Radiant Floor Heating/In-Floor Heat, Boiler vs. Water Heater
We had our house built this past spring, and had wirsbo installed before the basement floor was poured. The basement is unfinished and about 1000 sq ft. There are 4 zones to the system, but its just roughed in with the tubing. We are not planning to finish the basement anytime soon. We would like to get the in-floor heat going, but we are not sure which way to go with heating it. We are not planning to use seperate zones, and we would like to keep costs down. All of our appliances are hi-efficency, so thats another variable in the equation.
Do we use a boiler, electric or high-efficency gas. Or do we use a water heater, electric or high-efficency gas? I have been told we could go either way, but not from any real experts, so please tell me your opinion.
Here in Minneapolis we have many air-over-radiant HVAC systems in homes built over the last 20 years or so. Since warm air is king in the US and it does get sticky here in summer, it is hard to get around the value of a forced air heating and cooling system for the upper levels.
As you have concluded, a basement or walk-out is the perfect place to install infloor heat with PEX tubing, as forced air rarely keeps any sub-grade level comfortable.
For low-load systems, such as yours, it makes no sense to me to use a dedicated boiler since there are currently none available with minimum heat outputs (lowest fire available) that would not suffer fairly short cycles (on and off), the results of which will be decreased comfort and efficiency while service may go up. Like cars, high efficiency boilers do best when they are running full time. Think highway vs. city mileage. This is most especially true if you take a low-load basement such as yours and split it into 4 "micro" zones. If each could call the boiler independently nothing good would come of it.
Cast iron boilers must be run at high water temperatures and therefore must be "mixed" down to properly serve low-temperature radiant floors. We re-fit many old cast iron boilers for radiant service but rarely design a system on purpose.
We also install a good many electric boilers here in the Twin Cities, as we have off-peak rates in the rural areas, along with propane and of course natural gas in the Metro. Which fuel you choose depends on the your cost for fuel. Around here electricity generally costs 3 times as much as natural gas so the bulk of our work is with natural gas-fired boilers and water heaters. Electric boiler are the relatively easy to design with and install lacking the requirements of venting, gas, drain and to a lesser degree control but still suffering that nagging high energy cost.
If you have natural gas, this will likely be your choice.
A gas-fired water heater is the thing. Now, which one and will it be dedicated (acting as a boiler) or combi (producing heat and domestic hot water from one burner/tank.
Tank-type storage water heaters afford the savvy radiant floor designer the option of serving micro-zones without compromise. These low-load systems can draw from the stored energy without calling the burner on and off too often. This saves money and adds to systems efficiency and reliability.
The least expensive water heater option is to use a dedicated water heater listed by the manufacturer for space and domestic hot water. This will likely be direct-vent water heater with an output matching the heating load. The operating cost will be modest but still likely run 20-40% more than its high efficiency condensing cousin. This unit will need fresh air from outside (combustion air) and will make some noise. You must also protect the water heater for "sustained flue gas condensate" by means of a mixing device or its serviceable life will be greatly reduced and it will likely create a CO hazard as time goes on.
High efficiency condensing water heaters recover most of the energy lost up the chimney by conventional "atmospheric" boilers and water heaters. With stack temperature around 100°F instead of 350°F+. Besides saving money on fuel, condensing boilers eliminate the potential for acid rain as the "acid" goes harmlessly down the drain after the bulk of the heat energy is transferred to you home.
Here is the rub. Though the bulk of our local business involves the use of high efficiency condensing boilers and water heaters, the majority of hot water heating systems in N.America are still served by low-efficiency appliances. The reason is simply one of economics.
A condensing water heater will cost nearly three times that of an atmospheric atmospheric water heater, so if you don't mind cold outside air coming into the house, noise and higher fuel bills or high efficiency is just not in the budget, a conventional water heater is the thing.
To combine or not to combine.
We design and install many "Combi" space and water heating systems and determine which we will use based on the demands of the home and the budget of the homeowner.
Bradford White has made the low-efficiency Combi-Cor for many years. This unit has a built-in heat exchanger separating space heating water from potable by means of a built-in heat exchanger. It is functional but requires considerable knowledge to trim and balance it correctly for service.
The other option it to build-your-own by using a sub-system with heat exchanger to drive heat from stored bathing water to the heated space. Any factory listed combi water heater can be used and fan-coils (think of the radiator in your car) can be employed to heat air if you wish.
In the end a qualified hydronic designer is needed to perform a proper ACCA Manual 'J' heat load to determine the size of the load and then consider integration match output to load and budget. These are the things that get me out of bed in the morning.
PS A zone is an area controlled by a thermostat. Each loop in a radiant floor manifold may cover a zone. If you want an option for 4 zones in future you will need a manifold with the proper trim to allow you to operate the basement as one zone now and add multiple thermostat/zones later on.