Solar single zone radiant heating system design help

rimrock

I am building a 320 sq. ft. detached workshop that has good roof-top solar access, along with water and electricity, no gas and no south facing windows or walls, so no solar heat gain. A single zone of Wirsbo ½" tubing has been installed in the new slab, and I need to design the rest of the system.

I have radiant heating in the main house, which uses a Navien CH-240 gas water heater for both radiant heat and dhw. I installed the closed loop radiant heat system in 1992 using Wirsbo tubing and zone manifold, a Tekmar 254 boiler controller and a Grundfos circulating pump. There are no mixing valves in the system; mixing is controlled by a pair of ball valves. This system has given me 25 years of excellent service, although it has been redesigned a few times along the way. I'm located in Salinas California, so I only get a few days a year below freezing temps.

For the new shop, I am considering a roof-mounted Sunbank SB40G solar water heater for the heating, with an electric on-demand water heater. This would be a closed loop system since I have no need for dhw in the shop. I would use a Grundfos circulating pump. The solar water heater has a built-in 40 gal. tank. I've never designed a heating system with two heat sources, so what else will I need?

Jamie Hall

You might consider looking into this:

https://images.search.yahoo.com/yhs/search?p=solar+staircase+norman+saunders&fr=yhs-mozilla-001&hspart=mozilla&hsimp=yhs-001&imgurl=http://www.irisdemauro.com/site/wp-content/uploads/iris_plan.jpg#id=0&iurl=http://www.irisdemauro.com/site/wp-content/uploads/iris_plan.jpg&action=click

It works...

hot_rod

got much south facing glass? That would be a lot of your heat

Probably a load of 5,000 but/ hr or less at design?

Jamie Hall

That 320 square feet of solar access will have a peak power of around 32 KW -- more than ample, if the OP can figure out a way to store the heat. Passive -- like the system I linked -- has some real pluses.

rimrock

Passive solar is not an option, as I stated in the description. There are no south facing walls or windows, no solar heat gain. Roof-top solar is the only option. Here is a photo that shows the new slab going in with radiant heating tubing in it.

hot_rod

Looks like a bit of shade over that location. Do you have a good shot at the sun from 9-3:00 during the heating months?

I don't see any rating or output data at their site to get some idea of the amount of energy you have available. I suspect the array is intended to heat that 40 gallon integrated tank.

I don't see that company listed at the SRCC Solar Rating site either. They certify collector performance at various load temperature and sun conditions.

Here is an example of a similar evacuated tube collector.

I would guess The SB may be similar, if so expect to see something like 8- 10,000 BTU per day maybe, in winter conditions supplying 80- 90 degree fluid temperatures?

It looks like it uses water, so you will need freeze protection for the piping, the tank seems to be super insulated to handle freezing temperatures that you occasionally see.

The best approach is a load calculation for the room, then see how the collector performance matches.

A wild guess is you will cover 25- 30% of the load with solar, the rest from the backup source. that is a common target for solar radiant designers, 30%. Beyond that the array size, and required storage becomes large and $$

Unless you have a DHW load you will need some over-heat protection plan for summer months, or drain it down.

Also shown is their combined DHW radiant concept.

Jamie Hall

Pity. The design I suggested is for that roof top you mention... but if you're not interested, that's OK too.

rimrock

The design you m

Jamie Hall said:

Pity. The design I suggested is for that roof top you mention... but if you're not interested, that's OK too.

The design you mentioned looks like a passive solar house with lots of solar gain. My question is very specific, so please tell me how the article answers it: how do you plumb a radiant heating system that has two heat sources: one being a tank of solar heated water (that may be colder than the inside temperature) and the other being an electric on-demand water heater?

If I made the solar tank a permanent part of the radiant heating loop, that would protect the solar heater against freezing. During summer I could just turn off the radiant heating pump and cover the solar panel. Possibly I could automate it so that when the pump was off, a cover would roll into place.

But I am hoping some of the experts here would have some additional suggestions.

Jamie Hall

How I would do it would be to use the solar staircase on the south facing roof, storing heat into a water store in the space under the roof. I would pump from that through the slab radiant and back up to the water store in the roof space. If the temperature in the store tank was low, I would fire up the auxiliary boiler and circulate through that, too, until the temperature came up to where I wanted it -- essentially a giant buffer tank.

I would not use an on-demand water heater; I would use proper boiler for longevity.

The solar staircase design does not need a shade in the summer.

hot_rod

Are you planning on an additional storage tank, the collector has a tank built in 40 gallons I assume?

Are you going to use this for both DHW and radiant, or just radiant heating?

One method to toggle between solar and the electric back up is with a three way valve. A motorized version would monitor when the solar tank is too low to provide heat, then turn on the back up. most solar controllers have that function built into them, two relay outputs.

A simple drawing, expansion tank, air elimination, relief valves need to be added of course.


Orange lines indicate solar input, green for heater input. The valve selects one or the other based on temperature.


You would need to cover in the summer or dump heat somewhere. Dumping heat requires pumping power, not my first choice. If the system is not providing DHW, drain the collectors or cover the array. There were some companies making nice canvas covers for those vac tube systems, similar to boat covers with elastic bands to keep it on. I'm sure you could have one made locally.