Help deciding between options for replacing gas tankless water heater
user-7828891
| Posted in Mechanicals on
We need to replace our gas tankless water heater, a Rinnai model manufactured in 2006. We are debating between a heat pump water heater (HPWH) and another gas tankless water heater (GTWH).
We already have worked to reduce our hot water usage. We don’t use warm or hot water in our clothes washer and our dishwasher is energy star rated. Though we don’t have low flow attachments on our shower or other water fixtures. We also are more of a shower every other day type family. We have two young kids so I expect our hot water use to go up as they get older.
Using my utility bills, I would estimate the ccf usage of natural gas for the GTWH to be between 5-10 ccf per month with the higher range being in the winter months. I used some data collected by folks on Reddit for their real usage of the HPWH for a similarly sized family as a basis to guesstimate our annual usage.
When thinking about comparing the HPWH with a new GTWH, I am finding that the CO2 emissions would be lower with the GTWH if I assume a worst case scenario of our electricity being solely from coal.
Here is my calculation:
GTWH – 90 ccf/yr x 11.7 lbs of CO2/ccf = 1054 lbs
HPWH – 1750 KWh/yr x 2.21 lbs of CO2/KWh = 3868 lbs
I obtained the emissions rates from eia.gov.
I’m hoping for some thoughts or guidance on whether I am thinking about this in the right way or maybe I’m missing something? Any advice you all might have would be greatly appreciated.
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Replies
What climate are you located in?
Generally, a tankless water heater is a bad appliance with 0 advantages over a tank, UNLESS you are sort of a few square feet and need the room. A tankless water heater is equal to or less efficient than many tanks and is often lower performing too, the worst of both worlds. But they are smaller.
It makes more sense to compare emissions on a per MMBtu (1,000,000 Btu basis), as you're using your data (reasonable) but comparing it to Redditor data (less reasonable). If you do the math: 90CCF = 2741 kwh/year, but the HPWH is modeled with an input of 1750kwh/year which is a COP = 1.5. That's extremely low for a HPWH.
In which case:
GTWH: 116.65 lbs/MMBtu / 95% efficiency = 123 lbs/MMBtu
https://www.eia.gov/environment/emissions/co2_vol_mass.php
HPWH: .86lbs/kwh *293 kwh/MMBtu / 300% efficiency = 84lbs/MMBtu
https://www.eia.gov/tools/faqs/faq.php?id=74&t=11
So about 32% less emissions based on the US average. The tank will of course have standby losses, probably about <365kwh/year (of heat) so 121kwh input and 105lbs of CO2. The more hot water you use, the less standby losses there are, so this is a high estimate. So if you use the lower end estimate of 60 CCF/year, you'll use 6 MMBtu, saving 6*39lbs - standby losses of 105. So 129 fewer lbs of CO2 using the HPWH.
The climate question matters though - if you're heating water using the summer, the HPWH provides cooling but in the winter it'll be using energy from inside the house.
Really appreciate the detailed response. We live in northern Kentucky, climate zone 6b.
We ended up going with the HPWH after I posted this (actually being installed as I type this). Ultimately, I just did not like the idea of committing to a gas-fired appliance for 20 years. Even if my electric is currently 100% coal generated, it hopefully won't be forever. But the GTWH would always be gas fired. So, we opted for the HPWH. And based on your explanation, it seems like we made the right call.