Gas Boilers - Modulating, Condensing,..HELP!

PS

I realize more information is needed, but for the brave... HW, mono tee, single zone, CI radiators (2 story Cape). Heat load ~46MBtu/hr + DHW = 60 MBtu/h IBR? Located in MA where temps go below 0F at night. I have done much research but can't seem to nail this down - I am looking to replace old gas boiler (1940) and add indirect-fired water heater with mid or high efficiency system. Any recommendations and reasoning for MY application - considering modulating burner, condensing, reset controls, etc? I know it is alot to ask but this forum seems to have very knowledgeable contributors.
BTW - Any info on elastomer gaskets vs push nipples on the boiler sections? I have read many bad stories about gasket propensity to leak.

Constantin

Brave?

I'd consider a condensing, modulating, low mass boiler if you want to stick with gas as a fuel. A number of manufacturers make interesting units in your size range, such as the T50 or T80 series from HTP, the Vitodens from Viessmann, the GB142 from Buderus, the WeilMc Ultra, etc.

If I were you, I'd have the pro confirm the heat loss you calculated to be sure. Then, find the smallest capacity condensing, modulating gas boiler to meet that need and be sure it's a brand that your installer and service technician are familar with. Installation, proper design, and programming is everything... With a large indirect tank, you shouldn't run out of hot water if you don't on a standard water heater.

Modern controls, like outdoor reset, are a must and will repay for themselves in short order. Condensing boilers can better take advantage of outdoor reset than regular cast iron (CI) boilers can though, as regular CI boilers can only tolerate return water temperatures above 140°F (to prevent flue gas condensation).

I wouldn't worry about the boiler section elastomers as much as ensuring that the heat emitters in your home are large enough to keep the place warm on a design day with supply temperatures below 140°F. That way, your condensing appliance can run at a very high efficiency and save you a lot of gas.

PS

OK

Thanks. Now for the hard part - I know from experience that on the coldest days, I need 170-180F HWS in my CI radiators to maintain 68-70F indoors. I am also looking to add an indirect WH. Doesn't the fact that I will need ~ 190-200F HWS for the indirect and at minimum 160F HWS for heating season (Nov - April) preclude any advantage in efficiency from the condensing feature? My understanding is that the HWR needs to be < 140F in order to find this extra efficiency, and I doubt I could ever get away with that as far as heating.That is why I am also looking at the benefits of modulation and reset controls.

Constantin

Consider several factors:

Design day conditions only predominate 2% of the year, a full 7 days. Most of the heating season, even a baseboard-heated house can allow condensation as long as the circulation is constant.

Adding baseboard is cheap. The more you have, the lower the emitter temperature can become. Some of the newer panel rads like the ones from hydronic alternatives disappear into the walls.

Insulation and proper weatherization will also reduce your potential consumption a great deal. Consider hiring a blower-door test to find the leaks. See energystar.gov or your local utility for more info.

BTW, do you really need 180°F hot water on the coldest of days? Is the circulator system running 24/7 during those cold snaps or does it merely inject 180°F hot slugs of water whenever the thermostat calls for heat?

PS

Factors

I am limited in ways to decrease heat loss, but have improved where I can before costs become prohibitive (65 year old house, 1+" rock plaster interiors, T&G sheathing w/ wood shingles and vinyl w/ backer board over that). I will be blowing in insulation when warmer weather arrives, but my envelope will never perform up to modern standards (long story). FYI - My old gas burner is rated for 300MBtu and it barely maintained 68F w/ circulator and boiler running non-stop during a recent cold snap (=< 0F all day for 2 weeks).

Anyway, I am not planning to replace CI radiators so i realize that probably reduces my options. The final choice will be dependent on several factors - system resulting in reduced annual (see indirect DHW) and annual (see AFUE) NG usage (aka operating costs), initial cost and payback, reliability. I realize anything will be better than what I have - i am just looking for the most bang for the buck via the boiler and DHW system.

BTW - I am looking at a Buderus GA124/23 or G124X/25DI, but their website doesn't provide a lot of info. I am not locked into anything, but I must say i am VERY wary of W-M. System is only as good as the installation. As an engineer, proposals with accurate heat load calculations will carry a lot of weight with me - as it shows attention to detail and thoroughness. But I am no expert - which is why i value these forums.