Inherited unfinished system

DGerrard

I have a small studio in my back yard that has the following components: a solar hw heating array of about 50 sf, a radiant floor heating grid of about 100 sf, a below-grade hot tub with a pex grid on floor and sides, and a heat exchanger for domestic hot water. The system is a closed loop design. The studio is 100 sf with a 50 sf loft, shed style with the back wall 12 ft high and the front wall 8 ft. It's well-insulated on 2 sides and the ceiling. The east side wall has a double door with double insulated panes and the rest well-insulated, the south wall is poorly insulated (full 8x12 window, iron frame with 12" x12" panes, half of it double-glazed).
The hot tub doesn't have a pump or dedicated heater. There is no back-up heater for the floor or heat exchanger, and I'd like to get a heat pump for this, hopefully one that will serve all three systems. There is a small recirculating pump available for the closed loop systems. The electrical panel next to the equipment has at least 40 amps of 240 spare capacity. There's also natural gas available there, although I'd rather not use it. I haven't been able to find anything smaller than a 10k heat pump spa heater, which I've been told is way more than I need, with the exception of the hot tub, which I don't care that much if it isn't heated during design limit days. The property is in west Contra Costa County, California, which is pretty benign temperature-wise.
First of all, I have no idea what the output of the solar heating grid is. During hot, sunny weather, can I expect it to provide all of the heat for the three systems? Second, it would be great if there's a way to provide cooling for the floor, and I don't know if there is any air to liquid heat pump that will work bi-directionally. Third, is it possible to use the heat stored in the hot tub to pre-heat the other two uses? Or should I just resign myself to having a separate (presumably hotter) storage tank?
Thanks in advance for any advice, David

hot_rod

There are solar simulator programs that can tell you what to expect from an array.

Input the type of collector, pitch, facing south? Plenty of rule of thumbs also if that is close enough.

If it is a manufactured, listed collector www.srcc.org has the output data for all collectors that are OG-100 tested and listed.
32 sq. ft is a 4X8 collector, 40 sq. ft would be a 4X10. 50 sq. ft. is an odd size for a collector?

Sounds like some passive solar also. Sometimes that can over-heat the space unless you have some movable shading.


To make solar thermal viable you need a year around load. Some suggest you get the most solar when you need it least, hot summer months. Your spa load is probable small in warm summer months.

DHW load would help use some of the array in summer. Rule of thumb is 2 gallons of tank for each square foot of collector.

Jamie Hall

You don't need a fancy program for that hot water array. When in full sun, it will have a theoretical output of 500 watts, or 1700 BTUh. That's theoretical, and if you can keep water circulating through it when the sun is shining on it and keep that water temperature relatively low -- say within 20 to 30 degrees of the air temperature, you can expect perhaps 75% or that. As the water temperature increases, the output will go down rather sharply and, of course, it will drop to zero within an hour or two of sunset or after dawn. You don't all that much cloud cover where you are, so that is good.

hot_rod

Here is the data on a 4X10' glazed flat panel.

Use Cat. C for spa or DHW heat.
So on a high radiation day around 36,000 btu/ per day.

DGerrard

The array is custom built, and it looks to be well-made, with double glazing, 1/2" copper piping 6" on center embedded in aluminum fins, 3" Styrofoam base. It faces south with about 25 degree angle. There are some trees that shade it early morning and late afternoon, with maybe 8 hours of exposure at equinox.
The loops all have solenoid valves in them, which I assume are there to control demand as well as prevent overheating or unwanted circulation when there's no output. The solenoids look kind of cheap, made out of plastic, and are sun-bleached, so I assume they will need to be replaced.

hot_rod

If the hot tub is inside the building, it will do quite a bit of the heating for you. You will need to address moisture in the space if the tub is uncovered.
Certainly an air to water heat pump could run all those loads. The temperature you need is ideal, and your climate is ideal also.

A 3 ton, maybe smaller would cover those loads. Pricey buggers those A2Whps

A solar day is considered 6 hours, so if the panels are unshaded from 9-3 you are good.

A drainback solar is my preference. So if you don’t have summer loads, you are not overheating and steaming. That may eliminate the need for solenoids outside.

Is this something you are assembling and piping?

DGerrard

The hot tub's outside. If I left it uncovered, could I use it for a heat sink and would it dissipate enough heat to make both solenoids and drainback unnecessary? A couple more existing features that might have a bearing: a small (~2 gallon) expansion tank, and a pressure relief valve at the high point of the array.

Any relatively inexpensive way to get the floor to act as a cooler?

Yes, I'll probably do the work. I'm an electrical contractor with a fair amount of plumbing experience.

hot_rod

Depends on how hot you are willing to allow the tub to go. Certainly not too hot to be useable?

One way to dump heat is running the collector durning the evening hours. Night time re-radiation cooling.
Basically the air temperature in the upper altitude removes heat from the collector by the same principle that the daylight radiation adds heat.

We used this nighttime cooling, most digital controllers have that function, on vacation homes that sit empty most of there lives. A bad use of solar thermal, by the way.
Dump the heat until the tank is low enough to take the next days heat with over heating. If not you cook the glycol the first sunny month.
A silly way to use free solar energy, running a pump to dump it 😳

Drainback would be a better match for what you describe. Heat the loads until they satisfy and turn the system off.

The old solar motto “use it or lose it”

hot_rod

Maybe pick the best load, and the most consistent. The outdoor spa for example. In warm climates typically you use them pool or spa pump to flow through the collector. A 3 way valve, often PVC, Pentair makes them, diverts flow around the collector when the spa is up to temperature.

In warm climates there was a draindown system also. Water from the well goes directly to the collector. When the HW storage tank is hot or at days end a valve opens and drains the collector onto the ground. I think SunStrand was one brand. Occasionally those draindown valves would stick however and a lot of wasted water on the ground.

I don't think you have enough horsepower (collector area) to cover all 3 of your loads. Probably not even the spa tub in winter months?