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Posted by HeatPro on September 09, 2004 at 09:58:27:
In Reply to: Re: boiler water temp (Questions for HeatPro) posted by marc on September 09, 2004 at 08:17:01:
: ***No I am not playing with any thing yet I just wanted to know if such a control existed but I think your response answerd to my question : the more the thermostat turns on the boiler the more the water gets hot...simple
***Exactly.
: but if it is that simple what does the optional outdoor sensor do (like the one offered by honeywell and which is standard on your weil -Mclain) ? Is'nt the same thing done by the thermostat automatically once the temperature as reached the thermostat setup the thermostat shuts off the boiler so the water doesn't get hotter ?
*** Frequent reasons for using an outdoor reset control are:
A. Megalithic dinosaur estimation. That is, it is simply too much trouble to do a heat estimate, or the supplier only has a big boiler available, or the customer wants to impress the neighbors with a huge thing, so the customer gets a boiler that is more than twice the input required by the house. So as the radiation is sized somewhat to the rooms, but the boiler burner is too large, so the water temperature shoots up. The outdoor reset limits the burner firing, acting as a variable high limit, to what is needed, in spite of the oversizing.
B. As in my home, I don't use a thermostat for multizone, so the outdoor reset control sets the water temperature maximum, and the automatic radiator valves control each room temperature. It is a great method for apartment houses. also. The outdoor reset acts as a variable low limit to asure enough heat for the rooms, but not excessively hot water.
C. Multizone systems have what is called overlapping demand as one, then the other zone calls for the burner. However, as only some of the radiation calls for heat, but the burner input is fixed at the maximum for ALL zones demanding, the water temperature remains higher than necessary. An outdoor reset acts as a variable high limit to prevent the water from getting overly hot.
A correctly estimated boiler in a single zone application only needs the thermostat.