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Midnight Brilliant Heating Idea! (or maybe not?)
#1
Hi All,

I have been playing with an idea and would love some feedback on it.

I'm looking at converting a cargo trailer and one of the issues I am looking at getting into is hot water (hehe, I get in enough as it is already)
I will be insulating the floor and all other surfaces (yes I know Bob doesn't recommend insulating the floor, I'm still hazy on why) and my thought is to install a freshwater hydronic electric heat pump through the floor of the trailer. The thinking is that if I size the system correctly (I haven't done the calcs yet) I can use the excess solar power (I should have ~1200W of panels) generated during the day to heat a super-insulated small tank and use a small food grade pump to circulate the hot water throughout the floor when heating is desired. If my understanding of the solar charge controller is correct, when the batteries are full they basically bleed off the excess power as heat, so I figure why not capture it and use it for the evenings. If I add into the floor on the inside some kind of thermally transmissive lightweight substance I will be able to predispose the heat flow into the trailer (thinking of maybe some aluminum foam, or even balled up aluminum foil), especially if it is insulated from below.
The water capacity of the PEX piping and water in it is relatively small (100 feet of 1" is like 3 gallons), so the bulk of the storage would be in the tank. I would set up a thermostat to regulate the pump and it would also serve as a hot water source and possibly refill automatically in the morning for the next days heating. I also thought about putting a loop up on the roof to allow some solar gain (controlled by a thermostat to disable in the evening.
In the Summer the whole thing could just be switched off for extra cold water storage. 
I looked into doing this with a propane water heater and the reading I did pointed to them being too powerful for such a small system.
I could also plumb it into the main water system but I would lose redundancy

The layers would look something like this: 
#=polyisocyanurate foam
O= PEX tubing running front to back down long axis of trailer
&= Thermally transmissive lightweight material

______________________________ Inside walking surface
##&&&#&&&##&&&#&&&#&&&##
###O###O###O###O###O###
#######################
______________________________ Trailer floor (outside)

Thoughts?

Pros:
Extra water storage in an independent source (redundancy)
Hot water on demand
No need for propane for heat (in theory)
Safety 

Cons:
Weight (I will be carrying water anyways)
Complexity and component failure (I like things automated...)
Freezing (need protection)
Need to assure over 140deg for legionella
Cost (needs looking into)
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dawnann7 (12-09-2017)
#2
If your water heater tank is inside then insulation matters less.  Any heat leaking out of the tank heats the space you want to heat.  

A small radiator and a 12 volt computer fan can extract the heat from the water.  That might be easier than floor pipes.

A big 40 gallon, 320 pound, tank heated to 210 degrees will release 45,000 BTUs as it is cooled to 70.  A little 4 gallon tank will only produce 4,500 BTUs.  Depending on outside temperature and insulation, a water tank could work but not a tiny one

Lead acid batteries will hold about 10,000 BTUs electrically (3000 watt hours) in 320 pounds.  Just heating the lead would be more but water is a lot cheaper.
You cannot bring about prosperity by discouraging thrift. 1 of Ten Cannots, Rev. William John Henry Boetcker, 1916.
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#3
The reason insulating the floor is pointless is because heat rises, so you actually lose very little heat through the floor. It's more effective to insulate the walls and especially the roof, where most of the heat is actually lost.

Living in "Ziggy the Snail Shell" since May 2015
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#4
(12-09-2017, 09:28 AM)lenny flank Wrote: The reason insulating the floor is pointless is because heat rises, so you actually lose very little heat through the floor. It's more effective to insulate the walls and especially the roof, where most of the heat is actually lost.

Insulate the floor. That was the first thing I did in my build. Park on a hot parking lot and you will feel the heat. Step on a cold uninsulated floor and you notice. That is why they invented carpet. 

As for hydronic heating, in my plumbing experience a simple roof pipe grid can heat the water for the cost of pump electricity. No gas needed. (Unless it keeps snowing).
Trouble rather a tiger in his lair than a sage at his books. To you kingdoms and armies are mighty and enduring, to him they are toys of the moment, to be overturned with the flick of a finger. G Dickson.
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Cammalu (12-09-2017)
#5
(12-09-2017, 12:30 AM)Redbearded Wrote: If my understanding of the solar charge controller is correct, when the batteries are full they basically bleed off the excess power as heat...

No, they simply shut off the incoming solar input to the batteries when they are fully charged. Solar panels are not damaged when the output from them is effectively 'open circuit' (no current flow) even if they are in full sun.  

Now, most wind turbine controllers DO have to dump excess power somewhere, so maybe your hydronic system should include some wind power, and since it's often windy when it's cold, you might have some 'free' power being generated all night long.
Never trust a camp cook with lots of shiny new pans...
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#6
My first thought is that floor heat is great for foot comfort but not so much for heating the space above it.

Second is you are going to need a big storage tank to get heat off of it for long. Not only will it cool faster in the tubes than you think, the now cooled water is going back into the tank cooling it. Now it takes even more water to produce the same heat result.

There are controllers that will divert power to a load once the battery charging allows it but not all. Those that do that I have seen or dealt with had limited amp capacities for that load like 10a. You can not heat much water in the hours after the bank is full at 10a.

I do use my 1185w solar system to heat my water primarily but I did not design it with just that in mind. It was a after thought as I wanted to run a air conditioner and other loads that pull 4-500w off of the solar output. I use a 120v Hott rod that pulls 450w and heats a 6 gallon tank to 150 F in two hours. You are talking around 40 amps per hour off of a inverter.

Insulating under the tubing would be critical or you would not get much heat going up.

A passive solar design with black tubing on the roof absorbing the suns light would be much more effective as it could run all day. You could use a heater core and fan or run the tubing around the edge like baseboard heat. You still have a storage issue.

Another type of floor heat is heated mats that I know of one person on RV.net using. The thing is he does not use it off of his bank, only on shore power along with other small heaters. My 675 Ah bank could run a 400 watt heater over night but that is not a lot of heat.

There are hydronic heaters that use the water heater and propane to heat it. The people that do it like the quiet heat but you do hear the pump AND the burner of the water heater. I can not say it is any more efficient than my furnace because the water heater is a huge drain on the propane tanks. That is the reason I set mine up to use the Hott rod and it has easily paid for itself already.

In the end you might see why propane is so popular as a energy storage concept. It would take a huge battery bank and solar system or passive system with a water tank to store the ability to heat as much as you can with a 20 pound tank of propane. Not only is it much cheaper to set up and fuel, it weighs a lot less and that affects your gas millage carrying all that water, solar and batteries.
Sparky created, (Sparkles on some Saturday nights)  Max the Wonder puppy approved

2011 Ameri-lite 25 ft bunkhouse with 750w of solar and a 675 Ah bank
2003 Ford Expedition with 435w of tilting solar.
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#7
Not to mention if you had a water leak buried under the insulation and floor at 2 am on a freezing night and your pump runs dry, the tank is emptied, the tank element burns up, the floor is ruined, and you now have no heat and an expensive repair bill. 

There are only two kinds of plumbing on any RV or moving vehicle or trailer:

Plumbing that HAS leaked, and plumbing that WILL leak.

Dodgy
Never trust a camp cook with lots of shiny new pans...
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Vesper (01-19-2018)
#8
Insulate the floor.

If ceiling is 4", walls can be 3 and floor 2 or even 1".

But not none.

If aircon'd space then equal all 'round.
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UptownSport (12-10-2017)
#9
Relevant link from Skoolies

http://www.skoolie.net/forums/showthread.php?t=19412
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#10
well that is certainly not following the KISS train of thought. for me it seems way to complicated and to many places to have failures. highdesertranger
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