Circulator Run Time / Outdoor Reset

[jeffesonm]

My house was built in 1957 and they basically slapped baseboard along every outside wall of every room, plus some more where it was convenient, for a total of 178' of baseboard in an 1800 sq ft house. A pessimistic heat loss estimate/calculation for the house comes in at ~75,000 btu/hr at an outdoor design temp of 10 (although given tonight's temp maybe that should be 0). Historical oil consumption roughly supports this estimate.

I am working out a design for my EKO 40 install and am trying to figure out about a zillion different things, one of which is how much heat I need to deliver to keep the house warm at a given outdoor temp. Here are a few tables I put together to try and figure out this relationship. The first is baseboard output at different temps from the Slant Fin documentation. The second is the house load at various outdoor temps, how much baseboard output needed to satisfy the load, and finally how





If we assume 480/btu/ft baseboard output at 170 avg temp (180 in 160 out), that makes 85,000 btu/hr, or 10,000 btu/hr over what's needed at 10 degrees out, and 47,000 btu/hr more than what's needed at 40 degrees out. At 170 avg temp I am figuring the load circulators would run 75/85 or 88% of the time at 10, and 44% at 40.

Does this all look correct? What is benefit/drawback to circulating cooler water more frequently vs hotter water less frequently to meet demand? Extending the usefulness of storage at the expense of electricity saved in pumping? A more stable house temp?

This relationship of lower water temp vs outdoor temp is why an outdoor reset exists, correct? My limited understanding of modcon boilers is they like to make lower temp water and therefore benefit from running lower supply temps... how is an ODR useful in a wood boiler setup?

 

[Vizsla]

My bumble bee pump uses 9 watts to move 6.8gpm in 3 zones and at peak 41 watts at 15.8 GPM @180-188 supply temp. Only 3-4 watts more at the lowest temp. A 007 used 32-33watts previously. So not really a problem with juice. My coffee pot uses 980w for the brewing cycle and 720 to keep it hot. Monitored on a kill a watt meter. Run the low temps, I find myself keeping the tstat 2 degrees cooler at least from when I was not using outdoor reset. 67-68 now feels warmer than the 70-72. Also eliminated the drafts we would feel. Good luck and nice math

 

[Vizsla]

An ODR is useful for all. Mod cons just are more efficient when run in condensing conditions , if and only if a CA has been done. If not throw out the 92-94% claim.
On Taco site they show a graph of the savings by using ODR. Basically the energy used to maintain 180 = X$. Now every degree less equals X$-excess btu. Your house needs that 75kbtu on design day not every day. So as weather changes so does the heat load, and the reduced temp matches the heat load if your programmed curve is close.

I hope I explained that accurate and well enough. On mine, anything above 19 degrees and my 2nd stage won't fire unless all 4 zones call and just to get to operating temp or if IDWH calls. Even now with low single digit and -10 wind chill it's pulling 40kbtu input instead of the 80 it always did.

 

[jeffesonm]

Thanks. I found this thread which was helpful too.

As for the circulators, I did some number crunching there too:



It does look like the variable speed ECM circulators would pay themselves back within a few years. Also when considering different system designs, the 20 year cost of one or two additional pumps (as in a P/S setup) is not inconsequential.