Best heat source using solar power

[PVCabin]

Hello all, I'm a newbie to the site and plan on going off-grid solar sometime next year in a small cabin up in the mountains. I have many questions, but will start with this one...
I will be in the cabin about 3-4 days/week (I travel quite a bit for work). When I am not home, what is the most efficient way to heat an 800sqft cabin so that the pipes do not freeze? There are propane wall mounted heaters, and I have also looked into a gas boiler/radiant floor system. What do all of you currently use and which one would use the least amount of power from the
batteries.
Thanks in advance...

[Sunking]

Well first you would never ever use the batteries to generate any kind of heat or cooling. A small convection LPG heater would likely be the best bet.

[PVCabin]

I did look into these, but do they have a thermostat and are they safe to operate while I'm away?

[TnAndy]

Yes and yes.

Probably something like a ventless propane 15,000 BTU blue flame wall heater for 800sqft ( assuming it's fairly open and decently insulated ). You can set that up with the heat, couple 100lb tanks and a dual regulator for 6-700 bucks....Waaaaaaay cheaper than a gas boiler/radiant floor system.

[PVCabin]

Thank you for the information. I was only thinking boiler so that I could use it for hot water as well. Is it possible to use an on-demand hot water heater (Rinnai) with a well? I would have a pressure tank to hold a few hundred gallons of water, but was wondering if the pump would be working too often with a tankless system.
Thanks

[TnAndy]

The pump will have nothing to do with the type of water heater.....it will run based on the size of the pressure tank ( and you might check that....never seen one that large....even 100 gallon is unusual ), and the number of times you open a faucet or flush a toilet.

Also, neighbor of mine works at American Water Heater Co in QC and does heater testing/development all the time.....he tells me a tankless system doesn't save a lick of energy compared to a tank model with a standing pilot, as the standing pilot keeps the tank temp up somewhat, whereas the tankless model has to start from dead cold water every time. In his opinion, the only thing you save is floor space with a tankless. That seems to fly in the face of conventional thinking, I know, but he has worked with these doing testing/evaluating for 30 years, and that's his claim, and based on that, I went back with a tank model when I needed a new one ( he did turn me on to the best insulated model the made ) instead of the tankless I was considering.

[Mike90250]

.......Also, neighbor of mine works at American Water Heater Co in QC and does heater testing/development all the time.....he tells me a tankless system doesn't save a lick of energy compared to a tank model with a standing pilot, as the standing pilot keeps the tank temp up somewhat, .....

I'll have to agree with this, I had 2, 50 gall water heaters (in series, #1 was 85F, #2 was 120F), standing pilot, and with 4 people, showers, laundry, and dishes/cooking, summertime gas bill was about $8. When the cooler weather kicked in, we'd get $150 gas bills from the heating. [ So. Calif Gas Company ]

[PVCabin]

Great information...thank you. I just assumed the tankless was more energy efficient, I have one now at my home, but I'm hooked up to the grid. I do like the fact that it takes up very little space, but saving money on setting up the solar is my main focus right now. I wasn't aware the pressure tanks were less than 100g or so...my neighbor said he had a 500g tank, but I'm assuming his is not pressurized and just uses it as a storage tank so that he doesn't have to run his well pump very often.
Thanks for all the responses.

[Sunking]

Also, neighbor of mine works at American Water Heater Co in QC and does heater testing/development all the time.....he tells me a tankless system doesn't save a lick of energy compared to a tank model with a standing pilot, as the standing pilot keeps the tank temp up somewhat,

I do not agree with this as third party testing clearly indicates otherwise. However I am not a great fan of the tankless heaters either even though I do own 3 in my home.

Testing indicates they will save the consumer about 22% in hot water fuel being either natural gas or LPG. Forget about electric. The problem is they do save you 22%, but due to the high cost of the units coupled with high installation cost (if it is a replacement or add-on) would take around 22 years to break even which is beyond the expected service life of the units.

If you want to read a little bit about them from a 3rd party testing agency try his from Consumer Reports

[PVCabin]

So based on all the information regarding tankless vs. tank water heaters, which would be use less battery (to run the well pump) if all variables were equal: Pressure tank, amount of water used/day, etc. Is this possible to determine? I was just thinking, if it takes the tankless a little more time to deliver the hot water, I would theoretically use more water (waiting to heat up for a shower), thereby draining the pressure tank sooner & as a result, activate the well pump more often. Valid theory? I would prefer the tankless, as long as it doesn't affect the batteries ($$).

[Sunking]

So based on all the information regarding tankless vs. tank water heaters, which would be use less battery (to run the well pump) if all variables were equal:

I do not think either would offer an an advantage as it requires the same volume of water.

[Naptown]

There are certain physics at play here that are not being taken into account. The biggest savings on a tankless heater are improved combustion efficiency and virtually no standby losses. The fact that they use outside combustion air and not a draft hood decreases the heat loss on the building. (some power vent units allow for this also)
whether a water heater is a tank type or tankless it takes the same amount of BTU's to raise the temperature of the water to where it is usable. The cold start aspect of a tankless using more energy is negligible at most.
Now as far as the batteries go a tankless will use more battery than a standing pilot old fashioned draft hood type water heater as there are electronics and fans involved that are not present on the tank type. A direct vent may actually use more batery as these cycle due to heat losses in the tank and not necessarily use.
There are also combination units available that will work as both a water heater and boiler. However a boiler will require a circulator again using more battery. ( Pump for that will most likely be 1/25 HP so not a huge load.)
I have used these with success and they are not hugely expensive http://www.navienamerica.com/product...?skin=combi_01

[PVCabin]

Great information, thank you...seems like a tankless is certainly possible & definitely my preference.

[russ]

Great information...thank you. I just assumed the tankless was more energy efficient,

They are more efficient when used as a point of use heater - otherwise - no.

[Naptown]

They are more efficient when used as a point of use heater - otherwise - no.

And how did you arrive at that conclusion?