GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted
BS* + Beer

The BS* + Beer Show: Hydronic Heating and Cooling

A technically rich overview of the role of air-to-water heat pumps in hydronic space-conditioning systems

This episode of the BS* + Beer show features engineer John Siegenthaler and special guest host, Kyle Macht. John shares a presentation looking at different aspects of and considerations with hydronic systems. He provides an overview of and points to advancements in air-to-water heat pumps (AWHPs) for heating and cooling, including different monobloc system types. Other topics of discussion include heating capacity of AWHPs; distribution systems and piping configurations; dovetailing older boilers with new heat pumps; ducting to integrate space and water heating; possibilities unique to AWHPs; and cost comparisons between new and retrofit system installs, among other wormholes.

The BS* + Beer Show schedule

Episodes air the first Thursday of the month. The next episode, “Grow Your Own Discussion Group” is on December 15, 2022, from 6-7:30 p.m. ET. It’s been exciting to see local BS* + Beer groups popping up around the U.S., as well as Canada and Australia. It’s a great way to share and learn about building science topics, build a local network and have some old-fashioned, in-person fun. If there isn’t a group near you already, you should start your own! It’s easy to do, with a few different models of how to do it. Join us for a discussion on starting your own local group. Guests will include some of the folks who started and have been running local groups successfully, including, Dan Kolbert, founding moderator of the original Building Science Discussion Group at Performance Building Supply in Portland, Maine.

Use this link to register for The BS* + Beer Show


Kiley Jacques is senior editor at Green Building Advisor. Photo by Anna Robinson, courtesy of Fine Homebuilding magazine.


  1. mtsolar | | #1

    Great presentation. Wish there were two hours of this that could dig a bit deeper. As a contractor, John's technical knowledge has greatly helped me over the years with hydronic, solar thermal design, and more recently with A2W heat pumps. I have been very impressed with A2W heat pumps in a variety of applications including residential applications discussed and also on apartment scale hot water production. One topic that was not addressed (and time may have run short) is the use of wall (or ceiling) hydronic fan coils. While not as simple as panel radiators, they do allow heating and cooling from the same emitter which is a pretty big plus. Another topic of conversion that would be interesting is slab/radiant in cooling in dry western climates were we are finding that dew point issues/condensation is not much of an issue.

  2. boxfactory | | #2

    Perhaps I missed this, but would there be any advantages to running an air to water heat pump, as compared to a ductless minisplit? Perhaps a bit more specifically, would there be efficiency advantages, or lower power bills, when the air to water heat pump can do the same job at a lower operating temperature?

    I’m sure this question can be worded better, I’m just not an HVAC person.

    Great presentation to watch. Very interesting.


    1. Danan_S | | #3

      The nice thing about an air to water heat pump is that it can store heat (as hot water in a tank), which let's you heat water when energy when it is cheaper (and hopefully cleaner), either from the grid or a PV array, and use it at a different time.

      The heat pump domestic hot water heater I have draws only 800w to run it's heat pump, and it does so during off-peak hours either at night or during the day while my PV system is producing electricity.

      Some models can serve both domestic hot water and hydronic heating functions.

    2. paul_wiedefeld | | #5

      There are some advantages, but many are slight. Air-to-water can be zoned really well - you could have a thermostat in every room if desired. Ductless heat pumps can do that too but with less success. Water is a good way of transporting heat, so you can run the whole house on smaller pumping energy compared to air transported heat. There's an opportunity to use the air-to-water to provide domestic hot water as well. Energy bills savings and efficiency gains are either not there or extremely slight and also dependent on storage. Storage is usually (not like there are many of these systems in American homes period) thermal, which could potentially be cheaper than storing electricity, which is better paired with air-to-air systems.
      Air-to-water systems scale up really well - an apartment building or block of attached homes could start to realize many of the advantages. The corollary of that is that they don't scale down well - the hardware cost is extremely high and if you're using ductwork for cooling, completely redundant compared to an air-to-air system.

    3. charlie_sullivan | | #8

      In addition to what others have mentioned:

      * Having the refrigerant only in one box, inside a factory sealed loop, means that you don't need a refrigeration tech for installation.

      * It also means there's less chance of climate-damaging refrigerant leaks.

      * And it facilitates the use of new low GWP refrigerants that each have different special issues that are easier to address in a factory-sealed unit for various reasons.

      * Ductless minisplits always run into issues with the smallest unit being too big for putting one in each room. You don't need one in each room, but sometimes it's hard to find a good way to locate the heads where it all works out well, or sometimes hard to get all parties on board with the idea of rooms with no heat source. You can get tiny hydronic emitters, in the form of fan-coils or panel radiators.

  3. boxfactory | | #4

    I can certainly appreciate heating water when the building is getting solar power. Makes a lot of sense.

    Is there any chance you have a rough idea of the savings from night time use? I’m guessing one has to balance the % of cheaper power / as compared to the reduced efficiency of obtaining heat from the colder night air.


    1. Danan_S | | #6

      > I’m guessing one has to balance the % of cheaper power / as compared to the reduced efficiency of obtaining heat from the colder night air.

      The SanCO2's COP at -12F/-24C is still 1.7 (or 170% "efficiency"). At 30F/-1C it rises to 3.0/300%. Peak efficiency is 5.5/550%.

      The best COP/efficiency an electric resistance heater will ever achieve is 1.0/100%, regardless of weather conditions.

      So on average it is 3 times cheaper to operate the heat pump than an electric resistance water heater.

      If you're comparing the operating cost and efficiency to a typical natural gas boiler, then the best efficiency for natural gas is about 97% and the price doesn't vary by time of day. Natural gas even at today's increased price is still 1/2-1/3 the price per unit of energy as electricity. So the operating cost is equivalent to a heat pump. But comparing a natural gas boiler to an electric one isn't an apples to apples comparison.

    2. paul_wiedefeld | | #7

      No real rule of thumb here, but as $/kwh delta increases, the efficiency difference matters less. The more energy you use, the higher the annual savings. Savings are higher at the lower end of the efficiency scale, which is a small portion of the year.

  4. [email protected] | | #9

    Where is the promised PDF file? It's not on the YouTube page, nor here.

    1. Expert Member
      Michael Maines | | #11

      What was promised? I haven't watched the replay yet.

    2. GBA Editor
      Kiley Jacques | | #14

      I just added a link to the requested pdf of John's presentation (see the show recap about).

  5. mtsolar | | #10

    Thought I would summarize some pros/cons of Air to Water (A2W) heat pumps based on the linked webinar, comments above, and personal experience. Feel free to comment as needed:

    Advantages of A2W heat pumps (generally referring to mono-bloc unit):
    -Cold Climate Temps: most work well to -10F and operate to -15F or so.
    -Efficiency: Nearly match COP of ground source heat pumps .
    -Heat/cool capable. Hot water in many applications as well.
    - Refrigerant only in outdoor unit (all refrigeration fittings factory sealed and no refrigerant lines run through house).
    -Simple controls; more advanced ones available as needed.
    -Easy to service compared to A2A HPs (easily accessible components including electronics).
    -Work well with low temp in-floor radiant, pre-fab radiant panels; euro panel radiators, wall mounted fan coils, and air handlers.
    -Very efficient distribution (low energy use) to hydronic emitters .
    -Most brands have multiple sizes (BTU output); several can be run in parallel for higher loads
    -Very retrofit-compatible with older in-floor radiant floor systems.
    -Compared to installing an hydronic boiler, A2W HPs are easier to install (and some of these benefits are associated with the buffer tank): no gas, no venting, no primary secondary/piping, no concern about short cycling, and simpler commissioning.
    -Compatible with electric or gas backup (tie-in to buffer tank). Can be quite easy.
    -Wide variety of buffer tanks allow for many applications (simple 4-port tanks; reverse indirect tanks, heat exchanger tanks, 4-port tanks with backup elements/DHW pre-heat coils).
    -Scaling up is quite feasible (multiple HPs in parallel; hydronic distribution to many rooms).

    Disadvantages of A2W heat pumps:
    -Lack of installer/contractor familiarity.
    -Not as plug-and-play as A2A heat pumps (too many layout options?).
    -Plumbing wholesalers basically do not stock them.
    -Can require 30 to 80 gallons of anti-freeze (HP->buffer tank->heat/cooling loops).
    -More expensive and not as simple as A2A HPs on small spaces.
    -Inherently less efficient/simple on forced air systems compared to A2A HPs (e.g. air->water->air).
    -Like an A2A heat pump, may need backup heat in some climates.

  6. here_to_learn_more | | #12

    Wondering the same as dackdotcom. A link for John Siegenthaler's PDF would be great!

  7. Expert Member
    Michael Maines | | #13

    Here are John's slides in PDF format:

    1. vivian_girard | | #15

      Thanks; these slides and the video are well worth one year of my GBA subscription! With his knowledge of hydronic and abilities to explains it to the masses, John Siegenthaler is a national treasure.

  8. boxfactory | | #16

    In reading Johns article

    It occurs to ask, if a house only had one heating zone in a concrete slab, would it be possible to not use a buffer tank?

    I realize that such a setup would reduce the stored thermal energy that could be generated before a cold event, and used as a thermal battery. On the other hand, I can see the appeal of a far simpler installment.


    1. Tim_O | | #17

      Great article, I like their points made about increasing buffer tank temperature in warmer months to supply 100% or nearly 100% of the domestic hot water. A good stainless buffer tank might also have an element you could use to store a couple KWH worth of excess solar as well.

      To your question - I don't see why not. As they mention in the show too, concrete is an enormous heat sink. To the point where you can heat the slab on off peak hours and coast into higher time of day rates. Disadvantage of that would be you would likely be heating at night when outside temperatures can be 20* or more cooler. But simply put, the thermal mass of a concrete slab is far more than a buffer tank, so I don't see why you would have issues.

  9. mtsolar | | #18

    Buffer tanks are needed to maintain minimum flow rates of air to water heat pumps. Lower output heat pumps may need 5-6 GPM and larger ones >10GPM. Since each loop of radiant floor tubing (assuming 1/2" pex) can handle only 0.6 GPM, you would need 10 loops or so to maintain the HP flow rate. So in theory a buffer tank could be skipped but usually it is preferable to have one, as in most cases the ability to have even a couple zones is desirable. Plus thermal reservoir benefits of a buffer tank are also a factor as heat pumps to do not have the instantaneous BTU output of a fossil fuel boiler.

    1. wastl | | #20

      For such an application (one zone) no buffer is required. As was already mentioned, the concrete slab stores more energy/°F than a normal/sensible sized buffer tank. With a heat pump you do not want high fluid temperatures anyway so a "hot" buffer tank is a no-no.
      If you use PEX loop, use enough loops of same length (at max 250 ft to limit circulator power-better less) to make the required flow happen and call it a day. Ok, you might need a manifold with more ports - but that is the only added expense.
      Use a outdoor reset and go with it.
      Mind that such a system is good for continouus heat. If you have a workshop that you want to heat in a hurry then a slab is preventing a quick heat-up. Maybe some light mass ceiling emitters would be more suitable (also hydronic), if you have an on/off heatpump then I would rethink the buffer tank. The ceiling emitter - something like the "PEX collector" in
      as a ceiling panel with a bit of insulation on top (Rockwool etc.) would do the trick.


      1. boxfactory | | #21

        That’s good stuff right there. Thank you for your help.

    2. paul_wiedefeld | | #22

      You could use a primary/secondary loop to meet the minimum flow requirements of the heat pump too. Higher GPM/low Delta-t through the heat pump, lower GPM through the floor with a higher Delta T.

      1. wastl | | #23

        Yes - if you accept that the HP will loose a bit of COP due to the higher fluid temperature required for a buffer tank with your GPM settings. I would not do the buffer tank if we have only one zone. You can always go "more loops" at a very low extra hardware cost + save a circulator and assorted hardware ( a buffer tanks needs valuable space and has to be bought).

        1. paul_wiedefeld | | #24

          I meant P/S instead of a buffer tank.

          1. wastl | | #25

            If you mean a hydraulic separator - it does not change the picture beside that it has a smaller footprint. You want to run the loops with low flow --> higher spread. So the HP has to create the higher temperature..

  10. boxfactory | | #19

    I appreciate both of your well reasoned arguments about having, and not having a buffer tank.

    Is the 0.6 GPM of flow though 1/2” pex, when the tube is long enough to cover an entire homes slab, as opposed to a short run to a faucet or shower head? If my question seems really basic, it’s because I’m approaching this topic from years of carpentry, and hours of plumbing experience. This discussion is greatly helping as I prepare to talk with people who will be installing systems in the build next spring.


  11. Arjuna | | #26

    It sounds like using radiators for cooling isn’t popular, but I thought I’d ask since John did bring it up as an option.
    I live in Zone 4 in a humid environment. I’m building a high mass passive solar house with a dehumidifier which will keep the dew point at 65 degrees or lower. I’m guessing that’s too high to use radiant cooling. I’d appreciate any insight.

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |