Hot Water Radiant Heat - My Installation

[Jammer]

Ambitious project.

The typical output for a radiant floor is 18-25 BTU per square foot. Your floor area is around 200 square feet, which means you'll have 5,000 BTU output, max, a little more than you would get from a 1500 watt heater. I think you'll find that it works really well down to 50 degrees or so.

Your 300' pex loop will hold around 3 gallons of water and so the water heater should shut off after around 5 minutes on a cold start. You shouldn't hit the time limit.

[Living31]

As luck would have it, this morning I was on a job and ran into the guy who taught me about radiant flooring.

He recommends using a glycol heating system with a heat transfer block between the hot water and the glycol. The water warms the glycol and the glycol is pumped through the floor. The amount of heat lost in the transfer is negligible (2 or 3 degrees). The benefit is, in event of a heater failing, the lines will not all freeze. He explained, frozen plex is a bad deal.
He showed me a couple of pictures of a recently installed hot water heater/glycol system and it is pretty neat. The heat transfer box was tiny for a house and for a trailer it could be smaller than and old style box of matches

[Living31]

Originally Posted by Jammer

Ambitious project.

The typical output for a radiant floor is 18-25 BTU per square foot. Your floor area is around 200 square feet, which means you'll have 5,000 BTU output, max, a little more than you would get from a 1500 watt heater. I think you'll find that it works really well down to 50 degrees or so.

Your 300' pex loop will hold around 3 gallons of water and so the water heater should shut off after around 5 minutes on a cold start. You shouldn't hit the time limit.

I am neither a heat (or any other type) of engineer, nor do I play one on TV or online, but I know a “typical” radiant heat installation with 1/2in plex is 25BTU a sq. ft. based on a 16 inch on center configuration. If you double up the plex (8 inch on center) then, using my higher math skills you get 50BTU a square foot.

Checking post #5, Malconium is using 1/2 plex 6 inches on center----making his configuration 62.5BTU a square foot. Using your provided figure of 200 sq. ft, that makes 12,500BTU. (The chosen heater provides more than enough to handle that load)

Standard BTU recommendation for the coldest climate zone in the United States, for a “well” insulated home is between 50 and 55BTU’s. This Airstream could be much less than “well” insulated and still fall within guidelines for the coldest climate zone, (Malconium lists Portland, OR as home and Portland’s heating recommendation is between 40 and 45 BTU’s).

All and all, Malconium should keep toasty to –10F. (give or take)

[malconium]

At long last my wife and I are going to be taking out our Airstream for a one week trial run starting tomorrow. We will be staying in Central Oregon where the projected night time lows will be in the mid to high 40's. This should give us a pretty good chance to test the system out. Hopefully things work properly since I have not really been able to test the heat output thus far. I did install the thermostat along with a relay that is able to switch on my hot water pump just fine. I will report in on the results - maybe even mid week depending on whether or not the advertised WIFI connection really works at the campground.

Malcolm

[Jammer]

Originally Posted by Living31

I am neither a heat (or any other type) of engineer, nor do I play one on TV or online, but I know a “typical” radiant heat installation with 1/2in plex is 25BTU a sq. ft. based on a 16 inch on center configuration. If you double up the plex (8 inch on center) then, using my higher math skills you get 50BTU a square foot.

Checking post #5, Malconium is using 1/2 plex 6 inches on center----making his configuration 62.5BTU a square foot. Using your provided figure of 200 sq. ft, that makes 12,500BTU. (The chosen heater provides more than enough to handle that load)

The problem with this line of reasoning is that making the spacing more narrow doesn't increase the BTU output by any significant degree, because 16" spacing is enough to heat the floor evenly. There aren't cold spots between the lines that could be heated up by an extra line. I've confirmed this with an infrared thermometer, and it's something that the industry relies on. The only way to increase the BTU output is to make the floor much hotter, because the BTU output is proportional to the temperature difference between the floor and the surrounding air. The 25 BTU/sf figure is based on a floor temperature of about 15 degrees above ambient. To get 62.5 BTU/sf the floor would have to be about 40 degrees above ambient or around 105-110 degrees which is both uncomfortably hot and detrimental to the life of the flooring.

Standard BTU recommendation for the coldest climate zone in the United States, for a “well” insulated home is between 50 and 55BTU’s. This Airstream could be much less than “well” insulated and still fall within guidelines for the coldest climate zone, (Malconium lists Portland, OR as home and Portland’s heating recommendation is between 40 and 45 BTU’s).

All and all, Malconium should keep toasty to –10F. (give or take)

Good luck with that.

Check the BTU thread I started. Airstreams have more heat loss than similar sized houses. It takes around 30,000 BTU to maintain a 30' trailer warm at -10 even with no wind.

[A W Warn]

Though I am not even a novice when it comes to this type of heat, I have a question/comment.
If the floor system does not suffice in colder climates, could an automotive type heater core with a fan be added, as a secondary forced air/convection heat source, that runs only when the radiant floor system is not sufficient? One more valve and a different thermostat would take care of the additionally need controls. This could be added up front near the heater or in the cabinet under the sink, or both, without modifying the floor.

Or, could a more simple solution be to add some forced air into the space beneath the floor with a grilled opening or two to let the air flow, allowing radiant and convection at the same time?

[malconium]

I have no desire to go camping in any place where the outside temperature is -10 degrees. I also am aware that the system that I have installed will have some limit as to how cold the outside temperature can be while still staying comfortable on the inside. I have a couple of back up strategies if I decide the radiant heating is not enough by itself for the type of places I want to go. I do have my eye on a fan forced radiator style heating unit that I can easily add to the existing system should I decide to do so. There are lots of places where I could tap off of the existing radiant hot water tubing. I have for example paid careful attention to where my tubing is in the floor so that I could even tap in somewhere in the middle of the run. That could be under the end of the bed in the bedroom, under the couch in the living room or under the kitchen cabinets. Here are a couple of sites that have some units to chose from:

Radex Hot Water Forced Air Heater for Boats.* Manufacturer: Dickinson Marine

Rear Air Conditioners Heaters

Welcome to ProAir LLC -- Auxiliary Heaters

If I am camping at an RV park with electricity I could also take along an electric heater of some sort for supplemental heat.

The long and short of it is that I am perfectly willing to take it a step at a time and see how it works out. I have a source of enough hot water to do the job one way or another so I think I can find a way that works for me with some experimenting.

Malcolm

[Living31]

Jammer, thank for your input. I read your BTU threads and you have done solid work.
While I have worked with different heating systems in home and commercial environments, this is my first Airstream. You presented factors, I did not consider, when working in this arena.

I did a quick search and found radiant heat websites, backing up my 25 BTU’s for 16 on center flooring and 50 BTU’s for 8 on center flooring figures, but I do not believe they assist my original proposition.

Please instruct me if I am mistaken, but I believe your position is based on the insulating properties of an Airstream and the properties of radiant heat flooring. For enough heat to rise from the floor and comfortably heat an Airstream in below zero weather, the floor must be heated to an absurd degree. (Or to restate) Regardless of the amount of hot water running through a floor, only a certain percentage radiates, so to achieve enough radiating heat to heat a very cold Airstream, you must have an insanely hot floor.

The answer then, for someone who wishes to utilize a radiant heating system in an Airstream, is either, refrain from camping in cold weather or super insulate their Airstream (reading the many, many, threads on the trials and troubles associated with that enterprise)
I suppose a third option, for those who like warm floors, would be to use the radiant heat system as supplementary. (The aforementioned options are not exclusive)

Did I get it?

[A W Warn]

Malconium,
Yesterday I was driving home around 5pm the temp showing outside on the truck was bouncing between 98 and 99 F. When I read your post, that you might need heat in your TT this week, it made me envious. Wish I was there!
Enjoy your trip this week! Let us know how it goes!
Alan

[Jammer]

Living31,

I think that's a fair summary for the most part although I would add that the physics of radiant heat dictate a specific maximum output level in BTUs per square foot for a particular surface temperature.

The other alternatives that are out there are to increase the area of the radiant surface by putting coils in the walls or ceiling, or to provide supplemental heat of some kind, such as the fan-forced solutions others are proposing.

[Living31]

Thanks for squaring me away.

[malconium]

Well I am back from the first trip ever with our Airstream. My wife and I spent a week in Central Oregon at the Thousand Trails campground just south of Sun River. Three of our four kids along with two spouses and 8 grand children also camped out there with us. In general we were very pleased with out Airstream even though a few details are not yet finished in my remodel.

I did get an unpleasant surprise though with the radiant heating part. I mentioned above that I had hoped this would be a great experimental time with the heating system. Indeed the weather got down to about the mid 40's at night so it would have been a good first test. I found out however that the water heater that I had chosen has a very specific problem relative to its use for radiant heating that I did not anticipate. I called the factory rep for debugging from the campground and we finally concluded that the unit will not work for what I want to do. The problem is that the burner is triggered on by water pressure - not water flow. At least 20 psi of pressure is required to trigger the system. That seems to be fine for the domestic hot water requirement but not fine for pumping water around in a loop. If I set the water flow limiter in the water heater to the lowest setting the heater would come on for anywhere from a few minutes to a max of 9 minutes. If I increased the water flow or even if I increased the temperature setting on the water heater it might turn off right away. This is even though the thermostat still has the water pump turned on. It seems that water pumping in a loop does not have sufficient back pressure to consistently reach 20 psi so as to keep the heater turned on.

I am currently examining other inexpensive tankless water heaters as a replacement. There are a couple that specifically say that they work well with even very low water pressure. I am seeing values around 3 psi advertised. My guess is that this would work since the current unit seems marginal at 20 psi. I am also speculating on perhaps using a smaller unit - and less expensive - for my system. Here are the current contenders:

Larger size:

~RV Tankless Hot Water Heater No Electricity Needed 10L | eBay

The EZ202 Tankless Water Heater by EZtankless.com

Smaller size:

~Marey Tankless Water Heater for Campers & RVs 5L | eBay

The EZ101 Tankless Water Heater by EZtankless.com

It is interesting to note that he first vendor listing Marey brand water heaters shows a picture of an Airstream in some of their adds. I might give them a call and see out of curiosity what they are recommending for installation with an Airstream. Marey water heaters are made in Puerto Rico and the EZTankless ones are made by a very large manufacturer in China. Both at least have people in the US that I can talk to.

My hot water pump is adjustable and can pump from 1 to 5 gpm. The smaller hot water heaters listed above range from 1.3 to 1.6 gpm capacity with about 34k to 42K BTU. The lager ones range around 2.7 gpm at 79k BTU to 3.2 gpm at 84k BTU.

Any thoughts on what size I should consider going with would be appreciated at this point.

Malcolm

[oilcan]

Buckminster was a brilliant man.

[A W Warn]

Originally Posted by malconium

Any thoughts on what size I should consider going with would be appreciated at this point.

Malcolm

Before buying a new water heater, why don't you experiment with restricting the flow to build pressure in they system that you already have? If the pump is sufficent, it could be that something installed just downline from the water heater with enough restriction to get just above 20psi would work. Maybe a pressure control valve. Maybe a gate valve that you could partially close. Or, something as simple as reducer fittings might be enough.

[malconium]

I did think about trying to restrict the flow some while I was out camping. The one thing that I did try was to run the hot water pump at its highest setting which is 5 gpm with the water heater set to its lowest volume setting. I don't know what the lowest volume setting restricts the flow to but it should be less than the 2.7 gpm that the water heater supposedly handles as a max flow. Those settings did not improve the situation. I did not think about the fact that I do have two gate valves in the system that I could have tried shutting down some. The gate valves are intended to allow me to balance the two parallel heating zones. I am not sure how I feel about artificially restricting the flow in that sort of way though. The same hot water loops are used to furnish the faucets and shower with hot water too and that might reduce the flow more than desired. I should do the experiment though just to find out what happens.

In the meantime I did seek out and order another water heater that might in many respects be a better choice anyway. It is this model here:

Outdoor Tankless Water Heater by EZtankless.com

I was able to buy a so-called scratch and dent unit for $375 instead of the full $524.95 price tag. When I asked about the condition I was told that it was never installed. The customer that returned it wanted a different model and this one was never used so it should be the same as brand new. The documented low pressure start up point on this unit is only 2.9 psi. The person that I talked to said that they know for sure it works down to 8 psi but they do not have a good way to test it at lower psi settings. The unit also seems more suited for an outdoor installation. It has a stainless steel case that should look a lot better on the outside of an aluminum trailer. The total capacity of the unit is somewhat larger too with a max flow rating of 4.4 gpm and a BTU rating of 112,000. One other major difference is that it has fan forced combustion that needs 110 volt power. For that I think I will consider installing an inverter that can run off of my battery if I can not plug into shore power. I have intended to get an inverter anyway so this would prod me to do so. The power consumption is supposedly a max of about 45 watts at 110 so that does not seem too bad. The new unit has a remote controller that I can put inside my Airstream that seems like a really nice thing to do. It also does not have a timeout at 20 minutes like the other unit so that is one less thing I have to worry about.

The vendor did caution me about the potential danger of the unit freezing if the outside temperature is too cold. That was going to be an issue with the other unit too. My thinking is that since I am intending to use the unit for running radiant heating that it will naturally be cycling on enough in the process of heating the floor to keep itself warm enough. As I mentioned earlier I am also not particularly interested in camping in really cold places anyway. I also have had in mind putting a temperature sensor in the heater that could be used to trigger the water pump to start up the heater if the temperature gets too low at the heater end. I will have to wait and see about whether or not that seems to be needed.

The new water heater should arrive early next week. I will try the water restriction experiment on the current unit and report back but I think I will still go for the new unit.

Malcolm

[brattwurzt]

Malcom, I have installed a radiant system on my house, and it is my opinion that the manufacturer is wrong. I have never heard of a tankless heater that requires pressure to operate. Just think about it, the original intent is to heat hot water for normal user (shower etc). In this normal use, the faucet is open and the water pressure goes to almost nothing and the water exits the system. It's always flow that triggers a tankless unit. I think your unit may be defective. Just my opinion.

[Carmilla]

Tis is the system I would like to install in my trailer- thank you for doing the experimental phase for me!!!
I VERY much want to use a tankess hearter for my aistream- there is so much difference between what the manufacturers tell you and how it actually works in real life.
Thank you! Can't wait to hear your further adventures.

Tess

[Wabbiteer]

Pressure to operate - think popping or sizzling noises as hot spots from poor design or mineral scale build-up flash to steam... having a pressure minimum may be a function of the heat exchanger design, once there is a hot spot it would only get worse with time. I've seen several on-demand heater killed from mineral water, the higher pressure would delay possible damage or changes to the exchanger assembly...

[malconium]

I am still working on the issues - now with the new water heater. My wife and I took the Airstream out this last weekend so I did not get a chance to do much on the water heater issue other than to finish attaching it so that it would not fall off during out trip. We were visiting our daughter and her family and were parked right next to their house while we were their. We did not need any hot water in our trailer during that time so I did not even turn it on.

I have done some further investigation relative to how this unit triggers with water movement. First of all I discovered from the vendor that it requires around 1 gallon per minute (gpm) to reliably trigger. My bathroom faucet is not passing enough water to reliably trigger the unit at the moment. Tech support suggested I try taking off the aerator and sure enough there is enough flow without it. I get a lot of splashing though. I clearly need some form of aerator but maybe I can find one with a little less restriction.

The unit still will not trigger with just the heating pump running. I did discover that I had mounted the pump in a position that was not recommended by the pump vendor. I changed that but still the water heater is not triggering. My next step is to find out what is really happening with the water flow from the pump. Yesterday I ordered a simple water flow meter so that I can monitor what is happening. Here is the unit that I ordered from Amazon.com:

Amazon.com: TOM Aquarium Flow Meter (measures 50gph to 250gph flow rate): Pet Supplies

I would have preferred something digital but was not finding anything at all reasonably priced. I just need the meter for a short while anyway. One positive thing about this meter is that since it is designed for monitoring flow in aquariums that implys that it should work fine with a pump.

Once I get the flow meter installed I will first check to see if the pump really is pumping at the rate that the dial setting on it indicates. The pump is supposedly adjustable from 1 to 5 gpm. At the moment I can not prove that it really is pumping at those rates. Once I determine for sure that the pump is working I will turn to measuring what his happening with the flow sensor in the water heater. This unit has a flow sensor that has a variable resistor in it. The unit starts at 21K ohms with no flow and the resistance reduces as the flow increases. The controller board in the heater monitors this resistance along with the temperature of the incoming and outgoing water. It then meters the correct amount LPG to heat the water sufficiently. What I am contemplating doing once I am convinced that the pump is actually working is to determine what the resistance value of the flow sensor is when it senses normal flow through it. I can then contemplate switching in some parallel resistance when my pump runs that will result in the sensor resistance appearing to be monitoring the correct flow. I guess that those of you that are not familiar enough with the electronics involved will have to sort of take my word for how this might work. The basic idea is that I will be tricking the controller into getting the reading it needs when my pump is pumping a specific amount of water. I think this should work but I have to go through the measurement process first to find out exactly what is happening with the pump, sensor and controller. Fortunately I have enough electronics background to feel like I know what I am doing. Time will tell...

Malcolm

[malconium]

I was delighted to receive a parcel from Amazon.com today that contained the flow meter that I ordered. This means that I should be able to start my flow experiments this Saturday. Hopefully all will go as planned and I will finally be able to get my water heater and radiant heating system working together correctly. I will report on progress when I have some more. I will also try to remember to post a photo of what the new water heater looks installed on the front of my Airstream.

Malcolm