Off grid Hydronics design help

Merrick

Looking for insights into design for off grid design of Hydronic in slab system.
I live at 7000' near Crawford Colorado. I have done the heat calcs and if they are correct are as follows:
-Straw Bale home 1077 sq ' with a heat loss of 12,422 btu
-Detached Garage/shop/ workout space (50' from Mechanical room) 1375 sq' with a heat loss of 17,196 btu

Although my plumber is leaning toward a propane combi- heater to supply DHW and radiant.... The electrical demands of an on board pump are too steep.
He believes that we can get away with one circulation pump for the 4 zones on the home and one circulation pump for the garage/shop/workout space.(3-zones.)
I'd like to see integration of Solar Thermal system. Possibly a Drainback. Another pump.
And since the Bathroom is approximately 50' from the mechanical room a timed re-circulation pump for the DHW.

The bottom line is that the electrical loads of pumps and additional controls add up.
It seems to me that a stand alone boiler with efficient pumps are the only way to make the system work with an eye to saving watts.

I have included my plumbers preliminary sketch of a design. He admits its rough. I'd have to agree!


And I'd like to get opinions on not using Glycol in the solar thermal system.

Thank you,
Rick

Jamie Hall

I wouldn't worry too much about the electrical load of an extra pump or two. The new ECM pumps just don't draw that much power -- comparable to a light bulb or two. The boiler is going to be the bigger draw on power.

As usual with off-grid power, the problem is getting enough of it -- and even more, storing it and bringing it back to 120 V 60 hz. Do not skimp on either your solar array sizing or your battery banks. Really do the homework on the availability of sunshine (should be pretty good in Crawford, but you have to crank the numbers anyway -- and be very aware of any potential shading effects) and maximum intervals without available sunshine (again, in Crawford, should be pretty short -- not like New England, for instance, where we have to figure on 3 days with no solar at a minimum!).

Also don't skimp on the inverter -- yes, it costs more, but you want something which is pure sine wave, or very nearly so, and has very good frequency stability.

Larry Weingarten

Hi, Will your home have a basement? If so, you might be able to use gravity to your advantage. I live in a tall off-grid home and built radiant heating into the walls. Floor heating would have required pumping, but walls don't as there is little to no possibility of air blocking flow. My solar heated water lives in a tank in the walk in basement, so convection drives it. Thermostatic control is done with modified greenhouse window operators, closing ball valves, so the heating system uses no electricity at all.

About using glycol in your collectors, I'd avoid it unless you're up to the necessary maintenance. Glycol can turn quite aggressive when it over-heats. Solar PV tied directly to a drainback system might work for you. Low voltage solar controls are available.

Yours, Larry

Merrick

Jamie, I have the PV side of things dialed in. Upgrading panels and batteries to handle additional electric loads.
Our inverter has been a workhorse over the years without issue. And we do have the backup generator if needed.
My Plumbers recommended NCB 180 E uses 2amps /200 watts which seems workable; but has been replaced by the H series which specs state a max power consumption of 15amps ac. Navien didn't have any real use energy consumption information or answers to why the the huge jump in power consumption.

Larry, no basement. Interesting use of the modified greenhouse window operators. Yes, a direct PV for the drainback pumps would be an option.

Jamie Hall

That Navien is rated to be fused at 15 amps. That is, it won't blow up if you could figure out how to push 15 amps through it. It will never draw anywhere close to that when it's running.

Not an uncommon problem with electrical ratings, particularly units (like those pumps) which have solid state components. When you really start to look into what various gadgets are rated for, what they can be fused for, what they should be fused for, and when they will self-destruct -- the numbers are all over the map.

Even something as simple as a table lamp plugged into a wall. The wall socket is protected by either a 15 or 20 amp fuse or circuit breaker (14 ga. or 12 ga. wire, repsectively). The table lamp, though, is only good for perhaps 8 amps steady draw -- and the light bulb in it is only going to draw perhaps half an amp once it's turned on. Momentarily, though, when you flip the switch, it might easily draw 30 or 40 amps until the filament heats up. What is the correct rating? What are the correct wire sizes? What is the correct fuse of circuit breaker?

Some devices -- like a lamp or a motor -- will have quite steady electrical draw once they are started, and are nice and predictable (an can, for the fussy, be protected by specialty fuses or motor starters tailored to their startup or stall characteristics, for instance). Other types of devices -- particularly those with switching power supplies -- will draw a certain low current averaged over a few cycles or more, but can have astonishingly high peak amperage, but for very short times (fractions of a cycle). Just as another say what example -- I have over in one corner a really nice high fidelity power amplifier. Part of its power supply is a big capacitor (like 30,000 microfarads). If it didn't have a soft start circuit built in which limits the charging current to that capacitor when it's first turned on the peak draw would be around 100 amps -- but only for the first two or three cycles. After that it settles down to no more than 10 amps at full power output, and a few tenths of an amp when idling.

Most of this is pretty arcane, but even in simple heating work one needs to remember that a single phase motor will draw from 2 to 5 times its running current when starting.

hot_rod

The reverse indirect tanks are another option, the tank water is used for the solar, boiler and radiant. DHW via a coil inside the tank.
Drainback would be my choice. It may be possible with a single 12VDC pump, although they are not high head type generally and pretty $$. Possibly two in series depending on the lift. Run both until the siphon is started, drop to one for circulating.

With a 120VAC ECM type circ you are looking at maybe 20- 30W per circulator.
Glycol systems tend to overheat and destroy glycol during stagnation conditions.

I have a bunch of 12 and 24VDC solar controllers if you want one. They have a drainback function and the ability to switch on the boiler when tank temperature drops, or solar cannot keep up.

Merrick

Bob,
Great information. I have a couple of questions pertaining to the storage tank and boiler sizing.
Size of storage / drainback tank for system?
I see only one exchanger coil in the tank for the DHW. Would I assume that the hydronics side would have an exchanger coil in the tank, an external heat exchanger or be glycol free? I know of one plumber in our area promoting glycol free systems in tight homes with some direct solar gain. The risks may outweigh any gains, should there ever be a extended system failure during the winter though.
As far, as the Mod Con boiler... for my location heat loss calcs and potential solar imputs what size boiler should I be looking at? The home will have one bath and one kitchen- no other DHW loads.
And finally I'm interested in your 24DC controller.
Thank you.
Rick

hot_rod

Here is a good read on collector performance sizing, etc. Typically 1- 2 gallons of storage per square foot of collector. 1 gallon in less sunny areas, 2 gallons in the sunny SW for example. The larger the storage the lower the temperature you can expect.

Ideally to leverage the best collector efficiency run them as low as possible, temperature wise. Pulling 120F from a cold winter sky is about the best you can expect with flat panel collectors. Low temperature radiant is the best match for solar thermal, anything above 130 required SWT, forget it.

Be realistic in your solar expectations in the winter months. The SRCC.org site will have the performance data on the collector. Just know the BTU output is per day! not per hour. A winter solar "day" 5- 6 hours.
So if the collector show 15,000 BTU, divide that by the solar hours. As you can see winter solar is pretty weak. The cold ambient air around the collector will grab most of the collection Copper tube under a single pane of glass in a poorly insulated aluminum box

Looking below you see on a 20° day supplying 95F, a flat plate collector will; run around 35- 37% efficiency. On a warm sunny day 80% or more. The closer the ambient to the collector fluid temperature the higher the efficiency.

For even a minor solar contribution for heating you want 5- 4X8 panels. If you only have a few collectors, I'd just plan on DHW pre-heating. Two collectors and a 50- 60 gallon tank would get you a 40- 50% SF solar fraction. Thats the amount of the DHW load you could cover. Probably 100% for the three summer months.


Evac tubes lose less heat to the ambient, so a bit more into the load, especially in 0 or below conditions. As long as they are not covered with snow.

Send me a mailing address PM or bob.rohr@caleffi.com and I'll ship a controller.

https://www.caleffi.com/sites/default/files/file/idronics_3_0920.pdf

Merrick

Thank you everyone for your insights.
Finally got the boiler up and running. Joys of in floor heat and hot water at the tap.
Electrical usage is manageable. Have some new questions about integration of Solar Thermal which I will start a new thread.

Mad Dog_2

Fascinating information and project.  Please keep us posted.  Mad Dog 🐕 

Solid_Fuel_Man

Agreed, there is really nothing like a high mass radiant slab. 

As an off-grid'r I'm surprised you are not using some type of biomass. Or is little available at 7000'? 

hot_rod

Solid_Fuel_Man said:

Agreed, there is really nothing like a high mass radiant slab. 

As an off-grid'r I'm surprised you are not using some type of biomass. Or is little available at 7000'? 

Plenty of beetle kill pine in the area? You would do well to have a small gasificationnboiler. Or even a wood fired stove inside. Maybe you do?

Merrick

No biomass other than our wood stove. Have been losing a few Junipers over the years due to mistletoe and beetles. Natures way of telling me I need to continue with fire mitigation by forcing me to thin out our land.
PV, small wind turbine and a backup generator. Veissmann boiler (propane conversion) with 130 gallon storage / buffer tank. Future solar thermal to supplement radiant and DWH.