Hot Water Radiant Heat - My Installation

[malconium]

First of all a little history:

It has been a while since I posted anything major on this forum. I was shocked to realize that I first purchased my 1973 31' Airstream in the spring of 2004. It has been a long haul doing the remodeling with many long stretches of inactivity in between. Life does happen after all. There have been a lot of people in the forums that have started posts on their remodeling projects as long ago as when my adventure started. Many of them have stuck with the same thread for most of the journey. I on the other hand have posted lots of different threads on various topics relating to my remodel. If you do a search in the forum under my user name you will see that I have posted on a lot of different topics including Polyboard for sub-flooring and reflective foil insulation - both of which I have used in my remodel.

I now am ready to post a thread on the topic of hot water radiant heating and how I am installing such a system in my Airstream. The installation is now almost complete with a few minor details left and a test run of the system. Since the radiant heating system is of the so-called "open loop" type it is very intimately integrated in with the rest of my fresh water plumbing so it all got installed at the same time.

Since there are a few details left to go and since I will still have to take a few more photos before I have all the details cataloged this thread will likely not come out all in one day. I will try to keep after it though and complete the thread as soon as I reasonably can.

I hope this topic is of interest to at least a few of you. I would like to have some feedback so that I know you are following along. One of the most interesting aspects of my Airstream remodeling journey has actually been this forum. I did not even know about it when I first bought my Airstream but I have since profitted much both from reading as well as sharing my experiences along the way.

By the way before I forget to mention it my goal is to have my Airstream road worthy by the end of July - of this year. My wife and I are pulling together a small family reunion for our 4 kids and 9 grand kids and intend to do it camping.

In my next post I will give some basic information on hot-water radiant heating systems and some background on why I made the choices that I did for my installation. I will also include a diagram that I drew up that proved to be an essential document during the actual installation to help keeps the details straight.

Happy reading,

Malcolm

[MDSilverado]

Looking forward to seeing your project!

[malconium]

For those of you that might not be familiar with hot water radiant heating systems here are some basics. I should also state up front that I am not certified in any particular way as an expert on the topic of radiant heating. My knowledge of the topic has been gained by a great deal of research and by some practical experience with an actual radiant heating system that I had designed for me and that I installed in a house I built for our family in the late 70's. If you happen to disagree with my commentary you are welcome to jump in with some comments of your own. I do think that I know what I am talking about for the most part but I may be missing some of the fine points here and there.

The main idea is to transfer heat to your floor (or some other surface) by pumping hot water (or other fluid) through tubing inside your floor. There have been many kinds of approaches taken over the years including the use of galvanized steel pipe, copper tubing and plastic pipe. Some systems embed the pipe in a concrete slab on the ground or in a layer of lightweight concrete poured on top of the pipes which might be themselves on top of a wood sub-floor. I have heard of other systems that use sand to surround the pipes and wood strips between the pipes to support the floor over the sand layer. Some systems use metal plates in contact with the tubing to transfer the heat to the floor. Some of these systems are installed from the bottom side of the floor and some are installed on top of the sub-floor with the finished floor then put on top of that. PEX tubing of one kind or another is the most commonly used tubing in systems that I am aware of today.

As a side note it is also possible to heat your floor using electrical wiring designed for the purpose. That type of radiant heating is very effective providing you have enough electricity available. The current demands would be too high to consider using that approach for off grid camping but could work well if you always intended to be hooked up to power when you camped. That type is outside of the scope of my discussion here but is never the less a viable approach as long as its limitations are acceptable.

The science of thermal dynamics tells us that heat moves from one place to another by some combination of three different forms of transfer. These three forms along with a brief and not entirely complete explanation are the following:

Conduction:

Heat transfers through a body in much the same way that electricity flows through a wire. How fast heat can transfer through a particular type of material depends on the type of material and its resistance to conduction. This resistance is called its "R" value. We can observe this type of heat by putting our hand on the back side of some object that has been sitting in the sun.

Convection:

Heat transfers from a warm object to a colder object through the movement of air between the two objects. We talk about convection currents. Most of us know that hot air rises and cold air drops. When this happens there are air currents created in the space which moves the air around giving it a chance to transfer some of its heat to colder objects it comes in contact with.

Radiation:

Heat transfers using what is essentially like radio waves. The most obvious example of this is the heat we feel when we step outside on a sunny day. It does not matter that the air outside might be very cold. We can still feel the warmth of the sun on our faces.

The reason that hot water floor heating systems of the type I am installing are called radiant heating systems is that heat transfer from the warmed floor surface to the people in the room is done mainly by radiation. From what I read radiant heating is considered to be one of the most efficient ways to heat a space. It is also interesting to note that a very large percentage of the heat that is lost from a heated space is also through radiant energy transfer. That is why I have argued so much in this forum in favor of reflective foil insulation for an Airstream - but that is another topic. Try searching the forum for "reflective foil" to find out more about that topic.

Malcolm

[malconium]

Hot water radiant heating systems fall into two main categories. These two categories are "open loop" and "closed loop" systems. The main distinction has to do with the relationship between the heating system and the rest of your plumbing.

Closed Loop system:

A closed loop system is isolated from the rest of your plumbing system. Typically the only contact with the rest of the water system is that it can get water from the regular system when it is needed. Sometimes closed loop systems use antifreeze or oil instead of water for their fluid. This is especially likely for radiant heating systems that heat outdoor surfaces such as side walks, parking lots and even intersections. The water heater or boiler may be entirely separate from the one used to heat the rest of your water. The boiler will have its own controls and the system will have its own set of valves, pumps and thermostats. This is the type of system that I installed in my own home in the late 1970's. The natural gas fired boiler was separate from my water heater and the water was pumped through copper tubing in the concrete slab on the main floor and through copper tubing on the second floor that was buried in 1-1/2" of light weight concrete on the second floor. These systems sometimes do have a special boiler that can also heat your regular hot water but the water still needs to be isolated from the heating water. It is my observation that in general a closed loop system is more complicated than an open loop system and requires more parts. Also the type of PEX tubing that is used is usually different. It typically is a type that includes an oxygen barrier. This type of tubing is usually more expensive and is not needed for open loop systems.

Open Loop system:

An open loop system is integrated with the rest of the water system. Because of this it can share a number of things with the rest of the water system. Most importantly it is possible to heat water for both domestic as well as heating needs with the same heater. Of course the water heater needs to have enough capacity to handle both. It is most typical to use a tank less water for these types of systems because they can produce a continuous supply of hot water on demand. One especially important consideration in an open loop systems is that it be designed correctly so that water in the heating loops can not sit stagnant when not heating. Otherwise the integrity of the whole water system might be compromised. This is done by making sure that hot water for both domestic and heating pass through the heating coils. I would have to say that at first this seems a bit harder to grasp the details but it can be done.

For more explanation:

For some more explanation about the closed and open loop systems check out the following website where I got some good information:

Radiant Heat From Radiant Floor Company - Sytem Types - Open System
Radiant Heat From Radiant Floor Company - The Closed System

What I am doing:

I have chosen to use the open loop approach for my Airstream. The amount of space that I am trying to heat is very small and I wanted a simple and economical system. I think that I have been able to significantly reduce the complexity of my system as I hope will become apparent in my next post where I include specific design details of what I am doing.

Malcolm

[malconium]

Now for some details on the design that I finally ended up with for my Airstream. As I already mentioned I decided to go with an open loop system. I have included a schematic diagram at the end of this post that shows the overall layout and positioning of the components I have chosen and how they connect to each other. I found that having a copy of my drawing available during installation was invaluable. While the system is not in principle very complicated it would be easy to take a wrong turn if you were trying to hook things up from memory.

One of the most important things that I was trying to achieve when I was making my drawing was to make it possible to entirely avoid having any PEX tubing junctions in the heating loops themselves except at their very ends. While PEX joints are likely to be reliable I did not want any joints under my finished floor. Hence all of my plumbing connections are in places that I can visually inspect should I wish to do so. They are mainly under the kitchen sink and under the bathroom sink.

Since I am using 1/2" PEX tubing for all of my plumbing and since my finish floor was to be laid on top of 3/4" tall strips of wood this meant that the space I had available in the floor for the radiant tubing was also only 3/4" tall. You will notice that there are no tubes crossing over other tubes in the areas under the floor. The tubes near the water heater that look like they are crossing over each other actually don't do so in the floor. The two lines that go to the water heater dive down under the sub-floor to get outside where the water heater will be located.

There are two separate heating loops. I probably could have done the design with just one but decided not to. For one thing it made it easier to find a place for the domestic water to get across from the kitchen to the bathroom without having to dive under the sub-floor. I added two valves that allow me to balance how much water is in the two loops if I want to. I also thought hard about using 3/8" tubing for my two heating loops and 1/2" for everything else. I finally decided against that to keep the type and variety of tubing and fittings to a minimum. Interestingly the cross section area of two pieces of 3/8" in tubing in parallel is slightly more than the cross section area of one 1/2" piece of tubing. This means that the flow should have been OK with 3/8" tubing for the two loops. I hope I made the right call here.

I decided to run my tubing at 6" on center. Most residential radiant heating systems run their tubing further apart. It is not unusual for them to be even as far apart as 16" or so. This does depend a lot on the insulation values in the structure and the way the tubing is installed. In the radiant heating system that I installed in my home in the late 1970's I think that the copper tubing varied from 12" on center to maybe as much as 18" on center in some places. The system also took about 24 hours to get to a stable temperature. That is fine for a permanent residence but would not work so well for a travel trailer. I picked 6" because I wanted the system to respond more quickly. I also picked 6" because I am laying 3/8" thick engineered wood flooring on top of the strips and wanted good support for that. The PEX tubing is not that expensive either. I think I spent about $100 for all of the PEX tubing in my layout. I bought 300' of red tubing for the hot water and 100 feet of blue tubing for the cold water. I probably have more than half of the blue left and not much red left.

The drawing will take a little study to understand how the water flows through the system during different scenarios. When a hot water faucet is opened water will be drawn from the cold water side all the way through the radiant heating loops, through the water heater and back to the hot water side to feed the faucet. The distance for the heated water back to the hot water side is short even though the distance that the cold water has to travel is longer. When the hot water pump is turned on by the thermostat it pushes water through the one-way valve into the end of the cold water line where it can then pass around and around through the heating loops and the water heater. The one-way valve prevents the cold water from flowing to the hot water faucets when they are opened instead of being drawn through the water heater.

See what you think about the drawing. My next several posts will show actual photos of various parts of my completed plumbing.

Malcolm

[malconium]

These photos show bare Polyboard sub-floor with 1" wide strips of 3/4" plywood glued and screwed down according to my design pattern at locations half way between where my radiant tubing was to be placed. I prefer plywood for such purposes because it will not split so easily when screws or nails are installed. Of course it helps to have a table saw to cut the plywood into strips. Fortunately I do have a table saw. Plywood strips typically can be cheaper than buying something like 1x1 strips that are of reasonable quality.

My youngest son was helping with this installation while I was working on some of the plumbing under the sinks. We were always trying to keep in mind that we had to provide support for the finished flooring when we placed the strips. The finished floor was to be laid from side to side of the floor so all of the strips ran length wise of the trailer floor.

See what you think...

Malcolm

[malconium]

I went around and made marks on the floor and the wood strips to indicate where I expected PEX tubing to cross the wood strips. My son Brian then followed up with a circular saw with the blade depth set to 3/4" and cut out all of the notches that I had marked. These two photos show some of the notches and some of the marks I made on the floor with a Sharpie marking pen.

Malcolm

[malconium]

The next step in the process was to install reflective foil insulation in each of the cavities between the wood strips. Brian cut strips of insulation and stapled them down as needed. The foil does two things. First it help reduce over all heat loss from the living space through the floor by radiant energy. Second it is intended to reflect more of the heat from the tubing up to the bottom side of the finish wood floor. This photo shows the reflective foil. You will see more of it in future posts where I also show the tubing installed. I used some of two different types of foil. Some of the foil is the type with bubble wrap in between the two layers and some is thinner and has re-enforcing poly fibers instead. I actually prefer the second type mostly because I can get if for a lot less money and in my opinion it works as well as the other type. I had some left over from a project in my house but I can not find a local place to buy it and did not want to wait to order more. Hence I had to buy some of the bubble pack type at Lowes so that we could finish the job.

I should also note that I intentionally left out the foil in areas that are directly above my holding tanks. This will allow some residual heat from the hot water tubing to leak down to the tanks and help keep them from freezing in really cold weather. The fresh water tank has some insulation under it in the form of the 1" plywood floor. My new gray and black holding tanks will also have some reflective foil below them.

Malcolm

[malconium]

You will see from these photos that I am using red PEX tubing for hot water. You may also note that I had to use a slightly unconventional approach to tie the tubing down to the floor. I found that conventional nail down anchors for 1/2" PEX are just a little too tall to fit inside of a 3/4" tall space so what I did instead was to us strips of plastic plumbers tape and pan head screws to hold the tubing down. You will also note that the tubing does not easily flex to a 3" radius. At first in my design process I thought this might be a problem but it suddenly occurred to me that the actual layout did not need to be exactly like the drawing. It was OK to have the tubing loops dog bone out a bit at the turns. If you look closely you can see the two different types of reflective foil that I mentioned using in the last post. You can also see some places on the floor where I left the foil out above my holding tanks.See what you think.

Malcolm

[batman]

Great timing. I have been looking into heating/hot water options for a while. Nothing quite like your solution. I have explored two main contenders Aqua Hot 375-LP and Precision Temp (twin temp-2 / JR). However I like out of the box thinking and will be watching for more technical details and photo's, lots of photo's. Who makes your pump, LP only? How does it vent?

[malconium]

I know there has been a lot of controversy in the forum about the potential use of LP tankless water heaters that are not fully approved by a US based agency for use in a travel trailer. The low cost of many of the units that are available these days however is very appealing. I admit that I am taking a risk with the approach that I have decided to take but it is a very carefully calculated one. For one thing I have decided to mount my water heater on the outside of my Airstream rather than opting for one that could be mounted on the inside. There are some that have direct vent connections and bring in their combustion air from the outside but for safety reasons I have decided to keep it all outside. The unit that I purchased a while back is in the link below. This vendor still seems to have the best price on this unit too by the way.

Eccotemp L10 High Capacity Tankless Unit - Lowest Prices GUARANTEED!!

Camping World is selling this model now too along with its smaller brother. This model has a capacity of 69,000 BTU at 3 gallons per minute. It is designed for an outside installation and has a stainless steel rain hat on it. I opened up the case a while back to see what was inside and was impressed both with the units overall simplicity and the fact that there is absolutely nothing that can bounce around when the trailer is in motion. It is true that it is manufactured in China. The company that builds it builds something like 1 million water heaters per year though.

When I first got the heater I wanted to see how it worked. I built a little stand for it and hooked it up to the propane tank from my BBQ with the included hose. First I tried hooking up the unit with a garden hose. Later I also hooked up the water pump that I am using to pump water through the radiant heating loops. I just had to put 2 D-size batteries in the battery box on the unit to activate it. When I started up the water flow either from the pump or the hose the water heater fired up right away and started heating water. Stop the water flow and the water heat shut off right away too. It works just fine.

The one issue that I am going to have to see how it plays out is that there is a 17 minute safety cutoff mechanism built in that turns off the water heater if the unit has run for 17 minutes continuously. This is a good safety feature but could be an issue for me if the unit needs to be on for more than 17 minutes at a time for the radiant heating. If this does become an issue I will have to add a simple timer that shuts off my hot water pump for a minute or so to reset the 17 minute timer. That will not be that hard to do but may not be necessary so I will wait and see.

I thought long and hard about where to mount the unit on the outside. I would actually like to mount it on the back of the trailer but the outward slope of the back wall is way to steep to be able to mount it there without mounting it further back than I wanted. I would also like to mount it on the street side of the trailer in the area where my frig and propane generator are located. There are no windows in that area and it would be closer to where I need the hot water. Having the unit sticking out on the side does not seem like a good idea at all. I even considered mounting it in a way that I could take it off for travel and put it back on when needed. That might not be a bad way to go if you don't mind the hassle but I finally decided against that. I thought about creating a recess on the side of the trailer to mount the unit it so that it would not stick out into oncoming traffic. That seemed like too much work but could also be an OK place if you feel up to the challenge. I finally decided to mount my unit up on the front of the trailer. I know the issues. It will be near my propane tanks but not really any nearer than my BBQ burners are to its propane tank. It will also be mostly below the front window. The chimney will likely come up above the bottom of the window. My plan is to just make that window fixed so that it can not open and collect combustion fumes. I will be mounting the unit off center toward the curbside to get as much distance as I reasonably can from the propane tanks and to keep it away from the electrical umbilical cord that connects in the center. I am also going to install a rock guard around it using perforated aluminum sheet metal.

I have not yet installed my water heater in the above mentioned location but will be doing so soon. Once I do I will include a photo or two. I did however run the two PEX lines that will connect to the water heater out the front end of the lower part of the body in a place that will be close enough to run braided metal connection hoses to the bottom of the water heater. The attached photo shows where these connection points are now. At the moment I have a flex hose just connected from one fitting to the other so I can test out the rest of the water system with out hooking up the water heater just yet. As you can see from the photo I have a little clean and painting to do on the trailer tongue.

Malcolm

[malconium]

I did quite a bit of hunting around for a pump that would work for hot water as well as work on 12 volt DC power. Most of the pumps that you can find for hot water radiant heating systems are for 120 volt AC power. I finally found the perfect pump was one that is made for solar hot water heating systems. I can run on power from a solar electric panel and it can pump hot water. It is designed to run on a range of voltages that you might expect from a solar panel that has more or less sun shining on it. There is an adjustment screw on the pump where you set the desired water flow - up to 5 gallons per minute I think. The internal controller on the pump tries to maintain this flow even as the voltage fluctuates. I was able to run the pump just fine from my 12 volt battery when I was testing out the water heater. Here is where I bought my pump. It is a little expensive but is actually a good value for the money compared to other pumps I considered.

East Coast Solar :: Laing D5 VARIO Bronze PV-Direct Circulating Pump $179.90

The pump is actually surprisingly compact, runs very quietly and the pump motor connects magnetically to the impeller so there is not actual mechanical connection to wear out. The motor can also be removed and replaced without disconnecting the plumbing. Overall I am very impressed with the pump and its specifications.

Malcolm

[malconium]

I wanted to be able to run my hot water heater at a temperature range setting for radiant heating that might be too hot for domestic hot water. Anything above about 110 degrees has serious potential for dangerous scalding. The water heater can run at temperatures up to 150 degrees. Something like 140 degrees is a good temperature range to run hot water to help minimize the growth of bacteria in you radiant heating pipes. So I can I run the water heater at 140 degrees or so and prevent the domestic hot water from being higher than 110 degrees? Enter the mixing valve. It has inputs for both hot water and for cold water. It has a setting knob for selecting the maximum temperature to be allow at the output. The unit mixes sufficient cold water with the incoming hot water to keep the output water from exceeding the set point. Problem solved. The mixing valve that I chose - and the place I bought it - looks like the photo in the first link but is actually the cheaper unit in the second link. The second link for some reason does not have a photo included.

AM100C-1 - Honeywell-Sparco AM100C-1 - 1/2 inch NPT Mixing Valves (80-120F)

AM100-1 - Honeywell-Sparco AM100-1 - 1/2 inch NPT Mixing Valves (70-145F)

Each of the inputs and the output are all threaded for 1/2" female pipe thread. I just screwed in a brass fitting into each that converts to a 1/2" PEX fitting.

Malcolm

[malconium]

I believe that I already mentioned earlier that I was going to install engineered strip flooring on top of my 1" wide and 3/4" tall wood strips that are between my radiant heating pipes. What I am using is maybe not the absolutely best choice for an Airstream floor but it is something I have had on hand from another project for several years now so it basically costs me nothing right now. The top surface is Brazilian tiger wood and the under lying layers are more like plywood. It is 3/8" thick. The strips are all about 3" wide and 42" long. They have tongue and grove on the sides and on both ends. I am sure that the darker color will be a bit harder to keep clean - especially in a camping context. We can use area rugs and door mats though to help with that if needed.

There are some good things about using this type of flooring on top of the type of radiant heating installation that I am doing. One important thing it is very easy to see where the PEX tubing is located when each strip is fastened in place. We attached the floor with both glue and air driven 18 gauge brads driven through the tongue of of each strip. We could be sure that we were not nailing into the PEX at each step of the way. The alternative would be to use 1/4" or 3/8" plywood sheets with some type of flooring on top. Cutting and fitting sheets of plywood would have been much more work than cutting the strips to fit. We found that the flooring strips are plenty stiff enough when support by 1" wide strips of wood space at 6" on center.

I only have the one photo at the moment that shows the early stages of the floor installation. It is all done now but we immediately covered most of the floor up with cardboard and etc. so we would no accidentally damage it as we continued working on other things. It does look very nice though. I will try to include more photos of the finished floor later on. My intent is to put a piece of some sort of sheet flooring over the part of the floor that is in the bathroom area. Yes we will be covering up nice hardwood there but if I didn't use some of the hardwood strips I would have had to find something else for that area. As it turns out the part that will be covered up is only about 50" x 52". Of course there will be other areas covered too such as under the bed and the couch in the living room.

Malcolm

Malcolm