Boiler opinion based on calculated heat loss

TT_Vert

I am heating a very large garage and bathroom w/ many windows. Based on my heat loss calculations I'm at 106,712 BTU total. 20,122 for the bathroom and 85,589 for the garage. My question is regarding sizing the boiler. I do NOT use it for DHW, it's solely for radiant flooring. I am looking at 2 units. First, isn't the DOE calc. just max input times AFUE? Does DOE take into account thermal losses in piping, etc, hence the lower ~92.7% values below?

• Unit 1 has a MAX input BTU of 110K but DOE is rated at 102K at 95% AFUE. It is not a COMBI.
  

• Unit 2 has a MAX input BTU of 120K and DOE is 112K at 95% AFUE. This a COMBI which I well never use the DHW side of. Should I steer away from a COMBI unit give i'll never use it for DHW?
  

Do you think unit 1 is undersized and unit 2 oversized given my heat loss calcs? .

Thanks much

SuperTech

I've read on here that the Slant Fin app usually shows a little more BTUs needed than you actually need, but it should get you in the ballpark of what you should look for.  
I'd look for the output rating, rather than input. Don't worry about piping losses, it should be minimal.  
I wouldn't consider any combis if you don't need your boiler for DHW, although an indirect tank is a great way to make DHW if you choose to go in that direction in the future.  
I'd look for boilers with a high turndown ratio, you will only need the maximum output on design days, its nice to match heat loss with boiler output when possible.  
I'm glad you are giving this a shot, you will probably be happier in the long run than going with the recommendation of locals. I knew from your other posts that you have the intelligence and ambition to get it done.  

EBEBRATT-Ed

Your calculations seem awfully high for a garage and a bathroom. I would expect 1/2 that amount or less

HVACNUT

The Slant/Fin heat loss calculator is pretty good.
Must be a big garage. What's the square footage, windows, insulation, etc. Tubing in slab? 
Is it being used as a garage? I can't imagine a scenario where I had a garage with a bathroom and not have hot water. That's just mean.

TT_Vert

Sorry guys, didn't see this in my spam folder.

EBEBRATT-Ed said:

Your calculations seem awfully high for a garage and a bathroom. I would expect 1/2 that amount or less

The garage is about 2100sq ft w/ 12' peaked ceilings. Bathroom is ~400sq ft w/ 12' peaked ceilings. It's quite a large space. Not to say I didn't do something wrong but i'm pretty confident I'm close. Is there an area people screw up? There was ONE area I was questioning and it's doors. I did list the garage door openings as doors as well and that did bring the # up some. The doors are 16'x8' albeit they are 2" thick insulated doors w/ a threshold and seals on all 3 sides.

@SuperTech I was also thinking that a COMBI wouldn't suit me well given I will never use this for DHW and it just seems i'm paying for more components I won't use. if I do opt for one for DHW down the road it'd be in a different location for sure and quite frankly I only plan to be in this house 7-8 more years. In the units I've been looking at I don't see "output" is there another term they may use in their spec sheets?

My concern w/ doing this is my lack of knowledge w/ the plumbing side of this. I do know I'll go w/ a primary/secondary loop (the existing plumbing isn't laid out that way) but there are things that keep coming up in my head that make me balk at tackling this. None of them major in the least but enough to make me second guess doing it. Right now i'm thinking about how i am going to do the intake/exhaust on this. I have existing 3" intake/exhaust plumbing which I have cut off at the ceiling and will use reducers since most seem to use 2" these days. However, given I have a rigid set of ducting already for intake/exhaust (Drywall is tight to the pipes) how in the world do I mount this unit and THEN get the intake/exhaust plumbing in there? is there enough area in the intake/exhaust ports of this to slide the pipe down into the unit enough to slide into the existing PVC and then just slide it back up to secure? Do they use rubber and hose clamps for securing the intake/exhaust to the unit? I may not be explaining that right but it is what popped into my head today while at the gym. Then i started to consider recirc pump placement, etc. I think I spent more time reverse engineering everything and running through the install steps than I did lifting today. Is there some online program that I can draw up a rough design of my system? If not I'm going to whip out CAD and spend way too much time doings this so please tell me there is something.

Thanks
Dave

hot_rod

From a rule of thumb approach, 25 -30 btu/ sq ft from a radiant slab in a shop, so 2100 sq ft at a high number of 30 btu/ft would be 63,000. Your load number of 85,500 wild require 40 btu/ ft. Seems high for a well insulated space?

You get about 2 btu/ sq ft for each degree difference 82 floor temperature in 62 space temperature would get you 40 btu/ft. Run your load numbers again to check that load.

TT_Vert

hot_rod said:

From a rule of thumb approach, 25 -30 btu/ sq ft from a radiant slab in a shop, so 2100 sq ft at a high number of 30 btu/ft would be 63,000. Your load number of 85,500 wild require 40 btu/ ft. Seems high for a well insulated space?

You get about 2 btu/ sq ft for each degree difference 82 floor temperature in 62 space temperature would get you 40 btu/ft. Run your load numbers again to check that load.

I'm not pro at this, in fact I didn't know what heat loss testing procedures were until @SuperTech pointed me towards the slantfin app. I do know the app takes into consideration a significant number of things including exterior building materials, window square footage as well as material/design, door design and opening linear footage, exposed exterior wall length, etc. I have no idea how much of an effect it has on the calculation but I can assure you I did enter this info to the best of my ability. Did you calculate a 11" ceiling? If i reduce my ceiling height to 8' I get ~67,416 BTU which is in your window. I have 12' peaked ceilings in both areas (10' wall to a 12' peak) so I averaged at 11'. Using a design day temp of 1 deg. here in my area.

Jamie Hall

I'm not going to contest your heat loss figures -- if you plugged your building into the Slant/in app correctly, and I have no doubt you did, it usually comes up with a pretty decent estimate.

So... Unit 1 would appear to be the unit of choice.

TT_Vert

Thanks much. I did find another boiler I think I like even more at this point. It's a Navien NHB-110 which has a DOE of 102K BTU. Not being in the industry getting units is proving difficult. Any suggestions on the best way to go about getting one ordered? Do you guys source your fittings/valves at your local HD or go to a dedicated pluming store? It seems my HD doesn't have bleed valves and the link in stock.

Funny enough I did go through my calcs and realized I had oversized my garage door openings a hair so i'm at 81,446 from the garage and still at 20,123 bringing me to 101,569 BTU. So if I "should" size to DOE i'm right there. I'd be curious what @SuperTech is referring to as far as output as I'm still not seeing that nomenclature on the spec sheets for these things (Maybe that's just DOE?). The other nice thing about this thing is that it has a turndown of 11:1 so on design days I'm right in this things max wheelhouse and on warmer days it can modulate down into the 10K BTU range. Let me know your opinions on this. I'm going to be picking you brains on my plumbing soon so I hope you guys are ready I'll probably create a new post for that when the time comes but I just want to make sure there isn't some water feed specification/flow restriction spec for 90 deg turns, etc.

ewang

Within Slant Fin's app, you can somewhat specify the wall, ceiling and slab insulation values to directionally point you in the right direction.