I spent some time going over this on paper and a little time in the basement last night moving the boiler around and thinking about what I am trying to accomplish. Part of my problem is the old good/fast/cheap issue (you can have 2 but not 3). I also spent some time talking about what we want and need out of this project with my wife.
Cheap is key. I mean face it, I’m not burning coal because I just love coal, I’m burning it to save money. Cheap for the installation means I’m going to have to do it.
I really want good as I purchased this stoker boiler with the idea that it will be much more user friendly then the Stelrad, and I want it to keep the whole house warm (at least to the coal-o-matics capability).
Fast is now important because it is starting to get cold around here. I would rather avoid using the gas for heat so I need to get the coal-o-matic in place relatively soon.
I ran through some of the calculations based on whats here in this thread, plus some other thread information and some information at the freecalc site. I used the assumption of 100’ for my piping length. Its somewhere between 25-30’ between the two boilers, then some additional elbows.
First I figured what my flow rate needs to be to move all 130k. Yanche gave me a 6.5gpm FR based on a 40deg Delta T. I don’t know what my gravity system was designed for, I know modern systems planned around 20deg Delta T based on what I have read here and elsewhere. So I ran the numbers for 20, 30 and 40 Delta T and came up with 13, 8.7, and 6.5 gpm respectively. My thinking is this accounts for the flow between the two systems and not what is going on between the main boiler and the radiators.
It did make me wonder what the flow rate calculations for a gravity system are? It seems like it would be constantly changing based on the needs of the house. Cold days would draw the heat out of the radiators faster, cooling the water faster and causing it to “sink” faster – faster flow rate. Warmer days, the water would move to the radiators then take some time to cool and “sink” – slower flow rate? I’ve read somewhere that when these gravity systems become older and not working as well one “fix” was to turn up the high limit on the boiler to induce a faster flow rate? Is there a “normal” expected flow rate for the gravity systems, or a “normal” Delta T for these systems? Sorry for the digression.
I also found an article here http://www.pmengineer.com/articles/8419 ... ?v=preview
that gave me an opportunity to attempt to learn how some of the piping information is found. I came about it while trying to find out the head loss for the Pex-al-Pex. I skipped over to excel to help with the formula as presented in the article. Based on the formula and information there ( I stuck with 100F water model) I came up with the following head loss for Pex-al-Pex
13 gpm = 11.7
8.7 gpm = 5.8
6.5 gpm = 3.5
Based on that calculation and the graph on taco pumps shown in the thread at Circulator Pumps
- which I did a cruddy plot on using paint and some interpolation of the points it seems that the intersection of the PaP plot and the taco pumps intersects the 007 pump around the center of the pump curve. Hitting the pump curve around 10gpm or so.
So, if these calculations are correct, (and my hand plot is not too far off) I can estimate a 10gpm flow through the PaP. From that I worked out that my Delta T should be 26 (Delta T = 130000(btu)/500*10(gpm))
So, after my paper analysis and a look at my basement and thinking I am preparing to move forward doing the following. Installing the coal-o-matic and piping it near boiler with 1 ½ steel pipe. I have some of this from the Stelrad. The intention is to pipe around 6’ away from the boiler to include the connection to the pump. Run 1” Pex-Al-Pex to a 2-3’ stub of 1 ½ steel (again from the Stelrad install) connecting to and from the old locations that the Stelrad connected to.
I think this will solve the cheap/fast part of my problem, giving me a working but not 100% solution to adding the coal boiler in.
I also think this may buy me the time to install a “good” solution. What is the main detail here? To me it seems that getting the takeoff and return to the main boiler in the best position. What is the best position? Would the best bet then be to pipe from the coal boiler to the original in larger steel pipe? There are some head space considerations that make me want to lean toward a smaller pipe. What added value is there in moving to a large pipe?
So, do the things above make sense or am I off somewhere in the calculations.