Best Of

You're welcome, @FirstTimeHomeOwner .

The Accel CS is a modulating condensing boiler and can modulate down to 18,000 BTU/hr.

We recommend that you zone comfort areas, so your idea of separating living areas and bedrooms is a good one, but you could leave your zones as they are if the heat uniformly. Just do not zone each bedroom separately, which would be micro zones (an exception may be a separate zone for a master bedroom with a master bath).

I am not sure if your quote includes the domestic hot water system, your heating pro would need to clarify that for you. Each Accel CS does come with controls so it is ready to add an indirect tank or plate heat exchanger and storage tank with thermal purge (which is much more efficient as the boiler finishes cold each hot water cycle vs finishing hot with an indirect tank).

Best,

Roger

@Kaos , you’re spot on about micro zoning - it will keep the boiler idling and waste energy.
But the most efficient state for a boiler is when it’s off, so the best design is thermal purge so no energy is wasted at the end of each cycle. Properly designed indirect tanks with plate heat exchangers and thermal purge will exceed the efficiency of combi boiler water heating. Last month's ASHRAE journal also had an article that highlighted why heat pump water heaters have major issues (water damage and mold in enclosed spaces of outside walls due to cold discharge air), they operate at outputs and efficiencies far below their ratings (the rooms cool off and drop performance as there is no external heat source to feed it). Further, Vermont has relatively “cheap” residential electricity ($0.225/kWh recently) compared to the rest of New England, but that’s still $6/gal propane and $9/gal fuel oil equivalent so the cost to operate the heat pump water heater is more even without including the effective impact of running a small air conditioner in your home in the winter. Most of the boiler country in the US does not have comparatively inexpensive Canadian electricity prices found in some provinces you reference.
Roger

@Kaos , the controls have to be designed to allow stratification of the tank so that continuous cold water make up feeds the plate heat exchanger. The entire system has to be designed properly or you end up with lower performance and the boiler finishes hot which wastes energy (as it does with a conventional indirect tank and combo/instantaneous systems).

Regarding heat pump water heaters, the article indicates this is a common problem that has to be solved before widespread use of heat pump water heaters. This makes sense because the rooms (and outside walls) are chilled as part of normal operation. This introduces a new mold and condensation problem in the outside walls that did not exist before the heat pump water heater was installed. The authors identified the following issues:

1. Inadequate heating capacity compared to advertised specifications.
2. Real-world efficiency significantly lower than rated efficiency.
3. Moisture-related damage and mold growth.

And believe the issues generally track back to two primary causes:

1. Insufficient access to the thermal resources inside the building.
2. Improper handling of cold exhaust air.

Roger

The important bit is not the max output as most will cover your load but the minimum output. You want this as low as possible to avoid cycling.

Around me propane heat is only a bit cheaper than resistance heat so you have to take some care to get actual cost savings.

This means setting an aggressive outdoor reset curve and not micro zoning. Ideally you want to boiler to fire end of fall and not shut off till after winter. That won't happen in the real world but you can get close with the right equipment and proper configuration.

Most hydronic installers will push for an indirect for hot water. These are not as efficient and most believe (best case is mid to low 80's) and more expensive than other options. Cheapest hot water is a heat pump water heater, next best thing if you water quality is decent, is a propane tankless. Power vented tank water heaters can also work but have similar operating cost as a resistance tank around here.

Go with a boiler that has been.

1. Properly sized
2. Installed correctly.
3. Can be supported locally with parts and skilled contractors. For example in my area IBC, NTI, Weil McClain (mod con) aren't readily carried at supply houses. Not that they are bad boilers, but if you need parts will there be a delay in getting them? I had a customer that had a WM Aquabalance that just needed a sensor, It took a week to get in. Two day shipping cost 3 times more than the part.

The parts and service for the life of the boiler should be considered. Each manufacture is going to have some issue at some point. As a contractor we looked at the support and parts network in our area and went from there.

Combis have a place, it's not my first choice for most applications. You often times have to oversize the boiler from a heating perspective to accommodate the DHW. I think we have installed one in the past 5 years due to space in the home for a remod job. Typically if we use a modcon and the customer wants DHW on it we install an indirect tank. That combination has been successful and left customers satisfied.