sizing a bosch greenstar boiler

roundrightfarm

Greetings,
I'm trying to size a Bosch Greenstar boiler for use with propane. It is new construction of almost 4000 sq-ft, in a 4 level building (32' x 32' footprint). We plan to use panel radiators throughout. The walk-in basement is concrete block and about half exposed. First and second floors are 2x6 R19 walls, and the lived-in spacious attic has 2x6 R19 gable walls, and R38 cathedral ceiling/roof. I will also put R38 between floors (14" I-Joists) and R13 in the interior walls. My preliminary plans are for one zone for each of the 4 levels.

I live in a rural area and no one I've found close by has experience with this type of heating system. I have a decent amount of experience with plumbing and electric, and plan to do extensive research into hydronic design. Plus, I want to try this DIY to save money and to learn something. I'm attracted to the Bosch Greenstar because there are very detailed installation videos. Also, I have used a Bosch AE125 electric on-demand DWH for several years and have been happy with it and their customer service.

I'm questioning my outdoor design temp. Can't find data near my exact locale, but we are higher in altitude than most nearby cities. (by nearby, I mean an hour or two away) Most cities in my state show outdoor design temps from 0F to 10F, however, we see temps in the negative teens at least once per winter (often more than once), and some winters we will briefly dip below -20F.

There are no building codes in our area. Based on the following, which I read in a greenbuildingadvisor.com article, and since the Bosch Greenstar boiler is modulating, I'm hoping you all can help me with a rule of thumb calculation to size the boiler.

"As long as there is no code requirement in your jurisdiction for a Manual J calculation for the type of work you are contemplating, you may not need a Manual J calculation. To understand why, we need to examine two myths. Both myths haves long been promulgated by energy experts. The first myth is that rules of thumb are inappropriate; the second is that oversizing of equipment is disastrous. - See more at: http://www.greenbuildingadvisor.com/blogs/dept/musings/when-do-i-need-perform-load-calculation#sthash.eFY6bmLE.dpuf

Since this house will have 3 showers/ 5 bathrooms and up to 10 occupants, I'm assuming the 4 gallons/minute DHW of the Greenstar 151 combi boiler, will not be adequate, and so I'm planning for an indirect fired DHW tank.

so I have 5 choices- 57, 79, 100, 131 and 151... Despite the modulating boiler, I'm hoping to avoid oversizing too much. I'd still like to get one that will modulate down to the smallest input when it is not so cold or only one zone is calling for heat, but still do the job well in the coldest weather.

Thanks

Paul48

Use Slant Fin's free download and do a heat loss. Rule of thumb only applies to the size of the stick you can use to beat your wife with, and using it today is foolish.

Ironman

Where are you located?

roundrightfarm

we are near far Western Maryland at 2600' elevation

Paul48

At that elevation, you will need a professional to properly set up the burner.

KC_Jones

roundrightfarm said:

There are no building codes in our area.

Who is telling you this? I grew up in Maryland and have a lot of family in Western MD. I just double checked before posting and every county in MD has a code enforcement bureau. You are putting up this whole building without permits?
http://mdcodes2.umbc.edu/amendments/amend04.htm

roundrightfarm

I am in WV, right on the border with Western MD. The only thing that is inspected here is the septic system and main electric panel. Western MD is a more precise description than NE WV (which could signify 3 different spots, depending on how you look at it) plus the town that we visit most often is Oakland, MD

KC_Jones

Ahhh WV that is completely different.

Rich_49

You certainly need a proper heat loss calculation performed . Based on that design load and your hot water usage will you be able to choose something sufficient for your building . With 5 baths however , I doubt that a combi is gonna be the way to go , no matter who makes it in a tankless type piece of equipment .

Ironman

Been a while since I've been to Oakland, MD. Did you know there are actually three Oakland, MDs?

I'm near Staunton, VA. RobG is near Winchester. Contact one of us if you need assistance.

roundrightfarm

Thanks Hatterasguy and Paul48 for making me aware of the SlantFin heat loss calculator. I have been playing around with it, and I have a couple questions to make sure I am understanding everything correctly.

I'm assuming a cold partition is a wall that has unheated interior space on the other side. This is not the case for any of my rooms, so this will stay at 0 for all rooms. I'm also assuming the floor and ceiling factors, for any floors/ceilings that have heated space on the other side is also 0. Are these assumptions correct?

To calculate the floor factor for my 4" basement slab, most of which is below grade, it says to use 0.04 per square foot of edge loss. I'm confused what this means. do I take the lineal feet of exposed floor edge (same as exposed wall length), divide by 3 to get the sq-ft of edge, (for a 4" slab) and then multiply by the 0.04 figure?

How can I calculate heat loss for an insulated attic space that is one large room? It has cathedral ceilings, so height varies. Should I take the average room height? Does the sloping ceiling affect the ceiling factor? what ceiling factor should I use for R38 fiberglass, 1.5" ventilation space, then 1/2 foam under metal roofing?

Thanks for your help.