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Radiant baseboard with Heat Pump Combi for DHW and space

JamieF Member Posts: 1
We own a 1200sf home that is tight, insulated well and live in a mild climate on the southern Oregon coast. We heat DHW and space (70' Slantfin baseboard) with natural gas boiler. DHW is one zone of boiler system through tank with heat exchanger. We typically only use one zone for space heating out of four. The boiler is near end of life and want to change to all electric (upgrading PV to match next load). Looking favorably but cautiously at the Sanden Combi system. Really like the CO2 heat pump. I have been in contact with Sanden representative John Miles and one Portland user who uses this system with a large radiator (Runtal 4'X8') in super insulated home. He thinks it will work for me as long as I follow the install/use guidelines for a high heat application. One of the stipulations for the heat pump is to not de-stratify the water in the tank, returning the water to bottom of the tank less than 90 degrees. If not possible, then to the top of the tank if above 110 degrees. The system for Combi uses a Taco X-block heat exchanger/2-pump unit.
Here are some specs from the company:
DHW & Heating Combi
• Application based on several factors
• Up to 176°F water can be generated by the Sanden unit at ambient temperatures down to 5°F
• Unit testing & flow rates show that a 100 Gallons DHW use per day only requires 5 to 6 hours unit operation to recover
• 150°F water is produced at a 3.5 COP @ 32°F ambient
• 83 Gallon tank or larger MUST BE USED, important to keep the stratification of the tank to ensure highest COP

I have about 95sf of uninsulated basement slab with PEX. I'm thinking (and taking advice from other user) to use other zones in house and this slab section (separate zone valve with feed-back from return water temperature) to reduce return water temperature to desirable level. I realize that the lower temperature water through Slantfin will be less productive, but willing to try. Suggestions? Willing to send more information to professional for hire before leap.
sandenwaterheater.com for more info.


  • hot_rod
    hot_rod Member Posts: 22,121
    A 3 pipe buffer tank piping is what you should look at. It is the best piping I have seen to keep from busting up that stratification, and direct to load capability. Place attention to capacity and efficiency at those high SWT conditions. I'd design for the lowest possible SWT.

    Not to change you mind on brands, John Siegenthaler put together a very detailed application guide for air- water HP, the Mestek SpacePak system. It has an answer for all your above questions :)

    WaterWorks is actually a series of publications, try

    John also has a winter's use on a Solstice at his place in upstate, NY. If you google his name you may find some industry articles he has written on the subject.

    Time to start paying close attention to HPs as hydronic heat sources.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream