Author Topic: Tankless gas vs heat pump electric water heater  (Read 426 times)

NonprofitER

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Tankless gas vs heat pump electric water heater
« on: March 05, 2019, 08:41:21 AM »
I've searched the forum but haven't found a situation where someone had gas but wanted to compare going tankless vs converting to an electric heat pump WH...   
People who are smarter than me:

Our WH is 15+ yrs old (as in, we can't find the installation date on it old) and we're researching replacements so we can be mobilized to act quickly when it eventually fails, or maybe even preemptively if the numbers make sense.

This is outside my realm of knowledge, so please help me understand the variables I need to collect to be able to compare apples to oranges: IE, what do I need to consider/compare re switching to tankless gas or heat pump electric in our situation? 
Facts known:
  • Currently have 40gal gas tank/old school WH. Gas bills are minimal.
  • City is offering $800 rebates on efficient heat pump electric WH's. These are first come, first serve, so likely only available in the first half of the year.
  • Current 40gal works fine in terms of supply/demand for our household of 3. When other people visit staggered showers/laundry etc are necessary, but that's manageable.
  • Gas company is offering $650 rebates on tankless gas WH's. Again, first come, first serve. Likely available until Summer.
  • City electric is tiered (I can break this down further if need be), with power adjustments during Summer months, since we're in the South (Austin, TX). We generally have low usage until AC season hits, which for us is May - Sept/Oct. In the hottest/worst times (114 degree days), we're paying $0.12 kwh. In the winter its more like $0.04 kwh.
  • Still sourcing the gas rates, as they don't list this info readily but its available "upon request" [eyeroll]. I know we do pay a customer/delivery charge of basically $15/mo regardless of what we use.
  • Because we're in a hot climate and the water heater is currently in the toasty-for-most-of-the-year garage, I'm thinking the conditions are excellent for a heat pump electric WH. Current WH is in a small closet in the garage though, so its possible a new heat pump electric would have to be relocated to the open air part of the garage to have sufficient clearance (increasing set up costs).
  • The garage is terribly hot during the Summer months, and my child's bedroom is located just above the garage and is difficult to keep reasonably cool - so it *might* be that a heat pump electric WH would have double benefit of decreasing the garage air temp slightly? Even a few degrees difference could make an impact, but I'm not sure how to find reliable info on how much a heat pump WH reduces heat in ambient air?
  • While we currently have gas, I'm not sure if the pipes are appropriately sized to convert to a tankless, so there may be some increased set up costs for that route too.

Unknown: The cost to convert/install either the gas tankless or the electric heat pump option. I'm going to have a plumber friend come and give me their take on the cost to install/make ready both options, to see what kind of differences there might be in initial outlay.

"The Internet" say gas is always cheaper than electric, but given that either conversion may have some initial set up costs, I'm not sure its that simple.   Is there anything else I should consider?
« Last Edit: March 05, 2019, 08:42:53 AM by NonprofitER »

robartsd

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Re: Tankless gas vs heat pump electric water heater
« Reply #1 on: March 05, 2019, 11:53:59 AM »
I found an average of about $1.50/therm (approximately 100 cf) for residential natural gas service in Texas (also seasonal like your electric; highest is summer/lowest in winter). One therm is about the same energy as 30 kWh, so with an average cost of 0.08/kWh, the equivalent electrical energy is about $2.40. A condensing tank less gas water heater is about 92-96% efficient (some heat always goes out the exhaust - your tank water heater is probably 50-65% efficient) - a heat pump water heater is about 200% efficient (heat pump mode only). My back of the napkin calculation indicates that the heat pump should be slightly cheaper in terms cost of energy to run.

You're right, a tank less gas water heater could require upgrades to your gas service (tank less need to burn fuel much more rapidly). If you don't already have an electric water heater circuit, one would need to be installed for a heat pump water heater. Assuming your load center can handle it without an upgrade, running an electrical circuit is probably cheaper than upgrading a gas line. If installing a heat pump water heater, I'd install vents to allow air to circulate between the garage and the water heater closet rather than moving the plumbing. Based on the 200% efficiency estimate of the heat pump, half the heat added to the water is sourced from the air; but I wouldn't count on it make a difference in garage air temperature.

I'd lean towards the heat pump water heater, especially if your current water heater is your only gas appliance and you can save the $15/mo connection charge by cutting out gas service completely.

NonprofitER

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Re: Tankless gas vs heat pump electric water heater
« Reply #2 on: March 06, 2019, 08:10:49 AM »
Super helpful comments, thanks!
It gives me good questions to ask the plumber too as we consider the two.