I do think an air-to-water could make sense for us, though it wouldn't be able to handle the coldest days because our radiators aren't sized to keep the house warm at the lower water temperatures it puts out. The main problem is figuring out who can install one, since it's a pretty unusual product here.

(You get used to whatever units you're using, and the US units make some calculations easier and others harder. If I could switch it all over to the SI system without massive transition costs I would, though!)

You might be surprised at how well cast iron rads can provide comfort at low outside air temps and low flow temps. (I'm in neighboring Cambridge in an old, poorly/non-insulated house.)

https://news.ycombinator.com/item?id=39144329 has a bit of details on the experiment I ran back in 2022 to prove 135°F flow would work for us. (If you have a condensing boiler, you can run this experiment safely; if you have a non-condensing boiler, you can run it, but not for very long as you'll be damaging the flue and boiler with condensation at these lower temps.)

My outdoor reset curve (sadly, on a gas combi boiler because of the "pretty unusual product" factors) is now set to 105°F at 55°F OAT and 154°F at 0°F OAT (which is lower than the design temperature here, but it gave me more resolution to tweak the line to fit the loss just right; it's spot-on on the lower end, with the system running 22-24 hours per day when it's cold out and stays that way up until around freezing, where the utilization falls off).

Matching the gain to the loss quite closely has resulted in a house that's the most comfortable since we moved in in 2007 and gas bills with the combi went down about 46% (versus a 1990s oil-to-gas conversion of a 1950s boiler, so not a realistic comparison for anything that wasn't built by General Motors [not a typo]).

We have one loop with cast iron radiators, but the other two loops are modern baseboard. When we installed a condensing boiler in 2015 I needed to adjust the outdoor reset curve up so the loop that serves the first floor wouldn't leave it under temp on cold days.

Even our cast iron radiators are smaller than you might expect for the age of the house, because they were designed for water above its normal boiling point (using mercury pressure: https://www.jefftk.com/p/mercury-spill).

I also have one loop of modern baseboard. Fortunately, it's in the attic conversion where they did insulate the rafters while doing the conversion, so it works even at that lower temp. I did do something slightly unconventional in plumbing that zone in a primary/secondary and it gets the water from the boiler "first" and returns it to the primary loop ahead of the main zone which is all cast iron rads. That means the baseboard gets the hottest water possible and the full potential flow from the boiler if it "needs" it. In practice, that zone tends to only run 4-5 hours per day while the main zone is running 22-24 hours, so either what I did works really well and/or I didn't need to do it in the first place.

But, you've already discovered your reset curve with modern equipment, so you know the right answer for your place.

Thanks for the story on mercury pressurization! Fascinating. I learned a lot about our old house (originally gravity circulated as well, but near as I can tell, pressurized only to the typical 12-15 psi and with an in-ceiling green steel expansion tank: https://structuretech.com/wp-content/uploads/2020/03/Old-sch... )

(And of course, sorry to hear about your contamination inconvenience and expense!)

Jefftk I'm in the same boat -- what I'm going to have to do if I want to do the air to water heating is bring in a booster/combo unit that will boost the water temp to make the house the right temp. So I'm still tied to gas but much lower amounts - heat pump does the majority of the work and I still have protection on very low temp days.

Hmm. I wonder if adding an air-to-water pump to the existing loop, before the boiler, can be made to work? Perhaps with a microcontroller intermediating the thermostats on both to turn each on at the ideal time?

There are products that exist for that, though as always: maybe not in your market.

Over here they're called hybrid heat pumps and are quite popular. (At least with those that offer to sell them.) They're used in improving existing gas powered solutions. The heat pump takes most (or usually all) of the heating, while the existing gas heater provides hot water and can add peak heat if needed.

Can you point to some of those. I am curious at other options for my own heating operations but the landscape is difficult to navigate as a consumer and then you need to find people capable of helping out.

Sure.

Popular brands here are:

- Remeha, with the types Elga Ace, Mercuria, and Mercuria Ace - Nefit/Bosch with Nefit EnviLine hybrid and Bosch Compress 7400i AW - Vaillant aroTHERM plus/pro/split

There's also the Quatt Hybrid, which I haven't heard of before, but it looks nice. And according to their website it has a large market share.

That might work - an interesting take. I haven't really thought about the pre loop to the older boiler.

I think the challenge is determining how much energy you save. I guess we could run the calculations and back out nat gas savings.

Managing the microcontrollers might be a bit of a pain though.