outdoor reset

[Deleted User]

Is there an easy way to calculate the reset ratio for an outdoor reset control?

Thanks.

Mike

Big Ed

Heat Curve

Do you mean the heat curve ?

Bill Nye

To oversimplify

You really need a heat loss. If you know the water temp. required to heat the house on the coldest day of the year,you look at the chart or curve for the product you are installing and... , there you go.

Base board and hydro air generaly need a higher temp. Panel radiators and cast iron sometimes a little less and radiant less temp. but, It Depends.

It boils down to, take a good educated guess, pick a curve and then fine tune from there. Seasonal adjustments are sometimes needed. My house is perfect down to 20° , below that I turn the parallel shift up a couple of degrees. This raises the boiler temp a couple degrees w/o changing the curve.

Jamie_5

I am only a homeowner, so my reply may only be worth what you paid for it, but, as I understand it, the reset ratio is equal to (difference between the water temperature necessary to heat the residence on the coldest day and the indoor design temp) divided by (difference between the indoor design temperature and the outdoor design temperature). So, for instance, if you are using or planning to use fin tube baseboard with 180 degree F water for the design day, the design outdoor temperature is 0 deg. F, and the desired indoor temp is 70 degree F, the reset ratio is:
(180 - 70)/(70 - 0) or 110/70 or 1.57. The reset ratio works because (1) the heat loss of the house is assumed to be roughly linear from when the indoor design and outdoor actual temps match, so that no heat transfer from the inside to the outside occurs, to when the outside temp hits the planned low and the maximum heat loss occurs from the residence to the outside, and (2) the heat transfer from the hot water heating system through the heat emitters is roughly linear according to the temperature of the water supplying those emitters.

I *believe* that the output of heat emitters relative to water temp is never exactly a straight line and that the heat loss of a residence is also not exactly linear due to
infiltration losses, which is why some reset controllers use indoor sensors and adjustment algorithms to "custom tailor" the reset ratio to a reset curve.

I hope this helps.

eleft(retired)_5

My observation through experience

Hey Jamie,

Only a base to start from.

All well and....how do you deal with the ambient or residual heat of the objects in a given space when the out door temp. rises faster than the calculated heat lose.?

"is the answer open the windows"?

There comes a point where calculations need an open window to throw them and replace it with trial and error eh!

al