Hydronic System Critique

Good day folks

First many thanks, especially to David, for your help in determining the needed BTU's to heat air.  It was a very good discussion for me which forced me to defend (I believe successfully) to heat my domestic water with a Hybrid Heat Pump Water Heater.

Now that the decision has been made to go this route for the domestic, I was able to greatly simplify the space conditioning system.  While I am not a professional and have never designed a system before, I do have a significant knowledge gained via a formal hydronic heating course, extensive review of Ziggy's Modern Hydronic Heating text, and through discussions on forums like this one.

I was hoping for some critique on the layout I have come up with.  Be as brutal as you like as long as you explain your disproval - if there is any :-)

I would be interested in any recommendations for tweaks or additions to the diagram.  For instance, do I have check valves in all of the recommended places.  

I have tried to include ball and purging valves wherever there is equipment that may need to be serviced and also at the bottom of multi-story runs.

This system will be providing heating in the winter months and switch over to cooling in the summer months.  There will be a significant portion of the year (shoulder months) when the system will be dormant due to internal heat gains and the high-performance building envelope it is being installed into.

Many thanks in advance for any guidance provided.  I will be happy to answer any questions if more info is needed.

I have attached the circuit diagram as both a PDF and a JPG




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  • What motors will go bad?   can the motor be changed?   Will the Heat Transfer Plats get full?  How will they get cleaned?  Aluminum fill up quick.  I find water system full of "MUD" with higher temps - how will water be conditioned?    

    • - if you mean circulators - yes of course they can be changed. No other motors beside outdoor heat pump.

      - heat transfer plates get full?  this is what heat exchanger plates are - no opportunity for them to 'get full'


      - If system is full of sentiment then there must be iron in the system that is corroding and breaking down and also a lack of dirt separators.  I do not plan on any water conditioning but will have a lot of air bleeders and dirtmags.

      ThermoFin Radiant Heat Transfer Plates — Radiant Design & Supply, Inc.
      Providing appropriate energy solutions since 1979, Radiant Engineering specializes in hydronic heating & cooling and solar thermal systems. Radiant m…
  • What is the output of the panels on 66 degree water? Especially with 75 degree air. When we do a chilled beam or radiant cooling, we always cheat with a blower coil or DX make up air unit. Also, my typical chilled water system is on a 45 degree supply water and 55 degree return water to make 55 degree air. With 60 degree water you are going to need a lot of "air" at the lower temperature differential.
    What is the reason for not going with an air to air central heat pump? Or a few water source heat pumps?
    Feels like you are streaching even with the low loads. Also the envelope improvements, are going to push more of your year toward cooling. I think your going to struggle to cool with the configuration you have described.
    • Lots of questions - some of the easiest ones first.

      I am firmly in the camp of hydronic space conditioning vs forced air. It is more efficient from an energy standpoint and radiant heat beats convection heat any day in my books.  It is also much easier and attractive to route through the house vs the bulkheads needed for ducts.

      The DX2W will be able to, and will be, creating around 40F chilled water for the buffer tank.  This will be sent to the condensing coil on the HRV intake duct.

      Based on modeling, the mixed up 66F cooling water sent to panels (resulting in 70.3F ceiling surface temp) will easily handle the limited cooling loads we will have (our room setpoint will be 76F). We have oriented our windows to make the most use of the solar energy.  So they are generally concentrated on the south elevation. This means it is easy to block the summer solar gain with appropriate roof overhangs, and fixed exterior sun shades above the lower storey windows.  At the peak of summer, only the bottom 2"-3" of the south windows see direct sun.  The rest of the window is in shade.  The coatings on the window tri-pane will also bounce much of the solar heat away when sun is high in the sky. We will have 0 infiltration into the house, so none of the latent loads that typically represents (target ACH50 is 0.25 or lower).

      The only significant solar gain that could occur, would be through the upper floor west window in the master bedroom (this room currently models as 10% of thge cooling load).  It has been planned that if this becomes a problem, we would install motorized exterior window blinds on an automation system to address.  The largest solar gain loads we will have will most likley be internal gains, so we are not dealing with a lot of load.

      I supect that the loads you design to are much higher, hence the requirement for much cooler conditioning water.  My average room heat flux is only 4.96 with my high being 9.58 and my low being 1.86.  I relaly do not need to move a lot of heat or cooling to make this place comfortable. 

      As far as the cooling season here, the vast majority of homes still do not have air conditioning.  We have a portable one and have used it about 4 out of the last 15 years for maybe a week max per season.  With the envelope of the new house, I do not expect to need cooling unless the outside temps are above 85F.

      If I do end up not being able to meet the loads with 66F panel water, then I can just turn it down, untill I do meet the loads.  I have a lot of room between 66F and the 40F that the heat pump can turn out.

      To give you an idea on the quality of the envelope.  It is currently 25F where we live, and has been below 32F for days.  The inside of the house, under construction, has been a consistent 52F with only a 1800watt heater for the entire 4455 ft2 dwelling.  This is without the stud bay insulation in place but with the roof and the exterior insulation installed and the air barrier about 90% complete.

  • Just in case it is not clear - the heat pump graphic was the only one available in the software used to draw this. It will be refrigerant and not water connected between the outdoor heat pump and DX2W

  • Thanks for reply Sean.  

    Yes - I will also be cooling via the radiant panels.  The cooled water will extract heat from the objects in the room.  This is not common in residential but is used a lot in commercial spaces.

    The water going to the ceiling panels will be at 66F (design temp).  The chilled water going to the condensation coil in the HRV intake will be much colder (probably around 40F) to provide the temp needed to condense on the coil in the HRV intake.

    Not really sure what you are stating in second paragraph.  'hotter water to unit'. 

    I choose ceiling panels in large part because they are a low mass system that responds quickly to temp changes.  So on a winter's day when the sun comes out, the stat will shut down the panels in the effected rooms. Within minutes the panel temps have equalized with the room and the room will rely on solar gain.  When the sun goes back behind the clouds, the panel will re-energize and take over heat load.

    Good point re isolation of condensing coil.  Have added - thanks

  • So just to get this straight - you are also doing radiant cooling via the panels? My only concern with that is will you get enough dehumidification if you run the same temp through the forced air unit?

    As for heating, hopefully the controls will help keep you from overshooting the mark & overheating especially during the day - my gut says lower temps to radiant panels with hotter water to the unit when needed

    As for the layout - sorry not my cup of tea but I don't see any shutoffs by the forced air unit so if you need to work on it / pull something....

  • I received a request to provide some additional information about the equipment and control logic.

    The space conditioning will be via an air to water heat pump system.  Outside will be a 3 Ton Air Source Heat Pump (possibly Goodman).  The interior component will be the ThermAtlantic DX2W module that uses a spiral heat exchanger to transfer energy from the refrigerant stream to water.

    The system will create hot water in the winter months and, via a reversing valve in the heat pump, chilled water in the summer.

    The heating and cooling water will circulate through hydronic panels in the ceilings.  These will be made with Pex-Al-Pex snapped into omega shaped aluminum transfer plates embeded into 3/4" of Foil faced Rigid Foam

    During the winter, the temperature of the buffer tank and panel heating water will be regulated by the mixing valve in the DX2W and the outdoor reset control.  During the summer, the buffer tank will be kept chilled to provide chilled water to a condensing coil that will provide dehydration of the air being suppled to the HRV (and as such provide latent cooling).  The water to the hydronic panels will be mixed higher so that it is always 2F above the interior air dew point via a dewpoint sensor inside the dwelling.  This will prevent condensation from forming on the ceiling surfaces.

    The BTU Meter, is just some testing I have agreed to do for ThermAtlantic and the Al transfer plate manufacturer, to monitor the efficiency of the ceiling panels.

    An ECM variable speed circulator will maintain a consistent pressure in the system regardless of how many or few of the zones are open.

    The buffer tank raises the efficiency of the system (allowing longer heat pump runs) and also provides hydronic separation between the primary 'boiler' loop and secondary panel loops.

    Due to the very high performance envelope, my heating and cooling loads are both around 18K Btuh (4800 ft2 dwelling). Heating panel supply water design temp is 85F and cooling supply water design temp is 66F

    I believe that covers the basics. Thanks for any input you can provide.

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