How to improve my DIY Webasto radiant floor heat

Keenan

Hello all!
So I built a new home for myself, it’s a 22ft diameter Tipi sitting on a 20ft seacan.
Turns out hydronic heating is much more complex than I assumed prior to designing this system. The system is not working as I had hoped, so I come here today to ask the experts how to improve my system.

My setup;
Webasto Thermo top Evo 5kw 17,500 btu water heater
Us solar pumps 5.3LPM 26.5 max head circ pump
30% hydronic glycol
3/4 heater hose in/out of heater.
400 ft +/- 1/2 pex routered in spiral pattern with 8” inch spacing into 3” foam board.
380 sq ft 1 zone single room.
Off grid 12v

The stock webasto circ pump wasn’t capable of moving the fluid, added an additional circ pump.
Reaches 40 C (104 F) within a hour and stays there, gauging on the return side. Heater spec says heats water to 76 C
Heat side enters on outside of spiral circulates inwards.
Pumped up to 15psi, adjustable with expansion tank.

I can pretty easily remove 2 “wraps” on the outside of the run to reduce the pex length by 120ft
I can change the flooring type overlaying the pex but would like to maintain light weight and removability.
I can take out all the 1/2 and replace with 3/4
I can switch direction of flow, currently outside inwards.

Currently working on increasing the insulation of the incoming pipes and structure. The run from seacan heater to tipi floor is a sh**tshow yes as I monkey with the system. Don’t mind the domestic water RV pump and pex.

Any advice is greatly welcomed! Looking for the most cost effective and efficient solutions as my setup is off grid so power is limited.
Thank you very very much!

Hot_water_fan

You really don’t want high water temps in a radiant floor, so I think a return of 104F is great, given that it keeps the place warm on the coldest days. 

That’s a huge piping head loss and you need to fix that for your off grid situation for sure. Could you make 4 loops of 100 ft? Or size up to 3/4” and possibly slim down the entire length? You’ll want to move about 3GPM total, so split it up and/or size it up to make the pressure loss less. 

mattmia2

Is that pump rated for continuous duty?

Jamie Hall

As @Hot_water_fan says. Plus -- that pump you quoted is good for about 1 gpm, max, and probably a lot less with the pressure drop you have.

How2eever, that size floor is only good for around 12,000 BTUh before it gets too hot to walk on anyway -- even assuming you don't have any rugs or whatnot covering parts of it.

hot_rod

The big question is what is the heat loss of a tipi?

380 square feet of radiant floor will give you maybe 10,000 btu/hr.

The loop length is working against you you would need to move 1 gpm through that 400' loop to get close to that 10K load. So a pump capable of 14' head.

Some better transfer plates would help. The heat from that tube will be pretty "striped" Even covering the entire floor with a heavy aluminum foil would increase heat transfer.

What is the R-value of those pads covering the tube. Those will hamper heat transfer also.

Start with some heat load calculation on the home, I suspect you will need some supplemental heat to maintain a comfortable temperature.

Keenan

Thanks for the expedient and helpful feedback everyone!

@Hot_water_fan
I can shorten the loop, getting it down to the 300’ mark if you guys think that will make a marked difference. Separate loops of 100ft would be difficult as I understand as I would have to make sure my loops are equal length, quite the pain in this orientation. I’m open to going to 3/4 next spring as I’ll be packing up the structure for the winter soon.

@mattmia2 @Jamie Hall
The pump is rated for continuous duty, here’s the link
https://www.ussolarpumps.com/catalog/circulating-pumps/c1a-circulating-pump/
CORRECTION; the pump is rated at 5.3 gpm not the 5LPM quoted earlier. I will be looking to remove the stock pump to save power, if I can make just this one sufficient.

@hot_rod
The heat loss is undeniably huge. Uninsulated canvas with only the floor insulated and a 4ft tall section of reflectix circling the base, with 18ft ceilings. Currently working on lowering the ceiling and have reflectix all the way up. Currently 39 F at night, just over the border from Spokane.
R value of the foam flooring looks to be R2
Thinking of changing the flooring, my parameters make me think of roll out Vinyl or 1/4 birch plywood, which would you choose?
I did the simple heat load calculators online, came out to about 12-18000 BTU’s the Webasto being rated for 17,500 so just on the mark.

Could someone explain to me why a flow rate too low doesn’t allow for “the heat load” ? The pex gets hot, almost uncomfortable when laying directly on it, but it’s just not heating the space. I also don’t have a lot of furniture etc to absorb the heat in this space

Trying to not have to do a full do over, but understanding I missed the mark on a lot of key aspects. So with this my next move as I understand should be to remove some of the circuit getting it down to the 300” mark and decreasing the heat loss through greater insulation.

And later on going to a shorter run of 3/4 pex and perhaps a larger boiler? Any further suggestions greatly appreciated.

Thank you all for your expertise on this subject. 

Hot_water_fan

Separate loops of 100ft would be difficult as I understand as I would have to make sure my loops are equal length, quite the pain in this orientation. I’m open to going to 3/4 next spring as I’ll be packing up the structure for the winter soon

Eh you’re not going for perfection right? Get them close, that’ll be good enough. Maybe 4 pie shaped loops? The shorter and fatter the tubing, the lower the pump energy will be, which probably matters a lot here. 

This obviously isn’t a typical situation. What’s the indoor temp you want? 70F or are you more flexible?

Could someone explain to me why a flow rate too low doesn’t allow for “the heat load” ? The pex gets hot, almost uncomfortable when laying directly on it, but it’s just not heating the space. I also don’t have a lot of furniture etc to absorb the heat in this space.

Well the low flow is a symptom of a high pressure drop. Generally, a floor aims for a supply minus return difference of about 10 degrees because you might be able to physically feel the difference otherwise. So if your return is 104F and you’re supply is 115F you’ll be about as good as can be. I suspect your supply right now is much higher than 115, so you have parts of the tube that are very hot. 

It’s probably not heating the space because the space is a tent for now. You’ll need insulation, supplemental heating, or adjusted expectations. 

Jamie Hall

The PEX gets hot, yes -- but the heat doesn't get into the rest of the floor. A radiant floor works only because the whole floor is warm (not hot). You have -- at best -- about 20 square feet of effective radiating surface. That's just not going to dump much heat -- perhaps 2000 BTUh at your water temperatures.

hot_rod

I'd do a simple load calc first. No use changing parts if the square footage you have will never cover the load after changes are made. How cold does it get in your area, how warm are you trying to maintain the space. Those are the variables for the heat loss calculation.

I suppose just the total square footage of canvas with any r value number would be a start, the door opening sealed?
Looks like a smoke flap up top? Those count as infiltration losses.

A good friend and plumber I worked with lived in a tipi in Whitefish Montana, heated by only the fire pit in the middle. I visited a few times in winter, you want to stay close to the fire and rotate occasionally to keep warm.

He did double wall the bottom 4 feet around the tipi to get some thermal break, a bit of an insulation value . He did leave enough air leaking in to supply the fire.

Looking around the www it seems a forced air unit outside ducted in is one common approach to the "glamping" tipi you see on Ab&b. Wood stoves, catalytic heaters, not any radiant heated examples I could find.

Google tent heaters to get an idea of the btu they suggest for various size tents.

I built a well insulated tiny home 160 sq ft. I barely had enough open radiant floor space to cover the heat load.

I'm feeling like your radiator, the floor, just will not be adequate.

With that heater you could connect it to a forced air unit like a 12,000 btu/hr kick space heater. That way you would deliver the full output of the Webasto into the tipi. Using parts you already have. At least you will be warm sitting right in front of it Hot air will want to rise out the top, maybe a false ceiling during heating season?

Fun project.

Keenan

Yea thanks guys. The vision was to have consistent heat at the press of a button, with low consumption and power requirements and none of the work/space or danger of a wood stove. But perhaps this is just the wrong application. Without mass to absorb heat, heating the air directly is probably the better way to go.

@Jamie Hall
Yep I see, the floor is not absorbing and dumping the heat, therefore it’s just the pex radiating it, I’m wondering what creative flooring materials I could use… perhaps i could find large tiles at a decent price.

@hot_rod
yep did a simple online load calc. I’m in the blue region (southern BC just over the eastern Washington border) put in no insulation for all parameters. 12000 btu at least.
I’ll be double walling the whole thing today and adding a false ceiling, all reflective bubble wrap. See how it feels. Good suggestion on the forced air, “matrix heaters” people use em in vans with this unit.

 I also have a multitude of cast iron radiators of varying sizes laying around the property, would you consider omitting the floor and running those instead?

Many thanks, it has been a fun project, lots of learning. Haha

SuperTech

Why the choice of a tankless water heater? I don't understand why people choose the wrong tool for the job.  Why not use a proper condensing boiler with outdoor reset capabilities?

mattmia2

It isn't actually a tankless water heater, it is really a diesel(or gas) block heater that is sort of touted as heating the vehicle through the vehicle's heater(a real mod con would need 120vac and a lot of it). A radiator would work really well except they take up a lot of space, you'd need a couple at least, and I'm not sure that floor could support them since they are a few hundred pounds or more each depending on size.

Hot_water_fan

Yea thanks guys. The vision was to have consistent heat at the press of a button, with low consumption and power requirements and none of the work/space or danger of a wood stove. But perhaps this is just the wrong application. Without mass to absorb heat, heating the air directly is probably the better way to go.

Don’t think about mass, that’s not your issue. You need to find the heat loss, your online calculator is not built for tents, so that number is garbage. “No insulation” means an empty stud bay which actually has a tiny R-value. Right now, you got basically nothing. You really need to adjust expectations! The energy costs could be huge here. 

hot_rod

Your approach is good, insulate. Lots of foam under that exposed floor also.
The fastest emitter will be a fan coil, low mass, low water content. It allows quick temperature changes if you let the home drop temperature when you are away

They look like this, and can be put in a wall cabinet

Connect it with heater hose and you could move it around the space   From dining to sleeping area

Those Webasto heaters often connect into small fan coils located  under the seats in 🚌