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bluefoxicy 12-31-2013 11:54 AM

Solar heating and heat pump
Is it reasonable in temperate (USDA zone 7-8) zones to use a 2 stage heat pump with no gas back-up, if you seal drafts and insulate above R20-R30?

I was thinking about this whole solar heating thing and I got to thinking on a forced air heating idea using a hydronic stage 1 and a heat pump stage 2 and 3. That's most feasible now; it's also feasible to do some fancy engineering to make a hydronic-solar heat pump stage, but uh. Let's start simple.

Dual Stage Water Heater

In a standard water tank setup, the cold mains comes into your water heater, and hot water exits it:


A hot water system may reach around 80F-90F in the winter when it's -10F outside and sunny. This provides a reasonable preheat for your hot water:


Warmed water enters the tank. Additionally, a thermostatic pump could recirculate the water if the solar tank is hotter than the main tank and the main tank needs to be heated:


 //                    \\

The recirculating thermostat is a separate idea, not critically important. It would decide whether to recirculate water or to engage gas/electric/heat pump water heating.

Hydronic Heating

For primary heating, a tank that's 2F degrees above home temperature would work. So if your solar hot water tank only runs at 80F in the winter, that's fine: the thermostat tries to keep 71F, and the house is currently 68F anyway.


 ||            ==================%%%%%===//
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Above shows the heating coil (%%%%%) tapping from the solar hot water output in a hydronic system, returning to the solar tank's cold input.

3 Stage System

If it's feasible to run from a 2 stage heat pump, then the above could represent a stage 1 heating system. You might want or need a big tank--I can heat 150L (40 gal) with 18 tubes on about 1/5 of my roof in a 45x15 town house, but a 300L system with 54 tubes would provide higher temperatures (more tubes per liter) and longer heat storage (more liters) in the winter.

Thus in a temperate zone, stage 1 could potentially be solar water heating into hydronic; stage 2 and 3 could be heat pump. In a less temperate zone, stage 2 could be a single-stage low-temperature or geothermal heat pump, with stage 3 using fuel (wood, oil, gas, electric).


If you want to re-engineer the whole thing, then you could run the solar hot water tank through a heat pump loop. The heat pump would distribute to the main hot water tank or to the HVAC heating coil. This is similar to combined heat pump units with two coils, such as Rinnai supplies, where the heat pump supplies space heating and hot water heating.

Essentially, if the solar tank is above 4C, loop its hot water through a heat exchanger with a heat pump; if not, just loop cold outside air (take steps to prevent freezing the heat exchanger coil carrying tank water). Use a coil in the main hot water tank and a coil in the air handler as needed to move this solar collected heat into what you want to heat (water or space).

You could still feed main hot water from the solar tank, as well as recirculate between tanks if the solar tank is hotter than main hot water. You could even use straight hydronic as stage one, since it runs just a pump and not a heat pump: consume less power to extract 30 units of heat in 1 hour, or consume more power to extract those 30 units of heat in 15 minutes.

Lots of Engineering

Yes, that's a lot of engineering. What I've described involves:

  • A thermostat to recirculate water between tanks if the source tank is hotter than the destination tank, and otherwise just kick in the heater on the source tank.
  • A hydronic Stage 1 forced air heating coil (Exists, semi-common)
  • A single heat pump driving coils in both forced air heating and water heating (Exists, common)
  • A heat pump automatically circulating an anti-freeze loop if it's above 4C, else not circulating the anti-freeze loop (you'd need to add the anti-freeze loop to a heat pump and use a thermostatic pump).
  • Solar water heating, meaning plumbing and control units and collectors on the roof (exists, complicated)

You could easily DIY the first one: set the tank to 130F, set a thermostatic pump to recirculate water to the tank if the tank is below 135F and the source solar tank is hotter than the main tank.

The fourth one would be something a manufacturer (Trane, Rinnai) would have to get in on: antifreeze-loop supply to the condenser with a thermostat telling it to only pump the antifreeze if it's entering above a certain temperature. You'd need to be awesome to DIY this.

The last one exists, but is considered an advanced DIYer project.

The trick here is that making all of this optional is relatively easy: you can add an antifreeze loop with thermostat to a heat pump and just not use it. So manufacturers could supply the ability to "hook your solar hot water system up to the heat pump". Heat pumping from a hotter source (or, across a smaller temperature difference) is more efficient (think COP of 5-10 instead of 3-4), so yes that would actually reduce your energy usage. And of course using it as a thermostat-controlled stage 1 hydronic system has obvious benefits.

Interesting thoughts, but nobody's going to do all that.

garykerr 03-12-2014 10:50 AM

Using solar power to heat your water is one of the most practical and cost effective ways to harness energy from the sun. Solar thermal systems absorb the sun's heat and use it to heat your residence's own hot water needs.

mayagreen 04-02-2014 04:25 AM

solar heating is ennergy saving and environmental-friendly.

jogr 08-18-2014 02:17 PM

I wish I could read your post but the stupid ads cover it up and there's no way to move or close them.

alexjoe 09-04-2014 05:36 AM

I think now a days these Solar Heating & heat pump are durable in terms of performances then other electrical appliances.

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