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Old 10-06-2017, 02:00 PM   #1
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Propane "Aqua Hot" Equivalent, or DIY?

So, I'm looking at options for backup heat to supplement mini split type heat pumps. We don't really plan to be any place cold, but it seems smart to have another option for heat.

My initial thought was that since the bus is already plumbed for engine coolant based heaters, and has several powerful heaters already installed, it would make sense to look at RV type solutions that operate on the same basic principle. It seems like there are several used in higher end coaches that use a diesel burner based system to circulate coolant (either engine loop or a separate loop) through heaters and optionally function as a hot water heater also. Aqua Hot and Webasco seem to be common brands. Problem is, they are down right stupid expensive. I've seen discussion that they are a $10-20k option on a new motor coach, and was quoted $3500 for a used aqua hot from an RV salvage yard.

So I'm wondering if there is a propane equivalent, and if they are any cheaper? And if not, is there any reason one couldn't create a similar system using a propane tankless water heater and circulation pump? If you wanted to get really fancy perhaps using a recirculating loop and water to water heat exchanger to run a single heater both for hot water and for coolant heating?

Rob
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Old 10-06-2017, 05:47 PM   #2
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Old 10-07-2017, 07:35 PM   #3
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Thanks Rusty! That's heading in the right direction price wise.

I'm seriously considering trying to eliminate propane from our plan all together, but I'm not sure I can get there and keep the budget from blowing up. Tankless propane water heaters are just ridiculously cheap per btu. Take the example above, this Westabo is 104,000 btu for $1800, though I think it does come with a pump for that price. I'm not sure the cheap ones will meet the spec demands of a hydronic system, but you can buy a 140,000 btu Eccotemp for under $400. Even a higher end Takagi or Rheem in the 140,000-150,000 btu range is $500-600.

As far as I can tell so far, setting up a hydronic loop is similar to what some folks already do with these heaters for radiant flooring or forced air hydronic systems. They seem to have some drawbacks for radiant flooring, but I believe this is primarily due to the relatively low heat transfer compared to a system using a water or air heat exchanger. In some ways it's also similar to a recirculation loop, which I was already thinking might be a good idea. Having a tankless heater in our home, we're aware that one of their down sides is the water wasted waiting for the heater to fire and then the hot water to get to you. This is not such a big deal at home, but seems like a really bad idea when you are boondocking and counting every gallon.

Basically it seems like you need a circulation pump on each side and a heat exchanger. The hot water loop is basically a recirculation loop, which it might actually be able to do double duty as, with the heat exchanger inserted into the loop. The second side of the heat exchanger would insert into the coolant lines.

What I need to spend some time on is figuring out how to size the heat exchanger, but I'm encouraged that what seem like fairly large units like this 50 plate 240,000 btu unit seem relatively affordable at $175:
Brazed Plate Heat Exchanger 3/4 Inch 50 Plates - LA14-50. Flat Plate
I just don't know how efficiently it would transfer heat at the flow rates and temperatures involved in this application.

If we thought we were going to be living some place cold and this would be our primary heat source, it might make more sense to just go the Webasco or Aqua Hot route. But as a backup for the mini splits and given we are not regularly planning to be anyplace it snows for long, the thought of doing it as ~$500 add on to our existing tankless plan is pretty tempting....
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Old 10-11-2017, 04:54 PM   #4
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Quote:
Originally Posted by miscrms View Post
Thanks Rusty! That's heading in the right direction price wise.

I'm seriously considering trying to eliminate propane from our plan all together, but I'm not sure I can get there and keep the budget from blowing up. Tankless propane water heaters are just ridiculously cheap per btu. Take the example above, this Westabo is 104,000 btu for $1800, though I think it does come with a pump for that price. I'm not sure the cheap ones will meet the spec demands of a hydronic system, but you can buy a 140,000 btu Eccotemp for under $400. Even a higher end Takagi or Rheem in the 140,000-150,000 btu range is $500-600.

As far as I can tell so far, setting up a hydronic loop is similar to what some folks already do with these heaters for radiant flooring or forced air hydronic systems. They seem to have some drawbacks for radiant flooring, but I believe this is primarily due to the relatively low heat transfer compared to a system using a water or air heat exchanger. In some ways it's also similar to a recirculation loop, which I was already thinking might be a good idea. Having a tankless heater in our home, we're aware that one of their down sides is the water wasted waiting for the heater to fire and then the hot water to get to you. This is not such a big deal at home, but seems like a really bad idea when you are boondocking and counting every gallon.

Basically it seems like you need a circulation pump on each side and a heat exchanger. The hot water loop is basically a recirculation loop, which it might actually be able to do double duty as, with the heat exchanger inserted into the loop. The second side of the heat exchanger would insert into the coolant lines.

What I need to spend some time on is figuring out how to size the heat exchanger, but I'm encouraged that what seem like fairly large units like this 50 plate 240,000 btu unit seem relatively affordable at $175:
Brazed Plate Heat Exchanger 3/4 Inch 50 Plates - LA14-50. Flat Plate
I just don't know how efficiently it would transfer heat at the flow rates and temperatures involved in this application.

If we thought we were going to be living some place cold and this would be our primary heat source, it might make more sense to just go the Webasco or Aqua Hot route. But as a backup for the mini splits and given we are not regularly planning to be anyplace it snows for long, the thought of doing it as ~$500 add on to our existing tankless plan is pretty tempting....
BTW, it's common practice on tankless water heaters in residential installations (at least the higher end ones) to install a "pre-heat pump" at the point of use. This pulls water from the incoming hot water line and pumps it t back through the cold-water line. It turns on when you push the button and off when the desired temp is reached. Same wait time but no water wasted. They're about the size of a fish tank air pump and easily installed.
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Old 10-12-2017, 09:01 PM   #5
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An interesting idea. Why not invest a bit in an Ecotemp tankless heater and set up a mock up of what you propose (worst case, sell the tankless heater and move on). I use one of the cheapie EC s on my bus and after some growing pains it is the cat's meow.

Growing pains:

these heaters will change incoming water temps only so many degrees. If the incoming water is real cold then the hot water will be sort of hot. On the other hand, if the incoming water is almost hot-- the output will be scaldiong. My system has a recirculating leg on the hot water. That saves a lot of water. However, when the tank is low, the recirculation leads to a hot supply. My output water temp got to over 150- degrees, softened the pvc water pipes and blew them out. The cure was to install an upper temp limiting switch at the heater output. Now I have constant temp hot water and no swelled pvc pipes.

Another issue with the cheapie tankless heaters is that they require both a minimum flow and pressure to ignite and to stay ignited. This takes some trial and error to figure out. If you go this route I'd really appreciate a PM as to your findings. Jack
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Old 10-12-2017, 09:47 PM   #6
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Propane "Aqua Hot" Equivalent, or DIY?

Thanks Jack, appreciate the thoughts. Once we get our bus home (another week or so hopefully) I'm thinking the experimental route is probably the way to go. I know I could probably figure out the math and do some calculations and maybe have a better plan, but there's something to be said for just trying it ;)

The heat exchanger is ~$150, so not too big a risk.

I'm thinking about the eccotemp 45h, which is temperature controlled and supports a much larger flow rate. I've worked with both a Bosch manual control and Rheem temperature controlled unit for home use, and there's definitely an advantage to the later. Never had the chance to do a recirculating loop though.

Good to know about your experience with pex at high temps. I'd read it was supposed to be good to 100psi at 180 degrees. May have to make sure I get stuff rated for that spec, as I gather there are different grades. Ideally I'd probably be running the loop at 160 degrees (which I believe is the max setting in the eccotemp thermostat) in hopes of getting close to the spec'd 150 degrees out of the exchanger to the heaters.

I did read the Webasco Scholastic is rated 47,000 btu and is supposed to do a pretty good job keeping a bus warm, so hopefully I can get at least that out of the 140,000 btu water heater and exchanger....

Thought these pumps looked pretty good for the hot water and coolant loops.
BACOENG 3/4'' 110V/115V Hot Water Circulation Pump /Circulator Pump For Solar Heater System With US Plug https://www.amazon.com/dp/B00O1N69VQ..._-Hc4zbVD0TPVW


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