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Sanden EcoCute + Instant Heater as backup radiant design

NetComrade Member Posts: 15
This project started out as a solar heating project, and you can read about it here

I wanted to use a Sanden unit as a backup to solar, but realized I will need a backup for it. So for now, I'd rather use it as a primary heating unit, and add solar 'prewarming' later.

This rough design is for heating my rather large basement. The zones are not reflected in the drawing, because I wanted to get the basics right and get some input. The slabs had radiant pipes (pex) installed as part of construction.

Looking for your feedback on this design.

Disconnect existing tank (T1) from hot water and make it part of my radiant system (looks to be mostly useless though, although its heating elements are capable of producing 15K BTU)
Replace T1 with B1 (Ecosmart Eco11 Instant heater) for hot water.
Setup Sanden as primary heating system for Radiant and Hot Water (prewarm well water via heat exchanger H1)
Use Eco11 as backup for Radiant, as it can generate up to 35K BTU (11kW).
Use Taco RMB Radiant Mixing Block (H2) as radiant circulator, but it will also turn on Eco11 (B1) in case SanDen is not keeping up (e.g. 85F) and add as heat exchanger.
Optionally/later add S1 and S2 heat exchangers to prewarm well water or radiant water by Solar

I know I am missing check valves, expansion tank in this picture, and something that will maintain pressure, water filtering facility (don't plan on using glycol) and probably other details but what are your thoughts?

I am not ignoring heat load calculations, but based on my analysis this will be more than sufficient (and with backup).

I am guessing Sanden would cover 70-90% of heat and DHW use, with a portion of that covered with Solar at some point, and Eco11 picking up some load when it's really cold.

Sanden (15K BTU) 4K
Taco RMB 1K
Eco 11k (35K BTU) $200
Pressure Pal MF200S $600
Heat Exchanger $150
Filter/Expansion tank $200
Manifolds x2 $300
Mixing Valves x2 $300
Misc Plumbing estimated ~1k
Total: ~8K + labor

Solar heat exchangers/sensors/pumps TBD later


  • Rich_49
    Rich_49 Member Posts: 2,766
    Before putting too much time into this I suggest getting in touch with John Miles at Sanden and discussing . The Co2 heat pump does very well making DHW , not so good at heat though . Relies on cold EWT to do it's thing . Not enough heat exchange at narrower Delta as I understand . Maybe something has changed since I last spoke with John a few months ago .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • NetComrade
    NetComrade Member Posts: 15
    Check out this report, they had a similar setup (page 9 for diagram

    (or google: combination space condition and water heating pnnl sanden)

    They do say at the top of page 10 "operates best at cold inlet temperature", but the overall experiment was positive.

    While they did not use a backup heater, they certainly recommended one.

    In my case, I would prefer the unit is on radiant side, not the DHW side, primarily because my well water isn't very good, and I wouldn't want it flowing through pricey radiant equipment (it's theoretically more efficient too, as the majority of load is heat, not DHW).

    I will try to research the inlet temperature and CO2 units. Better efficiency at lower temp, doesn't yet make sense to me.
  • NetComrade
    NetComrade Member Posts: 15
    FYI, I did speak to someone at Sanden, but i don't think it was John. I also emailed them with questions on why them limit the unit in documentation to 8000BTU load, but haven't gotten an answer (the setup I drew wouldn't be supported)
  • NetComrade
    NetComrade Member Posts: 15
    These statements confirm my logical thinking, but they're not made about CO2 based systems


    It’s important to keep the water temperature as low as possible, because most air-to-water heat pumps struggle to make high-temperature water. (The exception is any heat pump that uses carbon dioxide as a refrigerant; in Japan, this type of air-to-water heat pump is called an EcoCute heat pump. The only such heat pump available in North America is manufactured by Sanden.) The lower the water temperature, the greater the heating capacity and efficiency of the air-to-water heat pump.


    The Lower the Better: Like any heat pump, the greater the temperature difference between the media from which heat is being absorbed, and the media to which the heat is being transferred, the lower the heat pump’s heating capacity and COP (Coefficient of Performance). A typical variation in heating capacity is shown in figure 3. Figure 4 shows a typical variation in COP.
  • tim smith
    tim smith Member Posts: 2,774
    The Sanden is what I have been planning for our smallish Island cabin here in the NW. Its #s work ok here for heat due to are quasi mild winter temps. Need to boost up some insulating at cabin to meet the output but #s look pretty good. We only have electric or propane there so the heat pump would be nice addition.
  • NetComrade
    NetComrade Member Posts: 15


    Tank Destratification
    • Occurs when the heat demand on the tank results in
    heat supply flow rates that cycle the storage tank
    making it all one temperature
    • Without temperature difference transcritical operation
    efficiency plummets
    • To maintain tank stratification:
    – Match load to heat pump output
    – Use a larger tank in combined systems—120 gallon is
    – Return water to tank location closest to that temperature
    – Like Site 10 use lots of hot water which pulls cold water
    into the bottom of the tank—maintaining stratification
  • NetComrade
    NetComrade Member Posts: 15
    edited March 2018
    more goodies

    pg7, more relevant text on pg12 (not quoted, essentially repetitive)

    A great deal of discussion took place concerning the best way to return water from the space heating
    distribution system to the Sanden tank. Sanden’s concern was that returning warm water to the bottom
    of the tank would: • Interfere with defrost function in cold weather, because warmer water causes the system to
    misread the temperature and turn off the defrost (this issue was solved in the UL listed system); and
    • Reduce efficiency in operation, which depends on maintaining a temperature gradient in the tank
    to deliver cool water to the outside heat exchanger.
    It was decided to return heating loop water to the top of the tank. This caused
    warm water to mix with hot, and resulted in some cool showers at the
    Bellingham site. A device called a diversion fitting was developed and built by
    WSU to direct the incoming warm water down toward the center of the tank
    so it could find its proper stratification level (Figure 5). A copy of this device is
    installed at five sites.

    The Bellingham site was retrofitted in early October 2015 to move the heating
    loop return from the top of the Sanden tank to the bottom, and to replace the auxiliary tank with a
    demand heater. Relocation of the heating loop return was based on the combined space and water
    heating lab test conducted by Ecotope in August 2015, which showed clearly that returning 70°F to 80°F
    water to the bottom of the storage tank is more efficient than returning it to the top of the tank or
    introducing it through a diversion fitting
  • NetComrade
    NetComrade Member Posts: 15
    Same document, and I'll have to digest this later

    Proper function of the split system depends on tank stratification, where cold water resides at the
    bottom of the tank and hot water is placed at the top. This allows the transcritical CO2 refrigeration cycle
    to perform as designed, with colder water going to the heat exchanger in the outdoor unit.
    The CO2 refrigerant in the transcritical zone does not condense at constant temperature as in typical
    refrigerant cycles that are below the critical point.. Instead, the CO2 cools as it transfers heat to water in
    the heat exchanger called the gas cooler. After it leaves the gas cooler at about the temperature of the
    incoming water, it drops down into the evaporator and goes through the air-to-vapor exchange at a
    lower constant pressure and temperature. The compressor then lifts the CO2 back to the high
    temperature and pressure transcritical zone, where it transfers the absorbed heat to the colder water.
    In normal operation, the split system heats water in a single pass to 149°F. There is, however, a catch.
    Figure 14, taken from the lab assessment of the combined system by Ecotope (Larson, et al., July 2015), shows the truncation that occurs when the water coming into the system is too warm. The efficiency of
    the transcritical cycle is reduced because the invested compression energy remains the same but the
    heat that can be transferred to the water in the gas cooler or absorbed from the air in the evaporator is
    Tank destratification means that temperature difference between the top and bottom of the tank
    decreases. This can happen if heating demand is high and sustained. In this case the X-Block will
    circulate enough water to exceed the tank capacity, thus causing the tank to completely mix. Also, if
    heating demand exceeds the rate at which water is heated in the heat pump the auxiliary heat will turn
    on and increase the temperature of the heating return water to the tank.
    Figure 14. Impact of Water Temperature on Heat Transfer
    The temperature ranges that optimize performance depend on system operation. In general, low- temperature distribution systems, such as radiant floors that return water below 90°F, have higher
    performance than higher-temperature systems such as radiant panels that return water above 100°F.
    Optimum performance depends on return temperatures no higher than 80°F, which implies a radiant
    slab for heat delivery and a moderate (68°F to 70°F) thermostat setting.
  • hot_rod
    hot_rod Member Posts: 22,491
    There is a way to pipe a buffer tank with a 2 or 3 pipe method to help avoid breaking up that stratification layering.

    John Manning of Phoenix Energy, a GEO guy has a You Tube on direct-to-load piping, 3 pipe method.

    You are correct to lowest possible temperature to a boiler, HP solar, etc is a goal, stratification can help along that quest.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • NetComrade
    NetComrade Member Posts: 15
    After reading the following..


    I decided to flip the Sanden to the DHW side of the heat exchanger.. Sanden really likes the water to be colder. I will need to filter water to minimize impact of well water to equipment.

    The long paper basically validated my design (as it has been field tested with backup), and my new design now looks more like the design used in this study.

    This will also allow me to pre-heat water for radiant use (but not DHW, although that could be figured out later, IMO), which would have been difficult if Sanden was on the 'heating' side of the plumbing.

    Diagram is overly simplified, and will need additional plumbing details by a professional,.

  • Icarus
    Icarus Member Posts: 143
    Bump older thread...

    I have been researching ATWHP for a boiler substitution, and posted some questions here on this forum. I discovered this thread after discovering Sanden CO2 HPs. Coincidently, I am in Bellingham, and would love to connect with some of the people involved with doing this, and or anyone else who has any experience with Sanden in general. Intuitively it would seem that with proper design/buffer tanks etc a Sanden should work quite well in my situation. The design load is probably too big for one unit, but might be perfect for DHW as well as CH with two units, perhaps with a slight gas back up for “extra” DHW as needed. Anybody with information or experience with this please feel free to get in touch.

    Thanks once again for a very helpful forum,

    Tony (Icarus)