Tankless Water Heaters

Matt_21

would a tankless water heater be more efficient in a new house than using an indirect on a condensing boiler. also any particular manufacturers i should stay away from on the tankless if i use it.

Constantin

Depends largely on usage pattern, IMHO.

If you're in the house constantly, then I doubt a tankless will offer a great efficiency increase over a indirect plumbed into a condensing boiler like the Munchkin or a Vitodens.

I see tankless water heaters as a boon to homeowners who have an old cast iron boiler with a coil. Under those circumstances, a tankless water heater will save you a lot of fuel, simply because the old CI designs have low efficiency and large standby losses.

Tankless systems also compare quite favorably with regular gas water heaters since a regular gas water heater maybe makes it up to 60% efficiency or so. Combined with relatively large standby losses (remember the big flue inside), and regular gas waters don't look so good. The standby savings only increase proportionally if you don't use the house much (think 2nd home, etc.)

The main benefit of an indirect water heater over a tankless one is the big slug of hot water it has on tap when and where you need it. If you own a jacuzzi or a similar water-hungry device (some of the new showers are somewhat ridiculous in their water consumption) then a tankless may not be able to keep up with demand, while an indirect may.

Look at the flow rates of the devices in your house and calculate the max GPM your water heater may need to sustain. Then compare that to the tankless GPM capacities at the maximum temperature rise the tankless will have to sustain (typically around 80°F)... As a consequence, a tankless may not be able to keep up with demand in the morning.

Uni R

Tankless water heater

Not unless you are normally never there or the house is so incredibly long that it would make sense to use multiple tankless units near the points of use.


An indirect does not lose much heat, and what heat it loses doesn't really matter during heating season. Also, how many gas burning devices do you want to maintain?

larry

An opinion for a non-professional...

I was under the impression that a modulating-condensing boiler typically ramps up to a relatively high temperature when it is servicing the Indirect DHW tank. As an example, when you look at the Viessmann documentation for the Vitodens, they show how boiler efficiency approaches 85% under these conditions as your probably not condensing at all. That efficiency is within of couple of % points of the efficiencies that the tankless-on demand systems (e.g. Rinnai) list in their liturature. To a first order of approximation, it seems like the efficiencies are pretty much a wash between the two ways of heating domestic water.

There is also the issue of capacity and use patterns to consider with an on-demand. One subtle issue with the on-demands is that they have a minimum firing rate, so you just can't generate a trickle of hot water if thats ever desired. Fill times on large tubs could also be an issue as well as problems steming from hard water. The other potential issue with the on-demand units is that they are VERY BTU hungry when operating at capacity. You might find that your existing gas piping within the house might need an upgrade depending on the BTU demands of your other equipment already in place.

Beyond that it comes down to standy losses. You can always do a back of the envelope approximation of the standby cost for a reasonable indirect DHW tank (i.e. 1/2 F degree/hr loss). As an example, a 50 gallon tank has a water weight of about 418 lbs. Since 1 BTU is the energy it takes to raise 1 lb of water 1 degree F, you can approximate that in 24 hrs at 1/2 degree F loss/hr you will have lost about 5,016 BTUs. In 1 year this is about 1,830,840 BTUs. With a boiler operating at 85% efficiency you would need to consume 2,153,929 BTUs of fuel to regenerate this loss. At about $1.60/therm the indirect is costing you about $34 in standby losses. I doubt you will ever make up the difference in installed cost between an indirect and an on-demand even if gas prices reach $3.00/therm given the expected life of an on-demand is about 20 years.

Uni R

$34

I'm impressed by the math Larry but I would further divide it by 2 unless that indirect tank is outside of the house, since that heat isn't really being lost during heating season.

larry

Good point! I'm sure some of that heat makes it into the living space, I'm just not sure how much. The insulation in the basement ceiling will tend to keep the heat in the basement, and there it might be eaten up by cold concrete walls, windows and other sources of basement heat loss.

If you look at the output of heat calc programs like HVAC-Calc you will see a large difference in heat loss between having basement ducts that are insulated vs. not insulated. This is implying that much of that heat loss isn't really making it into the living space. In similar fashion, I'd guess the same would apply to the indirect tank.

Constantin

Be careful where HVAC-Calc gets its results from...

...one gentleman discovered that the alleged heatloss through his ductwork in the basement exceeded the heatloss of the entire basement, were it part of the conditioned space! It appears that "exposed" ductwork is assumed to run through exterior conditions... not an enclosed crawlspace or finished basement.

So while I think HVAC-Calc is a terrific program, it is appars that it was exclusively developed using some Manual-J tables that are based on assumptions that may or may not reflect reality. As best as I can tell, HVAC-Calc seems to do nothing more than interpolate some Manual-J tables based on user input.

It may also explain why when I called with a tech support question that the author of the program was initially at a loss to determine what setting I should use for a wood fireplace that had a Locktop damper in the flue. In the end, he agreed that using the "Best" setting probably was the most appropriate, even if we weren't going to use exterior combustion air or glass doors on the fireplace.