Best Of

Allow me to amplify a little on @Ironman 's and @pecmsg 's comment -- with which I agree completely.

The Nest -- and it's occupancy sensing kin -- were designed for, and operate well with, forced air heating and air conditioning systems. They were not designed for any other type of heating system, and they do not work well with them unless they reprogramed to disable all their smart features (which can be done, although not easily) -- at which point you might as well have bought a regular thermostat for about $40.

If you do really want a Nest, that's your choice, but be aware that unless you do manage to completely defeat all of its occupancy sensing and time based setbacks, your nice boiler will not perform as expected, as its controls and the Nest will constantly be fighting each other. At best, expect higher fuel bills -- perhaps as much as 10 to 15 percent higher.

In eight years our WM modcon has required nothing but routine cleaning. I am a retired telco tech and took an interest in how this boiler works and why it works so well with the cast iron rads in our old house. The boiler is programmed to run between 88 and 140 degree supply water which is good to minus 20 degrees outside. It is pretty much condensing all the time in heating mode.
Do a heat loss survey, add up your radiator EDR and see if you stand a chance of running in the condensing range most of the time. Off the top of my head our heat loss is 62k btu and the radiators are capable of putting out 102k btu at 180 degree supply water. This is a 1740 sq ft house built in the 1830's. Modern windows, doors, insulation allow for lower supply water temps. We have ten radiators.
I bought a basic gas analyzer and do my own maintenance. Over the last few years I've picked up some spare parts, a blower, gas valve, circulator, spare maintenance kits. I haven't needed anything but the maintenance kits so far. The main board is quite expensive and would blow a lot of the fuel savings if that ever had to be replaced. Fingers crossed.
The modcon replaced a fuel oil cast iron boiler. Our local HVAC company installed it. The following year I installed an indirect WH to replace a very old electric WH.
Taking heating degree days into account and trying to make as fair a comparison as possible, we spent 38% less on home heating the first year with the modcon.
I'm hoping to get 20 years or more out of this modcon by keeping an eye on water quality and keeping it clean. When the time comes to replace this modcon, maybe air to water heat pumps will be something to consider?

Your list does not include FUEL COSTS.

Mod Con high-efficiency boilers provide substantial FUEL COST SAVINGS which your list of cons does not even include. If you are only looking at the installation and maintenance cost, but not the efficiency and fuel cost benefits, you are only kidding yourself.

We have 3 sister buildings, each about 45,000 sq ft, only one of which was retrofitted 16 years ago with Mod Con Boilers, Indirect DHW Tank, modern Ventilation systems, Driveway Melting and advanced process controls.

Fuel use on the two sister buildings with the 30-year old original natural draft boilers with ON-OFF control is DOUBLE that of the retrofitted building, because they also only think maintenance cost is important. I am quite happy to save about $20,000 / year on the fuel bill for the retrofitted building.

So the answer is - do the full economics - consider total long-term cost of ownership.

Doug

I'd look very carefully at circumstances for the fuel savings that are being presented. It is very likely that most if not all of the existing boilers were grossly oversized for the heat load and probably well past thier prime. This causes a very large reduction in efficiency.... especially when looking at atmospheric gas boilers. I have found that simply going from a 2x oversized atmospheric boiler to a properly sized atmospheric (in residential) is about a 12% reduction in fuel use. Tales of 36% saving are probably true, but how much savings would there be by just properly sizing the equipment? I have done many commercial projects where we replaced existing boilers with 2 or 3 staged atmospheric boilers and saw typical savings from 35 to 40%.
I just replaced my 60% oversized stack damper equipped atmospheric boiler in my home's high mass conerted gravity system( which I combustion tested and it was running about 30% excess air and about 84% efficient) and replaced it with an about 60% oversized tiny power burner gas boiler with stack damper ( firing about 30% excess air and 84% efficient) and it looks like I have reduced fuel usage about 20%.
A simple on/off cast iron boiler connected to a single zone high mass system is a very efficient combination. There is no need for outdoor reset, as the mass of the system automatically resets the water temperature, and the boiler will have very long on cycles with very long off cycles for very high operating efficiency. About the only gains you will have with a condensing boiler is due to the condensing, which is probably worth about 12% reduction in fuel use. These mod con fuel savings will also be offset in part due to higher electrical usage of a typical modcon ( 2 pumps, blower motor and onboard electronics running nearly 24 hours a day all winter long.

Also, Mod cons have been around a lot longer that 15 years or so. They have been in use in Europe for more like 30 to 35 years ( the Triangle Tube models are one example) and the typical life is in the 12 to 15 year range.

In my book mod cons are simply not cost effective. Focusing investment on reducing heat loss of a structure is likely much more cost effective, because it has a 4 fold impact....Lower heat loss at a given outdoor temperature, a reduction in the length of the heating season ( a building is self heating due to internal gains) , cooling loads are probably reduced, and you are more comfortable year round.