ODR

Unknown

I think the Munchkin reads temp inside the boiler. This might not be the temp going out to the radiators, especially if you have a P/S piping arrangement. Where are you reading the temp?

Couderay

ODR

After watching my boiler perform, after the last three years, does ODR prevent my mod/con from getting to the upper set point 140*F. I guess what I'm trying to say is when I lowered the limit from 180 to 140 boiler still never quite gets to 140. the coldest it's been when watching the boiler run is 15*F out side.When boiler fires up it ramps up tapers off (flame thru sight glass) runs untill thermostat is satisfied. System works good, great heat, and no short cycling is noted. Upstairs is fin tube down stairs is in floor radiant.

Timco

Is your design day setting in the ODR set for 15* or 0*? Have you checked the supply temp sensor's accuracy? Is the aquastat on the boiler set for 180 or 190? Just some thoughts...

Tim

Couderay

ODR

Set for 15* and I lowered the set point from 180 to 140 on the programer. Highest temp noted on readout from boiler was 137 no matter where the upper limit was set and system thermometers backed up boiler readings

Timco

Boiler sized correctly to load? Maybe oversized but modulating down and working properly?

Couderay

Oversized boiler

Yes Boiler is oversized but snow melt system not functioning yet. I was caught between sizes when planning snow melt and went bigger. In retrospect probably not the wisest choice.

Brad White

If the house is comfortable, you do not have a problem, Joe.

Just because you may have a heating curve or upper-range limit to your boiler does not mean it has to get there. Your car speedometer has the capability but no one is suggesting you actually use it.

Your ODR is designed to take what it needs. All the upper limit does is set the curve, against which the boiler will index it's supply water temperature to the outdoor temperature.

If it never reaches your perhaps arbitrary limit of 140 in the coldest weather, I would submit that you could lower that, create a shallower curve and save even more for the same comfort. Your observed 137 degree high temperature tells me that is really what it needs.

Try it and see.

Glenn Sossin_2

sounds like

All is well. You should only reach that 140 point in theory at design conditions, or if for some reason, the boiler is sensing difficulty meeting the heating load demand and going into a boost mode.

I would be very happy with the conditions you mentioned. Think of your boiler as having a gas pedal similar to your car. It is using a combination of burning gas and setting a high limit to meet your heating loads. I agree with Brad, you could try lowering the system design temp of the curve. It will have a tendency to increase the length of your heating cycles, cause your boiler to condense even more, and increase your overall system efficiencies.

If you never reach the design temp, it simply means your boiler is producing more heat energy than the heating system can absorb/radiate at that current output temp. Through the use of sensors and monitoring the rate of change in supply and return temps, it recognizes that it doesn't need to make the water temp any higher to heat your home.

Enjoy your fuel savings!!

Glenn

Mike T., Swampeast MO

First: No comfort problems. 137 max supply temp in the coldest weather of a season when driving a system with radiant floors on the ground level and baseboard on the upper is VERY good! Sure sounds to me like whoever designed the system did an EXCEPTIONAL job!!!

What mod-con?

Many mod-cons have separate settings for high limit and the reset curve itself. If this is the case in your situation than changing the high limit can only affect the reset curve by "knocking off the top" when the outside temp is cold enough to require higher temperature. Usually the high limit is set such that it's never reached regardless of how cold the weather, but there are sometimes special cases where you do want to limit the temperature.

If your mod-con does have a separate high limit adjustment and you find that the supply temperature tends to stay near your high limit setting even in moderately cold weather, then I would suggest that you use a lower curve. Most mod-cons have a dual component reset curve based on both ratio (how much to increase supply when outside temp drops 1 degree) and shift (how much temperature to add--or subtract in the case of negative shift--at all outside temps). Adjustments to the reset curve are best made in small increments with days if not weeks between changes. Just remember that shift affect the curve at ALL outside temperatures while slope affects how the curve changes WITH outdoor temperature.

If it's only nearing high limit during the coldest weather, then I'd suggest that your present reset curve is close to ideal.

One last thing: 137F isn't very far from 140F. While a few mod-cons do offer true analog modulation (within the min-max range allowed by the particular model) most approximate modulation via a series of discrete steps. If your mod-con falls in the latter group, there's a very good chance that 137F is as close to 140F as the boiler can achieve...