Best Of

Assumption is that COP of 4 would mean 50% cost of the BTU from heat pump.

Swapping 864,000 BTUs over to an a2w heat pump would require more land space, more money and more electricity than the building currently has available. Ideally we'd start with something and see how it goes before 100% committing. Too often projects are contemplated in an all or nothing scenario resulting in nothing.

We have our domestic preheating with 3 HPWTs before final heating with a gas hot water tank. The proterra units are each 4200 BTU. The gas hwt is 385,000 BTU. While the HPWTs aren't providing a huge portion of the hot water it did enable us to reduce from 2x 385,000 BTU tanks to one and leave the redundant one as backup. The setup likely was overkill to start with but the hybrid solution has saved ~10% off the overall gas bill with minimal impact on the electric side.

In order to justify the project I'd want to see making the money for the installation back in 10-15 years (ignoring interest). I can't see a full swap ever paying back so a full swap point could only happen once the current hydrotherm hits end of life. 1985 cast iron - it might be around for another 25 years? The heat pump tech is still in flux so it is nice to start small and realize the improvements in the technology rather than bleed out on the cutting edge.

The HPWTs are an easy win as they are used year round whereas the boiler is only used Oct-April and I think the heat pumps will generally have a hard time heating water at 127F a lot higher before the boiler while maintaining a high COP.

My sense is that given the project requirements a heat pump can't work for this application but I thought I'd post and get a sense from the experts. Can assume 46F+ as air temperature during the majority of the heating season.