Outdoor reset problems??

tom_49

In the last 2 days here in NE. I got 3 calls regarding heat not keeping up. Also my own house couldnt keep up.

1st house has radiant w/ kick space heater backup.
2nd house has slab radiant,1st floor w/ hydro-air 2nd & 3rd floors,.
3rd house is BB.

All these systems including my own have outdoor reset controls.

They all heat fine when the temps are around 10o or higher, but when the temps start to drop towards 0, which they did here in the last couple of days, they cant. It seems they cant react fast enough to the rapidly dropping temps.

Is this just a matter of changing the curve on the OR?

It seems to me that if I raise the high point on the reset control for when its REAL cold, I would be wasting fuel on the other end when its normal temps.

Any thoughts??

Thanks, Tom

Brad White_184

Curve and Parallel Shift

I have noticed this too sometimes, Tom, but mostly in the warmer hours oddly enough. Cold weather gives me the hottest water but when it is cool in the house and it is playing catch-up when it is 40F outside, it takes some time. ODR sometimes defeats fast recovery as you know.

The other part of this is recovery from setback- is this perception of falling behind occurring during a morning warm-up period or is it a general loss over time once warm? Oh, and does the boiler shut off during this time, indicating that the ODR is governing and holding out on you?

Depending on the sophistication of the controls, you may be able to enact a Parallel Shift for a portion of the curve. This will "skew" or bend the curve to a higher setting during certain periods at either end of the curve.

Absent that, yes, a curve increase may be necessary. The parallel shift, if you have it, allows you to not penalize the entire curve for the sake of these relatively few hours.

What I would do is check your design parameters- the HWS temperature on which your system is based. Is it making that temperature? And if not, is there remaining room on the scale to do so?

My $0.02

Brad

jp_2

this agrees

with my up bring that heatloss is not linear and that infiltration is dependent on temperature as well as wind.

tom_49

thanks Brad.

The house w/ the rad/ slab is loosing temp over time, in the early am hrs.I was there today and the boiler is just running non-stop, which I dont like much. I did a man.J for the house so the boiler should be plenty big.

2 of the systems are Viessmann kw2 controls so they have par.shift capability. I have adjusted ( upward shift ) at my house and the other job as well. We'll see, even though its going to be 50o this w/e.

The other 2 are Loch. knight boilers so I would just have to adjust my high temp. on the curve upwards.

These ODR jobs are nice but require some tweeking. I guess I could just crank up the curve when we are done , but Im trying to max. fuel savings.

thanks again. tom