Solar powered hydronic heated floor

[blackfly]

Hi all, what I'm looking for here is feedback if I'm crazy to want to install a hydronic in floor heating system off grid.

My wife and I are starting the building plans for a 480 sq-ft off-grid cottage that we plan to use in the winter and summer. We are located in Winnipeg, Canada and the cottage will be a few hours drive away to the east (close to Kenora)

I'm in the process of building up the power demands as we will be off-grid solar. I'm pretty new to this.

I was thinking we would use a standard hot water tank (4500w) and recirculation pump (80w) connected to a thermostat to cycle the solution through the floor.

What I need to know and can't find through the internet, is how long per day will the HWT and circ pump be running? I know that question is dependant on many variables including size of cottage (480 sqft) and insulation values (R30 walls) so I'm just hoping for some directional advice - is this a realistic load to put on a off-grid solar system?

I appreciate any advice or suggestions!
Thanks, Chris

 

[blackfly]

I suppose I haven't got an answer because there are few with experience with hydronic (glycol) heated floors.

Can someone answer me this - is a massive system required if I need to run an electric hot water tank (4500 watts) for 2-3 hours a day?

I've run the number through a calculator I downloaded, and the answer was yes I'm just hoping someone will sanity check that.

 

[BentleyJ]

What you are proposing actually sounds interesting. However, in order to even begin to answer the question you have to find some way of estimating your total electrical demand for everything including the floor heat vs. average amount of seasonal, solar output for your Lat & Lon using a solar calculator to figure how many solar panels are needed and if they fit on the roor of your cottage or if a ground mount system is needed. Conceptually is fairly easy, Energy in from the Sun has to EQUAL Energy you wish to consume. (and a back up generator is probably a very good idea) Not sure what your budget is? Realistically its cheaper in the long term to save a KWh than it is to produce one and save it in a battery for later use. Your insulation value sounds good so what about a ground source heat pump. Of course the initial cost is going to be substantially higher than a conventional water heater but the COP of a HP is in the range of 3 to 5. Meaning you can put 5KW worth of heat into the water tank using approx. 1.5KW of electricity. Feel free to respond with a list of your other appliances and electrical demands maybe we can help find a reasonable demand estimate.

 

[blackfly]

Thanks for the reply.

Attached is a screen shot of the calculations I've done so far. The file is opimized for winter use. We plan to there for a couple days at a time and in the summer we'll spend more time. We won't use much for appliances - our stove will be propane, and the house hot water will be wood fired (solar electric in summer but I haven't included that here)

I like the idea of a heat pump. My budget is not unlimited however, all though I plan on doing this all myself. I've yet to break ground there and we are on the Cdn shield so we may be sitting on a huge rock with a veneer of dirt - I'm not sure yet.

Looks like I need 5-6 batteries, 2 inverters, and 15 or so solar panels. And a back up generator

 

[acdoctor]

The very first thing you should do is a manual J. (A heat loss calculation on the structure.) I would guess at least 10,000 but per hour, but could be a lot more. You are way up North from what I am used to.
10,000 btu per hour is 3000 watts per hour. A cold cloudy day ( 3000 watts x 24 hours is 72,000 watts or 72 Kw per day plus the 2 Kw for the pump running for 24 hours.
To contrast that Lp is approximately 100,000 btu per gallon. A 95% efficiency tankless water heater can produce 10,000 btu per hour for 24 hours with 2.5 gallons. 240,000 x 1.05 / 100,000=2.5
To build up the system the way you want then run generator to to make up losses could take 5 gallons of Lp generator's aren’t very efficient at turning fuel into watts.

 

[newbostonconst]

Water tank stores about 4kwh of energy and 4kwh of batteries is half the price of the tank. Buy a Minisplit and get air conditioning too out of the same unit. Solar -> Battery Bank -> Inverter -> MiniSplit.....

If you use a water tank you are still going to have to buy batteries to store the energy over night.

 

[acdoctor]

Average low temperature of -5 and record lows of -45 I don’t think mini split is a good fit.

 

[AndyRonLI]

So you should seriously consider making a super insulated house with as much passive solar as you can. One load that Solar Electric just does not do that well is heating. Thermal solar can capture as much as 70% of the energy from the sun as directly usable heat. It has its own infrastructure issues, and you will need PV to run pumps and controls, but way less money than PV, Battery, Inverters etc. In floor radiant is likely to be a good match as generally the target fluid temp is not as high as perimeter convection (radiators). Thermal mass and insulation are your friends. Store heat as heat, not electrons. My 2 cents

 

[newbostonconst]

Average low temperature of -5 and record lows of -45 I don’t think mini split is a good fit.

Still way more efficient then a electric water tank by at least a factor of 3..... COP of 4, 90% of the time....

For that small of a house it is a perfect fit....pick one that will heat at the temp they plan to be there for and good to go.

 

[acdoctor]

Mini split with low temperature kit is only rated to -5 just can’t defrost. Below +10 the COP goes down the tube. Meaning it still consumes the same watts but produces much less BTUs. Not a good fit unless you have a second heat source to use below 0 deg.

 

[newbostonconst]

Mini split with low temperature kit is only rated to -5 just can’t defrost. Below +10 the COP goes down the tube. Meaning it still consumes the same watts but produces much less BTUs. Not a good fit unless you have a second heat source to use below 0 deg.

If you buy the correct one as I said they can have a toaster coil in them and heat at the same efficiency as the hot water tank for the short time that is needed in very very low temps.....you will gain in the long run because them majority of the time the COP will be greater then one and likely in the 3-4 range.....you are trying to discourage this and I don't see your advantage you are trying to say.....A hot water tank has a COP of 1 that is the worse you can possibly get and you are trying to encourage the OP to do that....makes no sense....sorry, guess you can't admit you are wrong....it is not a huge house it is 480 sqft...

COP of 1 means for one unit of electricity you get 1 unit of heat
COP of 4 means for one unit of electricity you get 4 units of heat.....that means you get 3 units for free from the outside air....it is a win win....

 

[upnorthandpersonal]

Can someone answer me this - is a massive system required if I need to run an electric hot water tank (4500 watts) for 2-3 hours a day?

I have hydronic underfloor heating in my place, but it's wood fired. The reason is that I don't have sun in winter to make this feasible, even with a 10kW array.

 

[acdoctor]

I definitely am not encouraging to use resistance heat. But you obviously don’t install hvac for a living. Their is a difference in reading a product flyer and real world application. Here if we get days in a row that stay below 30 deg and get freezing rain sleet and snow. Mini splits with the base pan heater can’t keep up with defrost. When outdoor coil becomes plugged with ice capacity goes down. If you continue to run it the fan starts hitting the ice buildup and you have to shut it down. I had a service call on Saturday that was the exact situation.

 

[Roswell Bob]

Hi all, what I'm looking for here is feedback if I'm crazy to want to install a hydronic in floor heating system off grid.

My wife and I are starting the building plans for a 480 sq-ft off-grid cottage that we plan to use in the winter and summer. We are located in Winnipeg, Canada and the cottage will be a few hours drive away to the east (close to Kenora)

I'm in the process of building up the power demands as we will be off-grid solar. I'm pretty new to this.

I was thinking we would use a standard hot water tank (4500w) and recirculation pump (80w) connected to a thermostat to cycle the solution through the floor.

What I need to know and can't find through the internet, is how long per day will the HWT and circ pump be running? I know that question is dependant on many variables including size of cottage (480 sqft) and insulation values (R30 walls) so I'm just hoping for some directional advice - is this a realistic load to put on a off-grid solar system?

I appreciate any advice or suggestions!
Thanks, Chris

You are not crazy - so far. I heat a 1200 sq-ft slab directly on the ground with zero insulation under slab and about 4" around the outside perimeter. I am in Southern New Hampshire. I run 80,000 BTU 80gallon Rheem LP water heater about 2-3 hours a day at about 1 gallon of LP per hour.

Your thinking to use water heater for hydronic heat is right on. It's very popular.

I have a slab on the second floor that I will heat with my PV array. The slab is about 600sq-ft. My array leaves me with about 14kWh of surplus power on a sunny day. The slab itself will store quite a bit of energy so no real need for batteries to store. I have a 12kW backup generator that I will use to charge battery array and heat floor on cloudy days. That will load up generator very nicely.

As for your situation I would advise you to get the slab insulated better than mine. If you can get the slab off the ground you will be better. If you are building with proper insulation in walls and ceiling then you should be in very good shape with the 4500W heater.

I wouldn't worry too much about storing a lot of power in batteries or water as the concrete slab will store quite a bit. I run my heater in the morning and again afternoon. Going off-grid will require an alternate method to heat. Firewood boiler or standby generator. You will see days without enough sun to heat the place I expect.

 

[400bird]

There was discussion of heat pumps. I agree an air source heat pump would be a bad choice at -45.

But, a ground source heat pump doesn't care about ambient temp. Sink a deep enough hole for the piping and you can have that 3.0 cop year round.

 

[newbostonconst]

From the way I read the OP they want to go to this cabin periodically. Would think they aren't planning on going in the worst weather. They also seem to be on a tight budget by the size being built.... trying to get something that will work with what they have stated.

They also don't have the space for a lot of mechanical equipment in their plan. A pump and dump ground source heat pump would work off a well but then they would likely have to winterize it when they leave in the winter without someone there to monitor. Other options like a geothermal well are double/triple the cost.

If things go bad and they are not there a minisplit just sits and can be powered right up latter. If the place freezes while they are gone for some reason they have to thaw it out to get to be able to flow water.

I would plan on a wood burner as backup and supplemental heat. There is also the option of turning on the stove to help heat...

If this were a 100 percent live in house there is no way I would do off grid solar in Canada.....not feasible....not very feasible in Michigan either.

 

[blackfly]

I appreciate everyone's replies.

The cost of going on grid where I want to be is close to $30k, so that ain't gonna happen. This will be a weekend place and we do plan on being there during coldest/darkest part of winter, but you're right if there is a blizzard we'll stay put at home so it will be unpredictable, meaning the place will likely need to maintain itself for weeks at a time. When we retire in 10-15 yrs we will expand the place and move there full time.

I suppose where I'm likely headed (for now) is a solar supplemented propane powered heating system to keep it above freezing. This will be probably be a heated floor. In the brighter but cooler months (Oct/Feb/March) I will try to use more solar and less fuel. I will also have a wood fired stove to heat the place up quickly for when we are there.

 

[blackfly]

You are not crazy - so far. I heat a 1200 sq-ft slab directly on the ground with zero insulation under slab and about 4" around the outside perimeter. I am in Southern New Hampshire. I run 80,000 BTU 80gallon Rheem LP water heater about 2-3 hours a day at about 1 gallon of LP per hour.

Your thinking to use water heater for hydronic heat is right on. It's very popular.

I have a slab on the second floor that I will heat with my PV array. The slab is about 600sq-ft. My array leaves me with about 14kWh of surplus power on a sunny day. The slab itself will store quite a bit of energy so no real need for batteries to store. I have a 12kW backup generator that I will use to charge battery array and heat floor on cloudy days. That will load up generator very nicely.

As for your situation I would advise you to get the slab insulated better than mine. If you can get the slab off the ground you will be better. If you are building with proper insulation in walls and ceiling then you should be in very good shape with the 4500W heater.

I wouldn't worry too much about storing a lot of power in batteries or water as the concrete slab will store quite a bit. I run my heater in the morning and again afternoon. Going off-grid will require an alternate method to heat. Firewood boiler or standby generator. You will see days without enough sun to heat the place I expect.

Thank you. This is very helpful.

 

[rogerius]

Do your homework before count on using solar. Check how many hours of sun do you have in your area. I'm located in Ontario and unfortunately not too much sun during the winter but a lot of snow.
I have a water heater tank heated by the PVs as a secondary source, connected to my in floor radiant system but is a challenge to get any warm water. I was counting and we had just 8hours of sun in the last 2 weeks.

 

[12VoltInstalls]

Thermal solar can capture as much as 70% of the energy from the sun as directly usable heat.

A problem is that the windows that allow passive heat gain get rid of it overnight and on cloudy days.

suppose where I'm likely headed (for now) is a solar suppose propane powered heating system to keep it above freezing. This will be probably be a heated floor. In the brighter but cooler months (Oct/Feb/March) I will try to use more solar and less fuel. I will also have a wood fired stove to heat the place up quickly for when we are there.

That makes sense. Solar electricity in winter is sketch to depend on.

The cost of going on grid where I want to be is close to $30k,

And solar in winter ain’t gonna be cheap provisioning for, either.