Combination Boilers, Pros and Cons

Brando

Hello again. I am putting in a radiant heating system and debating between 1) a tankless combination boiler, 2) two tankless water heaters (one for showers/sinks and one for heating floors), and 3) keeping my tank water heater for sinks and showers and adding a tankless heater for the floors.

The combination boiler would seem like the elegant solution. But some commentators feel that the technology is too new, repairs can be expensive and time consuming, and if your system goes down you lose both heat AND hot water.

What are your experiences with combination boilers? My contractor is recommending a Navien NCB-240, which has a GPM of 4.5. The house has two full baths and a full kitchen. Current residents number four. Only about 800 sq. feet will be radiant heating (the rest of the house, or another 1000 sq. feet, will remain forced hot air for the time being).

Any advice would be sincerely appreciated!

Hot_water_fan

That’s a small space - why radiant heat at all? It’ll be a challenge to size a boiler big enough for the domestic hot water and small enough for the heating load with a combi, even considering the modulation. 

Jamie Hall

I presume that this is the same system for which you have another thread going? And I will give the same answer I did there: using a domestic hot water heater to heat a radiant system is just wrong. Period. So kindly forget the idea of using a tankless or any other kind of domestic water heater for the heating side. You might get away with a tankless for the domestic hot water, if your hot water demands can be met by one.

The two options you mention using domestic water heaters for the radiant part are just wrong.

You ask specifically about combis. They work, and work well, provided that the relative loads are reasonable, so that both the domestic hot water side and the heating side are within their rated capacities. They are hardly new, and are no more -- nor less -- trouble than any other arrangement of tankless water heaters and heating boilers. The Navien is a good unit; whether 4.5 gpm of hot water will be enough depends on how you use hot water and what your fixtures are like.

Hot_water_fan

@Jamie Hall I think we know that tankless domestic water heaters and combination boilers are different, but I’m not sure that’s always clear to the poster. They are both tankless after all. I wonder if contractors are using shorthand. 

Alan (California Radiant) Forbes

I like:

3) keeping my tank water heater for sinks and showers and adding a tankless heater for the floors.

because your DHW demand is large compared to your heating load. But I wouldn't use a "tankless". Get a small boiler that will modulate down to 10,000 BTU or less. For that small of a space, I've seen people use a tank-type water heater.

Also, if your power goes out, you will still have plenty of hot water.

EBEBRATT-Ed

And stay away from Navien. There are you tube videos to watch

hot_rod

I'd have no problem using a dedicated hw tank for that small radiant load. Electric or gas fired. A second tank or tankless for DHW.

There are some DHW tanks that have hydronic listed stickers on them if codes are a concern. Not as efficient as a mod con or boiler but for a 16K load is operating cost an issue? 4 grand for a mod con, $500 for a tank water heater.

The Bradford White CombiCor are still available as special order. A tank with hydronic coil inside, for a single appliance solution.