Change tank, or add tankless?

[Mitchell-DIY-Guy]

Hi all, soliciting some opinions here. I'm having a new home built in TN. Sadly the upcharges from the builder on just about everything are exorbitant, particularly on things they are unfamiliar with. So I'm having to take things I do not want, but resolve to change them.

Despite natural gas at the home site--they have chosen to NOT provide a gas water heater, nor do they provide a gas outlet for a dryer in the laundry room. I'll add the latter myself after I move in.

So the house has a 50g electric water heater included. Nobody should be heating water with electricity if gas is available. So after move in I plan on doing something about it. Here are my two options.

1. Remove the electric water heater and sell it for salvage or as used, though it will be essentially new. Replace with a 50 gallon gas model with power vent. I can easily do the gas work, the plumbing work, and the vent work. A gas power vent unit runs around $1,000.​

-or-

2. KEEP the electric water heater as a storage tank, and use a small gas tankless to feed it. You do not need those large 199,000 BTU tankless models. Don't yet know what these will cost.​

So, thoughts anyone? Obviously selling a used water heater may take a bit of time, and I don't yet know which might be the best tankless in this operation, hence the questions.

Any thoughts would be appreciated.

 

[Dana]

Nobody should be heating water with electricity if gas is available.

Heat pump water heaters can have a lower operational cost than natural gas, and MUCH lower associated installation cost than a gas tankless. Since it takes the same or (usually) less power than a standard electric tank.

Using a gas tankless to heat an electric water heater is reinventing the modulating gas tank type water heater, but won't be as efficient. An HTP Phoenix Light Duty is a small burner (=cheaper smaller gas plumbing) all-stainless (= 2x the lifespan) commercial modulating condensing gas water heater, which might be comparable in cost to install and longer lived than the small-tankless + electric water heater kludge.

A 50 gallon heat pump water heater runs about $1300 at box store pricing, and uses about 1/3 of the electricity of a standard electric tank. During the summers it will lower the latent cooling load (humidity), during the winter ~2/3 of the heat going into the water heater comes from the home's heating system, which is usually higher efficiency than a plain old gas tank. Noise-wise they're comparable to a refrigerator, quieter than many mid-sized tankless.

The primary down side to heat pump water heaters is slower recovery time in heat-pump-only mode, but for a 2-4 person household that usually showers rather than taking tubbies a 50gal. is usually big enough.

 

[Sponsor]

 

[Phog]

+1 to what Dana says above. The one big, tangible advantage that gas tank water heaters have over heat pump hybrid electric tanks is recovery time. If that's not a big deal to you, then you don't get anything out of switching to gas. And I can't see how your builder would upcharge you too much to have hybrid instead of the standard electric tank. The only additional installation chore is usually the condensate drain line (although there can be other issues if it's supposed to go into a small closet with bad air flow).

The HTP tank w/ modulating burner that Dana mentioned is also a great choice. Depending on your home style and size, those can also sometimes be paired with a hydro-air coil (or radiative heat such as baseboard or radiant floor) to have a single highly efficient gas appliance serve the entire home DHW & space heating needs. I do realize you're trying not to reinvent the wheel here, just mentioning the other possibilities that make more sense than a hodgepodge of electric tanks and multiple gas burning appliances.

 

[Dana]

Since it's a new house and you have some control, you might be able to convince them to NOT oversize the furnace & AC.

A typical reasonably tight code-min house will have a cooling load of about a ton per 1400' of conditioned space, and a ton per 1000' would cover the VAST majority of new houses and might still be OK from a comfort & efficiency perspectives, yet somehow the HVAC hacks seem stuck on a ton per 750' or even 500', which makes for noisier less comfortable and less efficient.

Similarly, heating systems on new houses tend to get grotesque oversize factor. A typical 2500' code min-house in TN might have a design load between 25,000 - 30,000, and would be OK with a 2-stage 40,000 BTU/hr furnace, but installers are more likely to install a 60- 100,000 BTU/hr furnace (sometimes to have enough cfm to the air handler to work with the ridiculously oversized AC.)

If it's already too late it's not worth yarding the equiment out. But if it's not too late, hiring a professional engineer or even a RESNET/HERS rater to run an aggressive (per the instructions in the manual) Manual-J heating & cooling load calculation would be $500-1000 well spent, saving a fraction of the fees in down-sized right-sized equipment. ASHRAE recommends no more than 1.4x oversizing of capacity for the heat load at the 99% outside design temp, which is enough to be able to cover Polar Vortex disturbance cold snaps, but still run a 70% duty cycle at the typical 99% outside temp. It's more comfortable to have the thing run long efficient heating cycles rather than the more typical hot-flash followed by the extended chill. When things are too oversized and running low duty cycles, odd rooms with different heat loss characteristics such as bonus rooms or sun rooms end up being underserved and quite uncomfortable.

It's worth reading Nate Adams' freebie chapters on the subject and viewing his short videos. He is a contractor in Cleveland Ohio who has made a business out of fixing comfort problems in homes, with a combination of air-sealing/insulation and right-sizing the HVAC. If the mechanical equipement hasn't been purchased or installed yet, this is an opportunity moment to get it right, and the sad fact is that it's almost NEVER right unless working with high performance home architects/contractors.

More on oversizing AC here. This graphic was compiled by a company in Decatur GA plotting the Manual-J cooling load vs. house size for a few dozen of their clients (mostly in the gulf coast states.) Only the very worst performing homes in the sample came in with a cooling load/size ratio worse than a ton per 1000' of conditioned space. Reasonably tight new homes will all be above that line.