Mod-con upgrade?

BrianPhilly

Greetings All,
Looking at a boiler replacement in the near future - while the existing 160k Laars is apparently in pretty good condition for being 30 years old, it's well oversized for 1200 and only exacerbated by recent insulation projects in this old Philadelphia masonry home

I'm rather tempted to pick up a new Laars 2-stage, JVS100 with ODR that I can get pretty inexpensively, not have to move any piping, and call it a day. The house is all fin-tube baseboard and I can't expect condensing return temps anyway.

What am I missing out on this opportunity to go with a wall-mount modulating unit? Is cycling between high and low fire to maintain a water temp really all that different, practically, than a modulating gas valve? It's certainly simpler. That and the chimney/utility room is in the middle of a finished basement, so venting horizontally is a challenge

Ironman

Where did you get the idea that a mod/con won't condense with BB connected to it?
BB has about 1/2 of its rated output @ 140*. If your ODR curve were properly setup, you would only need 140* or less when your outdoor temp is 35* or above. A mod/con would condense @140* or below. If you'll check the average mean temp for your area, you'll find that it's probably higher than that. That means your boiler would condense the majority of the time.

A 100k btu boiler would be 3 times the size your house needs. Not a good investment, no matter how cheap it is.

BrianPhilly

Hmmmmmm.
Well, I enumerated the zones and radiation

1a) living room, kitchen - 30 ft @ 500/ft @ 180 - 15k
1b) dining room 250sf radiant wall 10k? (more on that science project later....yes the supply is 170 deg)
2) 3 bedrooms, 2 bathrooms - 45 ft, 22.5k
3) finished basement - 30 ft, 15 k
4?) - future finished attic space, ~300sf radiant floor, 10k tops

Even with everything on at full output (though the basement almost never needs heat, and the attic is 2 years out), 75k btu. I see what you mean, 100 is definitely still overkill. (The temptation was that it is smallest model with low and high fire)

So I guess the options are either
-Do nothing
- a "new" 75k 85% single-stage, which would debatably be not much of an upgrade from keeping the existing 160k [which is turned down to about 90k input, I forgot to mention]
-Somethling like a Lochinvar, Buderus, Triangle tube, etc, ~70k modcon
-Go a little higher, a combi into the 85k range, and toss the 15yr old DHW unit in the trash. Though 85 seems too low to potentially support 2 simultaneous showers without some storage

gschallert

That's just your radiation capacity, what's the actual heat loss of the home? Until you know that you won't know how many BTU's you actually need.

Dave H_2

Gotta do the heatloss otherwise you are just guessing.
Case in point, I installed a new mod-con in my mother-in-laws home and then measured up the radiation.
With those numbers, I was way over-radiated so then I was able to run the boiler with a MAX water temp of 150 F. and the minimum was set for 120F.

Dave H

BrianPhilly

Thanks for all the insight guys - this is really helping clarify things
I work in the commercial cogeneration/boiler world so I'm familiar with these concepts, but primarily installation /startup /commissioning, (hence considering this install myself), not sales or equipment selection (hence asking for advice!)

The boiler is almost never running 100% duty cycle at 180SWT. It takes a rather cold (~10 deg) day to keep from bouncing off the high cutoff.
That said, I still have to deal with 1100 SF of uninsulated drywall-lath-block-stucco exterior wall that I haven't (and probably won't) touch, and a number of old windows. My first crack at a heat loss came up with

Worst case (70 deg delta)
58k for the home, 12k est for infiltration

0 deg outside, 70k
32 deg outside, 38k
50 deg outside, 20k

Based on this calculation, a 69k with 4:1 turndown looks like the ticket.

At these low BTU levels, a combination unit seems out of the picture. Maybe just replace the $500 B-vent HWH and call it a day.

Out of curiosity, Why not get something like a Laars Mascot FT 140k (input) or Triangle Tube Challenger 124k (input) combination unit. Output ranges 25k ~ 130k - obviously they would be operating in the 25-70 for space heating, and only use the upper end for DHW and heating at the same time.

There doesn't seem to be any cost or installation savings premium compared to a 70k heat only mod-con. I can see the obvious downsides to being oversize in on/off, however not so clear for modulating