What is more efficient – Radiant Heat using Heat Pump Water Heater, or a Heat Pump ducted system?
Assumptions:
– 2000 SF home (2 level, conditioned crawl space)
– Pretty Good House: Tight, r26 walls, r60 vented roof space
Assuming the home is going to have a ducted heat pump system and a heat pump water heater, would there be any significant energy savings to using a PEX pipe radiant floor system for winter heating instead of the ducted heat pump system?
It seems like the general consensus is that a ducted heat pump is usually more efficient because the radiant heat source is usually assumed to be electric resistance strips, or a hydronic system powered by a resistance / propane water heater. However, in the case where a build specifically includes a heat pump water heater as the radiant source, is there significant energy savings over the ducted heating, or has this calculation ever been made?
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Replies
The big question is which heat pump water heater? Most are not rated for space heating, The ones that are, are incredibly expensive and come with other potential problems.
In theory, the answer will come down to the efficiencies of the heat pumps (water heater vs ASHP). I'd be surprised if the water heater comes out ahead, or if it does that it would be far enough ahead to justify the extra expense and complexity.
If at the root of your pondering there is the idea that radiant heating in and of itself is more efficient than ducted heating, toss that out. It's a myth.
If you're going to heat with the HPWH you have to use one that has an outside coil. Most of them pull heat from the space which isn't going to work.
Right sized ASHP with a good ductwork distribution will be very comfortable.
Skip the radiant, but make sure any slab that touches the ground or an outside wall is insulated (bottom and outside edge).
If zoning, do it minimally (don't micro zone), maybe upstairs and mainfloor as separate zones but don't go further. Keep your ductwork in the conditioned envelope (no attic ductwork unless it is in the envelope).
Seems to me the outdoor heat pump must bring 100% of the required the BTUs into the house for your idea to work. Once the BTUs are indoors. I see no benefit in spending energy in a HPWH to concentrate the BTUs in water for the purpose of redistributing BTUs around the house.
Walta
Can you explain a bit more about what you mean by “heat pump water heater”?
Like a Rheem that heats DHW using indoor air? A sanden that heats DHW using outdoor air? Or a Nordic that heats DHW and/or non-potable water using outdoor air?
Efficiency can be calculated in various way, are we talking economics or physics? An economic model is based on $ per kWh, a physics model can express that as COP. Then there is the comfort component and the aesthetic component. The model changes over the temperature range -30F to 100F.
I am biased toward hydronics, but you need to run ductwork for your ERV, kitchen vent hood and other stuff. So it is good enough in most zones. We have Warmboard - but typically heat the house from the bathrooms and the foyer (tile floor). The other zones kick in only when it gets below -12F.
The fun part starts when you look at the framing plan-can you easily run ductwork? I can run insulated PEX from an air to water heat pump through a 2x4 wall to a fan coil in the ceiling. The same duct work may end up reducing the size of a closet. You can run insulated refrigerant lines - hopefully they don’t leak.
With hydronics you can have the heat pump store hot or cold water in a buffer tank - using low priced energy then use it when it is needed. Not so much with an air to air system. A hydronic system also can do heating, cooling, and DHW and move the KWH in a 1 inch PEX tube (or less) to where they are needed. If you go dual fuel, then only use the boiler when it reaches the lowest modulation point which is typically 8,000 BTU per hour. Above that point an air to water heat pump variable inverter is much more effective and can modulate lower. Again, the climate zone and the design day temperature really is the key thing to think about. My worst case is -29F, if your worst case is 8F then just use a heat pump (air to refrigerant or air to water).
But again, talk with the installer and find out what they are comfortable doing. A well installed system is more efficient than a poorly installed installed system.
If you look at the amount of heat you can store in a tank, and what that tank costs, you'll realize quickly that it's not practical.
Yep - that was a bad example. Sizing of a buffer tank is more dependent on the time to switch between heat pump modes. 40-gallons typically buys 15-20 minutes which should be good enough for most applications.
>A well installed system is more efficient than a poorly installed installed system.
To your earlier point that "efficiency can be calculated in various ways," ultimately the purpose of an HVAC system is to provide comfort, so the real measure of efficiency would be the amount of comfort delivered relative to the inputs.
The problem is that comfort is hard to quantify so instead we rely on secondary measures.
As others have noted, the device you want is called an air to water heat pump.
Check out this post, "The Hunt for the Most Efficient Heat Pump in the World:" https://www.greenbuildingadvisor.com/question/the-hunt-for-the-most-efficient-heat-pump-in-the-world
All they talk about in the linked article is air-to-water. The efficiency of a heat pump is largely determined by the temperature delta, with a hydronic system you can design for low temperature water and get lower deltas.
The latest BS* & Beer episode was about air to water heater pumps for heating/cooling, and was posted here on GBA a few days ago. Good info about efficiency and temperature deltas.