Hi folks. It’s been a while since the last update, but for good reason! I’ve been busy building. More on that in another post. But first, I’d like to discuss plumbing.

(A little aside: Unless you’re willing to spend hours and hours designing a system from scratch (and likely still get it wrong), please just hire a professional. I’m only offering here the resources I’ve found helpful in the hopes it’ll be helpful for other DIY’ers. I certainly wish I had more examples available to study as I dove into this adventure.)

Since the tiny hacker house will be heated via a closed-loop in-floor radiant heat system, the plumbing is a bit more complicated than your typical tiny house. One heater will need to provide hot water for both the fixtures and home heating, so a “smarter” system is required to ensure everything flows properly and stays hot. Unfortunately, that means a bit of back-of-the-napkin calculations are in order.

Water heater sizing for mortals

The first order of business is figuring out a “worst case” scenario for hot water usage. This will serve as the maximum heat load for the system, measured in BTUs / hr. If you have a stationary home with traditional heating, this is pretty easy – just throw in one or more electric tank heaters (depending on the size of your family) and you’re done. With mobile homes and radiant heating, though, it requires answering some questions first. Tank or Tankless? Planning ski trips? Having house guests? Using an automatic dish washer? Everyone’s scenarios are different.

My “worst case” scenario is taking a 110°F shower in 0°F weather while keeping the house at a comfortable 75°F.

To calculate the required BTUs per hour for this load, I simply need to know 4 things: the starting temperature of the water I want to heat for my shower, the flow rate of the shower head, the area of my walls, floors, and ceilings, and the insulation R-value for each.

If I assume the temperature of the water for my shower comes from the tank at 32°F (hopefully not frozen!), we can estimate the BTU/hr required to heat the water to 110° through a 2.2 GPM shower head using the calculation:

BTU/hr = 500 * water flow rate * (temperature inside - temperature outside)

BTU/hr = 500 * 2.2 * (110 - 32)

BTU/hr = 85,800

Now, to estimate the BTU/hr required to heat the home, I can use the following equation:

BTU/hr = (wall surface area / R-value) * temperature difference

Shown below is a table summarizing the R-value and area of my walls and windows. I’m assuming a very conservative R-value of 20 for all my walls and 1 for windows. With more efficient windows, thermal barriers, and 3” of spray foam insulation, it should be much higher in practice.

Location Area (sq) R-value A / R
N Wall 75 20 3.75
S Wall 55 20 2.75
E Wall 130 20 6.5
W Wall 130 20 6.5
Ceiling 116 20 5.8
Floor 106 20 5.3
Windows 70 1 70
       
Total     101.1

Notice the majority of heat loss occurs through the windows due to their relatively low R-value. Increasing the efficiency of the windows to R-2 or even R-5 can help dramatically. But in this worst-case scenario, the required BTUs / hr to heat the house is:

BTU/hr = 101.1 * 75 = 7,582

So the total BTU/hr needed for my “worst case” scenario noted above is

85,800 + 7,582 = 93,382 BTU/hr

After a bit of searching around, I decided to go with the Takagi T-KJr2. Its 140,000 BTU capacity and 82% efficiency means it’ll be able to supply up to 114,800 BTUs of heat – more than enough for my maximum heat load scenario.

Let's get pumped

After the water heater’s been decided, we need to find a circulation pump to move the water through the radiant floor loop, heat exchanger, and water heater at the proper flow and pressure. With a pump too small, the efficiency of the plate heat exchanger suffers – I may not be able to reach the 110°F design temperature for the domestic hot water. With a pump too large, the efficiency of the system suffers – pressure drops exponentially with higher and higher flows.

tiny hacker house plumbing taco bumblebee curve
Pressure drop and temperature rise charts for a given flow for the Takagi T-KJr2 water heater.

At this point, a professional plumber would calculate the required flow for maximum heat transfer for my heat exchanger in my worst case scenario, the pressure drop through the radiant floor loop, and accurately size a high pressure, low flow pump based on this information.

We’re not going to do that.

Instead, I’ve opted to go with the Taco Bumblebee – a variable speed, low power programmable pump. At just under $200, it’s in the ballpark of most conventional circulation pumps, and uses as little as 9 W at the lowest speed setting. Pretty efficient if you ask me.

Except the issue with this pump (or rather, my system) is that I have a relatively high pressure drop compared to most conventional radiant floor systems. This is because of my choice of 1/2” PEX pipe, which I chose because of its 5/8” outside diameter – my subfloor plywood in which I’m laying the radiant piping is only 1 1/8” thick. With 3/4” PEX I need a 7/8” groove in the subfloor, meaning for long length-wise runs of my floor, I’d have only 1/4” of subfloor plywood supporting the load above. If it cracks or breaks, the hardwood floor above would likely crack soon after (it’s running in the same length-wise orientation as the heating loops), and I’d have to rip up the whole floor down to the trailer frame to fix everything. In a conventional home, I’d have plenty of space to use larger pipes. But with a tiny house, there are trade-offs I have to make.

Here’s a table of my estimated combined max pressure drop vs flow in the radiant loop, calculated from the Takagi chart above and the pressure drop tables from the residential pex design guide (PDF):

Flow (GPM) Pressure Drop (ft. of head)
1.0 6
1.5 11
2.0 18
2.5 29
3.0 40
3.5 52
4.0 65
4.5 81
5.0 96
6.0 140

Experienced plumbers and hydro engineers may look at this table and say, “get a cheap diaphragm pump, dummy! High pressure, low flow!”. But the Bumblebee is programmable, that’s like marketing kryptonite for software engineers such as myself. It has 3 modes of operation! 2 temperature sensors! Bring on the Arduino shield baby!

tiny hacker house plumbing taco bumblebee curve

The good news is the Bumblebee will work for my system, albeit at a maximum flow of 1.5 - 2 GPM. But this reduces my electricity usage anyway, and provides the maximum heat rise from my water heater. The only issue now is the heat exchanger loses its efficiency at that low flow.

Theoretically, roughly the same amount of heat is applied to a fluid through a heat exchanger regardless of flow, but in reality, faster flows generate more turbulence across the plates, covering more surface area and transferring more heat.

My solution to this final puzzle is to buy the largest heat exchanger feasible – a 1,000,000 BTU/hr copper-brazed plate exchanger (sometimes the brute force approach is best). The difference between the small and large ones is a matter of a few hundred dollars and a few inches, but the heating capacity gains are ten-fold. It’s an upfront cost that will save on both electricity and propane costs for the life of the house.

Tap supply

To supply cold / hot water to the fixtures in the house, there are two possible scenarios to take into account.

If the tap water’s already coming from a pressurized source such as a garden hose, city water, or RV hookup, you simply need to filter the water before running to your heat exchanger and taps. It’s also recommended to install a water pressure regulator in case you happen upon any extreme pressures in your travels.

If, however, it’s coming from the tank, you need to pressurize it, feed it to a buffer / accumulator tank (to prevent rapid pump cycling and surges) then feed it to the rest of your tap lines.

Many choices for parts abound for scenario 2, but what’s most important is the GPM rating of the pressure pump and its power consumption. I’ve selected one rated for 3.0 GPM at 90 W, but they come in all sizes.

Mixing electricity and water

Finally, we need to add a bit of smarts so that everything is automatic.

The state of the plumbing system can represented by 2 “bits” of information: fixture (shower) on / off and radiant floor on / off. So there are four possible states the system can be in: both off, both on, fixture on, or radiant on.

Using an HVAC thermostat and flow switch, we can control the system with a simple “truth table”:

Thermostat requesting heat? Hot tap flow? Pump state Radiant zone valve
No No Off Bypass
Yes No On Floor loop
Yes Yes On Floor loop
No Yes On Bypass

So if I’m showering and it’s cold outside, the pump is on and the thermostat switches the zone valve to the floor loop. If I’m showering in the summer, the pump is on but the zone valve is set to bypass the floor loop so as not to heat up the house. The water heater fires up automatically when it senses flow above 0.5 GPM, so the pump triggers the water heater in this setup.

Add in a couple relays to provide the switching and I should be good to go.

Putting it all together

I now humbly present to you, the schematic:

tiny hacker house plumbing schematic
Ok, so I like to color outside the lines a little

If any professional plumbers are reading this and feeling generous, please offer your critique in the comments below!

Update:

The reddit discussion concerning this design has prompted me to reconsider going with a dual zone heater, known as a “combi” boiler. The problem is, they’re very pricey – this Rinnai E110CP is about 2x the cost of the setup shown here for a similar BTU rating. It would be nice to save a bit of space and plumbing complexity, but if the pump, heat exchanger, or zone valve fail I’m stuck with OEM replacements.

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