Can I do this to my modcon?

[Dana]

After the mistake is made is when everyone scrambles to find a cure and they invariably mess with the boiler when the problem can't be solved there. Even adding radiation won't cure the issue because the heated envelope can't shed heat fast enough.

That's what thermostats are for, eh? :-)

With low-mass emitters like fin-tube you're not going to get a monster temperature overshoot with just 2x the min radiation required to heat the zone, and if you did a T-stat with tweakable anticipation would usually fix it. With excess high-mass radiation you may have to resort to smarter PID algorithm thermostats if the radiation is grossly oversized & very massive, but that would be a huge boost in comfort too.

[Tom Sawyer]

Many years ago I had a rep that promoted ringing the exterior walls with baseboard which allows lower boiler temperature. Of course homeowners weren't keen on the plan but it does work. Unfortunately many see radiant heat and mod con's as a panacea but thermodynamics is thermodynamics regardless of the wiz bang technology. Funny that the Europeans are re thinking high mass cast iron boilers again. Maybe the old timers had it right. I install a lot of mod con's and I'll say right off the bat that sizing the boiler to the load and sizing the load to the envelope is way more critical than running copper fin tube and slapping a C.I. Boiler in ever was. I'd say that 90% of mod con/ radiant problems stem from poor or no calculations being done. Bigger is not better. I have seen far too many oversized boilers feeding insufficient radiant tubing. If the system is close it is possible to tweak thermostats and modulation settings. If it's way far off though, you are screwed.

[Dana]

A simple-designed ~2400' rancher built to current MA code min (IRC 2009) comes in under 30KBTU/hr at MA outside design temps, for which there are remarkably few boilers "right sized" to the whole house load at the 99% outside design temp, let alone what happens to individual zones at the average when they insist on micro-zoning it like this one. (It doens't take a very big napkin to sketch out the problem with THIS installation, even when writing in lipstick! :-) ) The "design by hackery, no math needed" approach is guaranteed to cause issues, independent of the thermal mass of the boiler. Short cycling a high-mass boiler is at least as big a hit to efficiency as low-mass mod-cons.

Europeans have far from abandoned modulating condensing boilers, and have quite a few tiny output mod-con options as yet unavailable in the US. I'm not sure where you're getting the impression that they're moving toward high-mass boilers on the right side of the pond. They're quite fond of low-temp radiation- panel radiators are the standard (and a good one), but high-mass low temp radiant floors are also quite popular amongst those who can afford it. But on new housing stock most European homes have a fraction of the design heat load of most new northern-US homes (milder climate, smaller houses). There is a burgeoning European interest in wood boilers and pellet boilers with high-mass thermal buffer tank storage, but for gas-fired goods mod-cons (and tankless-combis) are still dominating the market.

With oil-fired burners as with wood-burners, the limitations of minimum burner size drive them to higher mass solutions than with gas burners. It's possible to make VERY tiny gas burners (modulating or otherwise), but with oil burners the practical low-limit for burner sizing is about 14-15kilowatts or ~50kbtu/hr, which is well over the design-day heat load of a brand new western-European house (and 2x the heat load of many newer homes even in MA.)

[ToddinMaine]

A simple-designed ~2400' rancher built to current MA code min (IRC 2009) comes in under 30KBTU/hr at MA outside design temps.

Okay, that gives me some assurance that the quick 'n dirty heat loss calc. I did for my house, a new 1900 ft cape and -2 deg design temp, coming in at about 27K BTU is in the ballpark.

I've put both radiant zones on one t-stat and have set my boiler to 65% max output. My burn times are now averaging 12 min, aided by the fact that the two small fin-tube zones upstairs rarely call for heat.
If I back max power down to 50% and it does the job during the sub-zero weather coming up, can I be sure that swapping this 100k unit out for a 50k will be the right move? My reset is set at 180 w.t. at 25 deg o.d. temp.

[Tom Sawyer]

I believe a 50k would perform much better in your case so if you can swap it, by all means do.

[Tom Sawyer]

A simple-designed ~2400' rancher built to current MA code min (IRC 2009) comes in under 30KBTU/hr at MA outside design temps, for which there are remarkably few boilers "right sized" to the whole house load at the 99% outside design temp, let alone what happens to individual zones at the average when they insist on micro-zoning it like this one. (It doens't take a very big napkin to sketch out the problem with THIS installation, even when writing in lipstick! :-) ) The "design by hackery, no math needed" approach is guaranteed to cause issues, independent of the thermal mass of the boiler. Short cycling a high-mass boiler is at least as big a hit to efficiency as low-mass mod-cons.

Europeans have far from abandoned modulating condensing boilers, and have quite a few tiny output mod-con options as yet unavailable in the US. I'm not sure where you're getting the impression that they're moving toward high-mass boilers on the right side of the pond. They're quite fond of low-temp radiation- panel radiators are the standard (and a good one), but high-mass low temp radiant floors are also quite popular amongst those who can afford it. But on new housing stock most European homes have a fraction of the design heat load of most new northern-US homes (milder climate, smaller houses). There is a burgeoning European interest in wood boilers and pellet boilers with high-mass thermal buffer tank storage, but for gas-fired goods mod-cons (and tankless-combis) are still dominating the market.

With oil-fired burners as with wood-burners, the limitations of minimum burner size drive them to higher mass solutions than with gas burners. It's possible to make VERY tiny gas burners (modulating or otherwise), but with oil burners the practical low-limit for burner sizing is about 14-15kilowatts or ~50kbtu/hr, which is well over the design-day heat load of a brand new western-European house (and 2x the heat load of many newer homes even in MA.)

I didn't say abandon but they are toying with high mass, low temperature models

[BadgerBoilerMN]

When teaching professional HVAC contractors to "believe" that radiant floor would heat houses (20 years ago now) I often started my seminars by asking water a particular house may present for a heat load. The results written down and passed to the front of the room ranged from 10 to 75 Btuh/sf. I have observed no improvement being in the field again these past 12 years.

First, the block heat loads and then the room-by-room to confirm radiation and design water temperature. We do not use fin-tube on new construction, period. If you can afford to be comfortable, you can afford radiant panels.

[Dana]

Okay, that gives me some assurance that the quick 'n dirty heat loss calc. I did for my house, a new 1900 ft cape and -2 deg design temp, coming in at about 27K BTU is in the ballpark.

I've put both radiant zones on one t-stat and have set my boiler to 65% max output. My burn times are now averaging 12 min, aided by the fact that the two small fin-tube zones upstairs rarely call for heat.
If I back max power down to 50% and it does the job during the sub-zero weather coming up, can I be sure that swapping this 100k unit out for a 50k will be the right move? My reset is set at 180 w.t. at 25 deg o.d. temp.

I've never seen a 1900' house in New England (even an older house) with glass in the windows that actually NEEDED a 100K boiler, yet most older stock came outfitted with 150-200K or larger behemoths, and subsequent installers rarely corrected the situation. (The boiler in my ~2400' house was less than 15 years old when I moved in- a 6-plate cast iron beast with ~120K of output for a heat load under 50K, now well under 40K after tightening up the house.) Moving to a 50K boiler would DEFINITELY be a move in the right direction.

[ToddinMaine]

Okay, here's the confirmation I needed. I set the boiler's max CH power at 50%. With all zones calling, it took 22 min to bring the supply temp from 140 deg. to 180, then it maintained 180 at 37% power. So. Assuming that 50% power actually means 50K BTU for this boiler (and I intend to call Utica to confirm this) I should be okay with the SSC-50.
I could spend money on a buffer tank and the replumbing for it, and still have an oversized boiler, or I can spend maybe the same (or less) money to trade for an appropriately sized boiler, practically a drop-in job, and live happily ever after. No brainer.

[ToddinMaine]

First, the block heat loads and then the room-by-room to confirm radiation and design water temperature. We do not use fin-tube on new construction, period. If you can afford to be comfortable, you can afford radiant panels.

Our radiant is clip-finned pex suspended under the floor - it heats the airspace beneath the floor so the design temp is 180 deg, a good match for fin-tube. It's a rather common set-up in this area.
Plus, we have alot of large double-hungs upstairs, so the space from the window sill-to-floor is limited making BB's a good fit.

[Dana]

A buffer would be a band-aid (and an expensive one, if going with a BoilerBuddy or ErgoMax). Right-sizing the boiler is a much better approach, but as pointed out previously, even with the -050 it'll still short-cycle on the fin-tube zones unless you marry them into a single zone or increase the radiation on each of the small zones.

At 180F out it's probably getting ~87% combustion efficiency. If it's running 180F out at 37% of the -100's max fire that's (0.37 x 0.87 x 100K-in= ) ~32KBTU/hr, and you're already at the highest temperature that the PEX is rated for. If your radiation can't take anywhere near the full output of the boiler without melting, it's dead-certain that it's the wrong boiler, eh? ;-)

[Dana]

Our radiant is clip-finned pex suspended under the floor - it heats the airspace beneath the floor so the design temp is 180 deg, a good match for fin-tube. It's a rather common set-up in this area.
Plus, we have alot of large double-hungs upstairs, so the space from the window sill-to-floor is limited making BB's a good fit.

The finned aluminum butterflies on PEX approach to radiant makes some sense if you have an oil-fired cast iron boiler that can't tolerate condensing temps, but is wasting the potential condensing efficiency of a mod con.

Convecting panel radiators come in baseboard-heights if necessary and deliver more consistent output at low water temps than fin-tube. Ten feet of 12" tall panel radiator delivers ~9-9.5KBTU/hr @ 180F AWT, and is still good for ~5KBTU/hr @ 140F AWT. A 10' section of 8-9" high panel is good for ~7500BTU/hr @ 180F AWT. It's more money than fin tube (even the cheaper ones), but it's also more comfort, and capable of reaping lower temp/higher efficiency operation out of the boiler.