Air to Water Heat Pump to Replace Aging Boiler for heat and hot water?
Hey Green Building HiveMind! I have a 1950s fairly well insulated 1750 st ft home in Tacoma, WA (zone 4c) for 2-3 people that has a nearly 20 year old slowly leaking Triangle Boiler with staple up hydronic heating for main areas+ 4 bedroom radiators. I’d like to get off of natural gas (and our electricity is $.09 and hydro) and would love air to water heat pump. I’ve gotten some bids (most HVAC/plumbers even in our area don’t do it) but have been reluctant to pull the trigger (I’d like to wait a couple years but don’t think the boiler will hold out) as I’m not super confident that the system and contractor can actually make it work. The current design value for the water in the boiler tank is 140 degrees at 20 outdoor temperature, and the house temp is 68.
The bid I like the best (maybe because it is one of the cheapest) is for:
- Spacepak Solstice 3-ton CC32-40 variable speed high efficiency R-32 Heat Pump Combi Water Heater (including domestic)
- Remove and replace existing boiler / install heat pump
- Install buffer tank and pump with all near equip plumbing
- Run insulated lines to heat pump unit on outside
- Run line voltage wiring to both buffer tank and heat pump
- New expansion tank
- Tie in water lines and purge system
- Fire off and commission equipment
- Includes required permit
- Includes first year maintenance
- 10 year compressor / 5 year parts / 1 year labor warranty
- $39,550
Is there something missing? Can it really do space heating and DHW (I’d like to not have to pay for another heat pump if it can indeed do both). Is there something more that I should ask or verify?
Thanks All!
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Replies
Looking at the ASHRAE data for SeaTac airport the heating design temperature is 29.5F. If you already have radiant floors installed, that outdoor temperature and the radiant floors is probably a good fit for a heat pump.
The big worry with a heat pump is running out of capacity when it's cold outside. If you look at the datasheet for the SpacePak, at https://www.literature.mestek.com/dms/SpacePak/CC32-0624.pdf , on page 6 they have performance tables. At any given outdoor temperature, the capacity drops pretty dramatically as the water temperature increases, as does the efficiency.
If you look at the 32F column, with water at 95F the SpacePak delivers 35.2k BTU/hr, but with a water temperature of 140F it only delivers 22.4k BTU/hr. With water at 95F it has a COP of 3.36 and at 140F it's 1.49. So you'd use almost twice as much electricity at 140F as at 95F.
To assess this proposal, you need two things: the first is what your actual heating needs are. You can estimate that from historical fuel usage using the technique in this article: https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
The second is what the water temperature needs to be to meet that heating load. The current boiler may be set for 140F but that might just be the lowest the boiler is capable of operating, it may be possible to heat the house using cooler water.
Those two numbers together will tell you whether it's possible, and also whether it's practical.
I'm not a fan of using air-to-water heat pumps for domestic hot water. A heat pump water heater will cost less to install, cost less to operate and take up no more space in your utility room.
From a dollars and cents point of view spending 40K on a fuel change is a foolish idea. If you are doing the fuel switch to make the numbers work you will need to replace all the gas appliances and disconnect the service. The $25 a month saving from cutting the service fee goes a long way toward making the numbers work.
If the cast iron part of the boiler is cracked and leaking then it is time to replace the equipment. You want two independent diagnoses both condemning the boiler and showing you the same crack, in real life and not words and blurry photos.
The fact that you seem to need 140° water is a problem In that the hotter the water your home requires the lowers the COP of any a ATWHP will be. Consider air sealing the house, upgrading the insulation and adding more tubing that will lower your BTUs required and let you run a lower water temp.
You could likely have a few mini splits installed for less money that would provide AC. AC would add to the homes resale value unlike ATWHP.