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Comment by sokoloff | Hacker News Reader

The prospects of higher fuel prices and a $1600 rebate from our gas supplier (MA, USA) helped me to decide to replace our ancient boiler (1950s oil, later converted to single-stage 200MBTUH gas) with a modern modcon combination boiler.

I haven’t compared degree-days of last year vs this year, but by the gas meter, through yesterday we’re using 40% of (60% less) on per-day basis as compared to last year.

It was many thousands out of pocket, but if the 40% holds up, it will pay back in about five years. (Our old system was dark-comically inefficient though. For most 80-ish% cast iron boilers, it doesn’t make economic sense to replace with a [theoretical max] 95% mod-con if the old one is still reliable.)

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I wish we could buy US-style furnaces and AC units. European combi boilers seem to give up the ghost after 10-ish years, regardless of manufacturer. You are very lucky to get 15 years out of any one without major issues. Planned obsolence maybe?

Mine is a Bosch, assembled in South Korea. From my research, these seem more common in UK and EU than over here.

It was also only $2550 for the boiler (much more for the trim, labor, pipes, pumps, etc., but the boiler itself was cheap). That was cheap enough that I considered buying an entire second one to just put on the shelf for the eventual need to replace it or replace major parts. I decided not to as the space required is probably more valuable than having a whole shelf in the utility room tied up for 10-15 years.

It's easy to buy a junk furnace in the USA. Even the good brands have begun to make low-end units with only five year warranties and few serviceable parts.

Is it because less heat escapes from the boiler or the burning is more efficient?

The overall burning cycle is more efficient. Even with tuning, my old boiler was putting out well over 200ppm CO (down from 2500/off-scale when they first tested it). That’s one source of inefficiency of getting BTUs of heat out of the gas and into the water. As a sibling mentioned, it was also sending 450+°F vapor-laden exhaust up the chimney and out the roof.

New boiler vents through plastic pipe and extracts so much more of the heat that the exhaust is leaving the boiler at around 150°F with the water vapor largely condensed (at least this time of year) and giving up that latent heat of vaporization to the water incoming to the boiler. Additionally, it pulls combustion air from outside instead of the basement. (If you pull combustion air from the basement an equal amount of outside air must leak into the house to replace it.)

New boiler also has a modulating gas valve and can burn in 5 steps between 25K BTU/hr and 149K BTU/hr (from memory, but close enough) to match the space heating demand. The old one was either 0 or 200K BTU/hr.

I probably went from 60% to 95% now plus the effects of not pulling millions of cubic feet of outside air in per year.

The computer controls on this one (and the ability of the boiler to not just tolerate but encourage condensation) mean I can heat my house at 32°F/0°C with 114°F/45°C supply water temps. Traditional boilers are harmed by acidic condensation so need return temps of 140°F which means supply temps of 160°F. That means hotter rads and pipes and more losses in the piping. As it gets colder, the boiler can compensate (via an outdoor sensor) and increase the flow temp so it’s just enough to heat the house. Based on 114°F flow (about 90-95°F return) being enough for 32°F, I'm virtually certain that I'll be operating in condensing mode (return under 120°F) even on the coldest nights we get here or if not, on all but one or two nights.

I have pump controls that can run the pumps for a bit after the zone and burner shuts off to put that (much smaller amount) of trapped heat into the living space rather than leaving it trapped in a massive hunk of cast iron to leak to the boiler room.

It’s a lot of small and medium effects.

Edit: it’s probably closer to a 1/4 million cubic feet of outside air pulled in for combustion rather than “millions”.

The old burner likely achieved ~100% combustion (not sure what you mean by the burning being efficient).

As to the other part, yes, less heat is sent up the chimney in modern designs. My boiler is getting pretty old but was sold as 80% efficient. As far as I can tell, the near drop in replacement would be 84 or 85%. Neither is a modern condensing boiler (which I'd have to figure out a new exhaust to switch and probably add active outside air exchange; tempting with the way the current one sucks all the humidity out of the house).

We replaced a 25+ year-old propane furnace with a high-efficiency natural gas furnace two years ago. The difference in waste heat is astonishing.

The old furnace vented through a vertical chimney vent and the exhaust was so hot that touching the metal pipe would burn your hand.

The new furnace is direct vented with PVC pipes and the exhaust outside the house feels barely warm — that’s a lot of additional heat that stays in the house instead of being wasted.

Between the cost difference of propane vs natural gas, better efficiency and a programmable thermostat, we easily reduced our fuel bill $$ by 50% with no noticeable change in comfort level. I did not track the difference in therms.

In a traditional design boilers/furnace you run hot combustion gases through a heat exchanger (HX) to heat water/air and then into a flue. The problem is that if it cools too much in the HX it can condense in the flue. With the condensate being extremely corrosive it will quickly destroy the flue and can cause all kind of issues from CO poisoning to fires. So the HX is small enough to keep the gases hot in the flue to prevent corrosion.

A more complicate design has a big HX and various pumps and fans to drain the condensate and push out the exhaust gases. It uses a PVC flue and drain line to withstand corrosion but at the end you still have moving parts exposed to the acidic condensate and it does not have the service life of a straight metal flue design. That, combined with the higher cost of a condensing heater (because of all the additional pumps and fans and corrosion protection), calls for a careful cost analysis since you need to burn a lot of gas to justify quicker replacement and more service calls for a more expensive unit.

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