would hope that your secondary pumps would continue to run consistently also. This will allow you to replace a btu for every btu leaving that particular heating space. In other words providing maximum comfort and also at the same time utilizing return water with a constant (let's say, dash) of hot boiler water. You in no way are wasting any of the btu that the boiler is making (you can't waste something that you don't need). How about making adjustments to your boiler control. Why are you running 200 degree water anyway? The boiler will fire when it reachs low limit (change it to 140, That's the minimum all boiler mfgs will allow back). Why not ask our boiler mfgs to add more insulation to the jacket to reduce jacket heat loss. Here's another question for you. Are you wasting hot water all night long when your in-direct isn't in use? The anwser is NO. Why? Because they only lose anywhere between 1/4 and 1/2 degree per hour. Why? INSULATION. Sorry. Now back to the question at hand.

You are not consistently cycling hot water back to the boiler. If you pipe it correctly and have done your homework you will find that you are cycling a mix of cool return water with hot boiler water. The only time you will be giving 100 percent boilre water back to itself is when the 3-way modulating mixing vlv boiler senses to cold return water. It will shut on the primary side and only allow hot boiler water to return to the boiler until it senses that the boiler will no long be shocked. See that 200 degree water is consistently being used and with a mixture of cool return water we are not completly cycling hot water back to the boiler.

As far as savings goes, for every 3 degrees below 180 you save 1 percent.

Bill N.

Outdoor air reset only saves if you are wasting now

You can only save any significant fossil fuel energy if you currently waste, as in overheating your building. If you don't overheat, then you aren't wasting very much. In fact, using outdoor reset and constant circulation (which is real home run for improved thermal comfort) may increase your electric bill, slightly.

If you are overheating though, you can save money by using OA reset, however I wouldn't put a number on it, since there are so many variables.

Good luck,
Bill N.