Concrete slab woes; need new water lines, radiant heat is leaking.

[Dana]

EGAD! The MINIMUM-fire input on the Dunkirk 95M-200 is 80MBH (80,000 BTU/hr) which is enough burner to keep my house toasty even at -100F, if I had enough radiation to emit that much! At the 200 MBH high-fire it's enough boiler to keep my house warm at an outdoor getting closer to absolute zero than my actual outside design temp.

HOW big is this place? (If it's under 10,000 square feet, it's too much boiler.)

The problem with an 80 MBH min-fire output is that there's no way to run a low-mass radiation zone without short-cycling it into lower efficiency and higher maintenance. Even at 190F output (does it go that high?) for a 180F AWT it would take 300' of fin tube to balance boiler out with radiation output. With slab radiant you have some thermal mass to work with (the thermal mass of the concrete), but it's probably short cycling a bit even on those zone calls.

There is clearly no way to install enough radiation on this zone to keep it from cycling. The best thing to do is to size the radiation to be able to deliver the heat at 125F it will fit, and let it short-cycle. The thermal mass of cast iron would help a bit, but not enough. When its time to replace this boiler (and it probably won't make it 20 years, maybe not even 15) be ready to install something more appropriately sized for both the load and the zone radiation. About 19 out of 20 houses could be heated adequately with a condensing boiler with less than 50,000 BTU/hr of maximum output, and 99 out of 100 with something with 75K max, which is your MINIMUM output.

 

[Nitroman58]

The square footage is approximately 3300. Usually the water temp goes up to 160-166F max.
I am not interested in buying another boiler at this point. I may have to abandon the radiant heat however. The line will be pressure tested next week.

 

[Sponsor]

 

[Dana]

I wasn't suggesting that you scrap a functioning boiler- even if it's short-cycling it's not economic to do that.

When asking the question about which boiler I was trying to get a handle on how much radiation you would need to suppress short cycling on zone calls, and the answer is "you can't get there from here":

Running the thing at its max temp you'd likely end up with an AWT in the 155F range. Fin-tube would be emitting about 400 BTU/hr per foot, so to balance with 70,000 BTU/hr of boiler output (87% efficiency) would take 175' of fin tube in that zone, which is simply not happening.

Radiators would be delivering about 120 BTU/hr per sq.ft. EDR, so it would take about 585 sq. ft. of radiator to balance. That's 585", or about 50 running feet of SunRad radiator width. You probably don't have that much free wall space on the zone.

And that's at it's max water temp, which is well above the condensing range.

Even if you had the space for that much radiation, it would be cheaper to install a small mod-con than to buy that radiation, even at scrap-radiator pricing. Installing more radiation than is necessary for condensing operation doesn't make economic sense.

Only when the gargantuan Dunkirks' day is up does it make sense to buy a new boiler, but when you do, install something right-sized. Even if you had the crummiest leakiest 3300' house in PA (and clearly you don't) you won't need more than 80K of boiler output. A typical 2x4 framed 3300' house from the 1950s with typical insulation and window updating would come in around 50,000 BTU/hr @ 0F. A boiler with 70K of output at max fire would be just fine, enough boiler to cover you through the all-time record low temps, but with a minimum-fire output sufficiently low to not short cycle on zone calls. There are several models that would fill the bill.

If you want to try running your own I=B=R load calculations to figure out the room-by-room heat loads, start here. If you're going to re-radiate the whole house, hire a HERS rater or an engineer (and NOT an HVAC contractor) to run a room-by-room Manual-J for you. It'll cost a good fraction of a grand, but safe you a lot more than that in radiators (and future boiler .) The whole-house load can be calculated from fuel use data.