Midnight Brilliant Heating Idea! (or maybe not?)

[Redbearded]

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)

 

[Trebor English]

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.

 

[lenny flank]

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.

 

[GotSmart]

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).

 

[tx2sturgis]

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.

 

[jimindenver]

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.

 

[tx2sturgis]

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:

 

[John61CT]

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.

 

[John61CT]

Relevant link from Skoolies

http://www.skoolie.net/forums/showthread.php?t=19412

 

[highdesertranger]

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

 

[Redbearded]

Thanks for the link John61CT!

That is very similar to what I'm thinking about except for the need for the diesel. I have seen how effective hydronic heating is and how nice it can be. No forced ventilation, just even heat that comes up from the floor. I noticed that he insulated the floor under the PEX tubes.

I guess I will have to set up some kind of system to register when the batteries are full and there is plenty of daylight left to pull power from the batteries to heat the water system. It should be pretty easy to set up a Arduino or something to do that.

 

[Riverman]

I would use a small RV water heater and a 12-volt circulating pump. it's way better to have too much heat, than possibly not enough. A small propane water heater would warm up the water in the loop fast, then stay off more than it ran. A simple system with it's on 6-gallon storage tank.

 

[UptownSport]

Great idea!


such a small space, insulated as well as you're planning, isn't going to require much to heat- Regardless of temps.

You will need some type of heat exchanger-
Going to the nines insulating and heating, then opening a window for ventilation unbalances all the work you've done.

Even so, living in a warm, oversized coffin might not be enjoyable for long, LoL

 

[John61CT]

I would not bother with the excess solar load dumping unless **lots** of panels, say a kW.

Yes gasoline or propane heater is just fine if diesel isn't your fuel.

Def want to tap into the propulsion motor when driving

Also a diesel genny or passive solar or solid fuel furnace if you had that (off-grid cabin or bigger boats).

 

[John61CT]

Whole point of all those controls is you stop circulating, or go slower, to a specific zone when thermostat shows it's warmed up.

For a smaller space just one radiator will do, but HWS is nice year-round.

Otherwise just use an air heater version, much simpler of course.

 

[tx2sturgis]

I would use a small RV water heater and a 12-volt circulating pump. it's way better to have too much heat, than possibly not enough. A small propane water heater would warm up the water in the loop fast, then stay off more than it ran. A simple system with it's on 6-gallon storage tank.

A typical RV water heater has a rating of about 10,000 to 12,000 btu INPUT.

But they blow a lot of the heat outside the coach. I figure about 25% to 50%. My estimate (if it is close to correct) means that you would get about 6000 to 9000 btu of heat inside the trailer. Maybe.

So they are very inefficient, but for 10 minutes use in the typical RV for heating up water for a shower, it's not a big deal. 

But using one for heating an entire RV or trailer all winter long, would get expensive....

 

[John61CT]

Those propane-fired instant "on demand" "tankless" HWS allow for 3-4 gallons per minute through rates, recirculating get temps very high.

Anything even the most efficient purpose-designed hydronic units will burn through a fair bit of lpg.

 

[Redbearded]

For my application this is a cargo trailer conversion, so no onboard engine. I'm attempting to stay away from propane, just as a theoretical exercise

The things I have read about using an on demand propane water heater is that the constant cycling would not necessarily be good to long term survival. Plus it defeats the getting away from propane idea.

I plan to have over 1kW of solar maybe 1200W so the idea is to use up the excess power generated when the system goes into float. I may choose to draw directly from the top of the batteries to heat the water (thus forcing the system out of float and maximizing the power). I may also adopt some kind of roof based solar thermal loop to boost the solar gain of the system.

As an example, 1KW of heat is equivalent to 3412 BTU/hour so if I can store the equivalent heat from the electric generation in addition to any solar thermal gain, I'm seeing if I can make do without the need for propane. I thinking if I can use the power excess for a few hours in combination with the solar thermal from the roof (say a few m^2) I can collect enough heat to maintain a comfortable temp as long as the space is well insulated and the temps aren't too extreme.

 

[highdesertranger]

since it's coldest at night and you have no solar at night, what are you planning on batteries? and if you aren't going to use propane how are you going to cook? electricity? highdesertranger

 

[WanderingCanuck]

I've also thought about having some significant amount of storage for engine waste heat.  The biggest difference is that I'd prefer to use paraffin wax or perhaps beeswax as a storage medium because its heat capacity is greater than that of water.  That said, water might be a good trade off between capacity and price, being exponentially cheaper than any kind of wax.  It depends how much weight and space utilization is acceptable too.

The more I think this one over though, it seems that simply relying on propane for heat is a heck of a lot easier.  For the amount of time, effort, and cost to do the heat storage experiment, I'm sure I could refill quite a few propane tanks.

Who knows, it can always sit on the back burner as an experiment to try once the "van life" begins for me.