The Money Mustache Community

Learning, Sharing, and Teaching => Ask a Mustachian => Topic started by: NonprofitER on March 05, 2019, 08:41:21 AM

Title: Tankless gas vs heat pump electric water heater
Post by: NonprofitER 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:

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?
Title: Re: Tankless gas vs heat pump electric water heater
Post by: robartsd 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.
Title: Re: Tankless gas vs heat pump electric water heater
Post by: NonprofitER 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.