setting the reset ratio

jimt

I have 25 year-old oil fired Burnham V14A-1 boiler with an indirect water heater. Effeciency at 80% (per my hvac tech). I use about 400 gallons of oil per year. Baseboard tube/fin on entire perimeter of exterior walls with some even on interior walls. HVAC tech advised me to add outdoor reset control to save oil.



Outdoor reset now installed:



Now I want to adjust the 3 controls. 1) reset ratio, 2) high temperature limit 3) differential



Any suggestions for me as to settings for each ?

Brad White

Questions

1. Is the outdoor reset just on the boiler? Or is it on both the boiler and the radiation loop? In other words, did you re-pipe and were they decoupled?



2. How much radiation do you have relative to heat loss? If you do not know the heat loss, what are your local Heating Degree-Days? (I can back calculate closely enough from that and your oil consumption.)



3. What brand of ODR was installed? Taco or Tekmar for example?



Reasons I ask:

1. If just the boiler has ODR control, your minimum boiler temperature is set, with a range of 130 to 140 minimum to avoid condensing. That sets the bottom of your curve.



2. Knowing the radiation to heat loss ratio will tell us the highest design temperature you will need. Your curve will be set between the low limit and that high temperature.

A shorthand way is to wait for a design cold day and reduce that highest temperature until you notice a space temperature drop. It gives you an idea at least.



You MAY find that with boiler-only ODR, your range is quite limited, especially if you have generous radiation and good insulation. Your design and low limits may be very close together. Solution is to affect ODR on the radiation side and deepen that curve but leave the boiler warmer.



3. The differential- Normally that is the same as the delta-T. If your system works at 20F, then that is divided around your setpoint, ten above, ten below. Old rule of thumb. But some controls such as the Taco PC700, will automatically adjust the differential based on outdoor temperature. Tendency is to widen when it gets warm out to reduce short cycling and exchange that for higher deviation from setpoint.



With an indirect, your setup should be for DHW priority and for that, consult your indirect manufacturers tables for your demand. Usually you need 160F minimum boiler water, but this could be up or down ten degrees.



My $0.02



Brad

jimt

reply

1) just boiler



2) unknown



3) Argo DPM-2

Brad White

Reply

1. Just boiler- as noted above, you are limited in range.



2. Your location? I can obtain the degree days for your location and back-calculate an approximate enough heat loss. Part of the equation anyway.



3. Read the Argo manual to see if there is automatic or fixed differential control. I fixed, I would start with 20 degrees. The purpose of the differential is to lessen short-cycling of the boiler by widening the temperatures where it is called on or off.