I love your practical breakdown, backed by the numbers. I have a heat pump HWHeater tank and added a small on-demand (about 3 gallons. The sink supply lines runs about 22’ under a concrete slab) heater under the kitchen sink. As a designer of homes, I think there could be recapture of heat from appliances to help ‘feed’ heat to a HP HWHT from refrigerators and cooktops/ovens. Yes, this would require a redesign of standard kitchen layouts by moving the refrigerator and HPHWT to an adjacent space, but what a great way to add overall efficiency to heating water and cooling, as well as housing noisy appliances away from open spaces in the home. I toy with this idea often, as I sit in my living room with the refrigerator humming loudly on one side and the HPHWT doing its thing off the other side, tucked into a closet.
Great thinking Bonnie! There are a few reasons why capturing heat from other appliances and "feeding" it to a water heater is challenging. Besides the practicality, in the winter, the heat from most other appliances is already in the building envelope, heating the space. This means a central HVAC heat pump would need to run less to heat the home. But the biggest problem has to do with refrigeration cycle efficiency limits imposed by the 2nd Law. It's about exergy, not just energy.
Authentic calculations would be a challenge, for sure, and planning for two seasons… I guess the real questions lie in how do we simplify systems while making them more efficient and then produce for mass distribution? I think the answer comes from reworking several components: efficient engineering (as you speak to), much better building envelope design (both in better R-values and planet-friendly mass producible materials), and lifestyle tweaks and managing expectations (the whole house doesn’t need to be 68 degrees in Phoenix in July, nor in January in Anchorage)
Appreciate the thoughts, and the hand drawn electric power use chart. A question: The analysis on heat pump water heaters seems…incomplete for the conclusion that is made. At high electric rates per kWh (eg $0.20-30/kWh), do heat pump water heaters get more cost competitive? If memory serves the COPs for heat pump water heaters operating in a conditioned space are ~3 or higher.
Good question, and apologies for not getting into all the details of heat pump water heaters, since I wanted to leave the bulk of the discussion about tank vs tankless. Yes, financially, anything that reduces electricity consumption when electricity prices are higher will have a faster ROI. A standard Rheem electric water heater at Home Depot costs $490, and the same size, same manufacturer heat pump version costs $1700.
I'm not hating on heat pumps. I LOVE heat pumps and am designing systems with them right now. But with heat pump water heaters, while a COP of 3 can be achieved, that's not always realistic. It depends on conditions and water heater set point. COP goes down (a lot) as the heat sink (water) temperature rises (see Sadi Carnot, 2nd Law, etc.) So, in practice, a COP of 3 is not realistic. Besides that, the noise and space issues can be considerable.
But the main point is that heat pump water heaters take heat from the conditioned space, necessitating more space heating. So even if it can achieve a COP of 3, 4, or 5, it comes at the cost of increasing the heating load of the house and/or decreasing the COP of the main HVAC heat pump. Dang thermodynamics!
Tanner, I have installed a couple of tankless propane fueled heaters in pool houses. I think that is a good application when the propane and minimial electrical were already available (propane was already there for pool heaters). There is a bath and shower, however I thought the actual usage would be so sporatic that this was justified.
Certainly propane tankless water heaters are capable of delivering much more heating power than electricity due to the nature of the high energy density of fossil fuels. And in that pool house situation, it sounds like you made a good decision. Tankless water heaters based on propane or natural gas have a place today and yesterday. In this article I tried to lay out the water heating tech of tomorrow—a tomorrow that doesn't include fossil fuels as part of a high-performance home. In a future article, I'll lay out why full electrification of homes is likely the direction were moving towards.
I love your practical breakdown, backed by the numbers. I have a heat pump HWHeater tank and added a small on-demand (about 3 gallons. The sink supply lines runs about 22’ under a concrete slab) heater under the kitchen sink. As a designer of homes, I think there could be recapture of heat from appliances to help ‘feed’ heat to a HP HWHT from refrigerators and cooktops/ovens. Yes, this would require a redesign of standard kitchen layouts by moving the refrigerator and HPHWT to an adjacent space, but what a great way to add overall efficiency to heating water and cooling, as well as housing noisy appliances away from open spaces in the home. I toy with this idea often, as I sit in my living room with the refrigerator humming loudly on one side and the HPHWT doing its thing off the other side, tucked into a closet.
Great thinking Bonnie! There are a few reasons why capturing heat from other appliances and "feeding" it to a water heater is challenging. Besides the practicality, in the winter, the heat from most other appliances is already in the building envelope, heating the space. This means a central HVAC heat pump would need to run less to heat the home. But the biggest problem has to do with refrigeration cycle efficiency limits imposed by the 2nd Law. It's about exergy, not just energy.
https://energyresourcedynamics.substack.com/p/understanding-exergy
Authentic calculations would be a challenge, for sure, and planning for two seasons… I guess the real questions lie in how do we simplify systems while making them more efficient and then produce for mass distribution? I think the answer comes from reworking several components: efficient engineering (as you speak to), much better building envelope design (both in better R-values and planet-friendly mass producible materials), and lifestyle tweaks and managing expectations (the whole house doesn’t need to be 68 degrees in Phoenix in July, nor in January in Anchorage)
Appreciate the thoughts, and the hand drawn electric power use chart. A question: The analysis on heat pump water heaters seems…incomplete for the conclusion that is made. At high electric rates per kWh (eg $0.20-30/kWh), do heat pump water heaters get more cost competitive? If memory serves the COPs for heat pump water heaters operating in a conditioned space are ~3 or higher.
Good question, and apologies for not getting into all the details of heat pump water heaters, since I wanted to leave the bulk of the discussion about tank vs tankless. Yes, financially, anything that reduces electricity consumption when electricity prices are higher will have a faster ROI. A standard Rheem electric water heater at Home Depot costs $490, and the same size, same manufacturer heat pump version costs $1700.
I'm not hating on heat pumps. I LOVE heat pumps and am designing systems with them right now. But with heat pump water heaters, while a COP of 3 can be achieved, that's not always realistic. It depends on conditions and water heater set point. COP goes down (a lot) as the heat sink (water) temperature rises (see Sadi Carnot, 2nd Law, etc.) So, in practice, a COP of 3 is not realistic. Besides that, the noise and space issues can be considerable.
But the main point is that heat pump water heaters take heat from the conditioned space, necessitating more space heating. So even if it can achieve a COP of 3, 4, or 5, it comes at the cost of increasing the heating load of the house and/or decreasing the COP of the main HVAC heat pump. Dang thermodynamics!
Good timing on this. I need to eliminate a boiler at storage and was going to go tankless for hot water but will do point of use instead.
Tanner, I have installed a couple of tankless propane fueled heaters in pool houses. I think that is a good application when the propane and minimial electrical were already available (propane was already there for pool heaters). There is a bath and shower, however I thought the actual usage would be so sporatic that this was justified.
Thanks for the great insights!
Certainly propane tankless water heaters are capable of delivering much more heating power than electricity due to the nature of the high energy density of fossil fuels. And in that pool house situation, it sounds like you made a good decision. Tankless water heaters based on propane or natural gas have a place today and yesterday. In this article I tried to lay out the water heating tech of tomorrow—a tomorrow that doesn't include fossil fuels as part of a high-performance home. In a future article, I'll lay out why full electrification of homes is likely the direction were moving towards.