Condensing Combi VS Condensing with Indirect Tank

[djwc.murphy]

Hey everyone,

Looking for some help on which way to go. I'll try to make a long story short and I'll be happy to answer any questions.

I have roughly 3000 sqft house that is heated by hydronic baseboards in three zones, one for each floor. Currently they are powered by an electric boiler and the cost is killing us. We had a couple $1600 bills over the winter. We live in British Columbia, west coast of Canada and electricity is very expensive here. So, I am looking to switch to natural gas. Speaking of location, our winters are considered mild, at our coldest we get down to about 28F/-2C. Maybe a bit lower in the last couple years

My questions is whether I should look at a combi condensing boiler or a condensing boiler with an indirect tank. I should mention the bottom floor of the house is a suite, so there is a tenant in there who I have no control over when/how they use their hot water. They have a washer and then just domestic taps, no dishwasher.

A contractor I am talking to right now is suggesting a Navien combi condensing unit, which after a short look online I am very skeptical of. All I see are nightmare stories from both homeowners and contractors on the units themselves and the support from the company.

Any insight and advice would be greatly appreciated!

 

[Dana]

At your fairly cool incoming water temperatures a tankless combi is going to be fairly flow limited, but if there are only two bathrooms and no huge spa-tubs to fill an NCB-240E might cut it.

Buj you may also be radiation limited on the independent zones for running the thing in condensing mode without short-cycling it to death. Measure up the baseboard on each zone, and run the napkin-math.

The minimum modulated output of the NCB 240E is about 17,000 BTU/hr. When running at temperatures that would actually deliver 95% efficieincy the baseboard only emits 200 BTU/hr. So to balance the heat emitted to the min-fire input to the system you're looking at ~17,000/200= 85 feet of baseboard. If a single zone has substantially LESS than 85 feet it's going to be cycling on/off during a continuous call for heat. How rapidly it cycles is a function of the heat being emitted, the thermal mass of water in the zone plumbing, and the temperature differential of the boiler. You probably have more than 85' total, but a couple of stubby-radiation zones can eat into both efficiency and longevity.

Unless yours is the crappiest house in BC whole-house heat load at -2C is probably no more than 30,000 BTU/hr, and could easily be under 25,000 BTU/hr, which isn't a heluva lot more than the min-fire output of the NCB-240E. Ideally the minimum modulated output would be no more than half your design condition heat load, so that it can actually modulate much of the time. If your whole-house load is 25K @ -2C, the load will be less than 17,000 BTU/hr whenever it's above 5C. I suspect +5C is roughly your binned hourly mean temperature for the month of January, and that most of the heating season hours will be above 5C.

There are a lot of assumptions in the above analysis, but it's probably not ridiculously far off.

Combi boilers are usually best suited for houses with modest domestic hot water flow requirements and pretty high heat loads. The NCB-240E has enough space heating output at high fire to heat my 2x4 framed 1920s antique 2400' house even if I moved it to the south pole, and probably yours too.

 

[Sponsor]

 

[djwc.murphy]

Thank you very much for the detailed reply! Much appreciated. It's crazy that I get better explanations from strangers on the internet than from the contractors I bring in to quote on this job.

That's the thing that is frustrating me most. Not a single company that has quoted on this has even mentioned doing a heat loss calculation or measured baseb

 

[Dana]

Thank you very much for the detailed reply! Much appreciated. It's crazy that I get better explanations from strangers on the internet than from the contractors I bring in to quote on this job.

That's the thing that is frustrating me most. Not a single company that has quoted on this has even mentioned doing a heat loss calculation or measured baseb

Unfortunately that seems to be state of the industry. In my area at least some contractors will measure the baseboard, but then specify a boiler capable of maintaining an average water temperature of 180F for the combined total lengths without questioning whether that would even be needed. While the amount of radiation is an important factor in a system design, specifying boiler capable of running it at 180F AWT is just plain wrong.

Still others use a really crummy rule of thumb based on the total floor area of conditioned space along the lines of "There's 3000 square feet of space, times 25 BTU/foot gives you a heat load of 75,000 BTU/hr, so let's upsize the boiler by one size and install something that can deliver 100,000 BTU/hr."

Both methods are completely flawed, guaranteed to oversize the equipment by a large factor.

 

[Jadnashua]

Trades in the USA tend to be very slow to change, and the apprentice learns from those before him. It's rare for someone to strike out on their own and actually embrace the 'new' stuff. Ages ago, energy was cheap, and people's expectations may not be all that high. We've learned that you can save money, energy, and improve comfort, but it takes some work. It's not necessarily that they don't care, they just don't know, and are relying on what their father's did and were taught. It bugs me that the non-pro ends up knowing more than the pros we employ to supposedly do it 'right'. There are some out there that do know what they should, and do it according to the latest industry guidelines and practices. They're hard to find. You need to do your homework, and you really shouldn't have to, but that just isn't they way things are.