Design of Radiant Floor Heat - Solar Option?

mjp24coho · January 2015

I'm working on some plans for adding an additional cabin to my offgrid property, and am considering doing hydroponic radiant floor heating. I'd like to get some thoughts on pros and cons, as well as answers to a few questions. A bit of background first. Cabin will be three levels: full basement and main floor, with a loft; 3000 sq feet total. It will be all solar, with backup generator and propane for water heat and stove, and wood stoves for main heating. It's a weekend use cabin, plus weeks at a time thought the year, but I hope to eventually turn it into a retirement home down the road. It freezes in the winter, so I'd like to use the radiant floor heating to keep the temperature above 40 degrees on all three floors when we're not there. Here are my thoughts/questions:

1. My initial thought was to use a closed loop system, with a separate loop on each floor (one zone per floor). I'd like to use a water/glycol mix, to prevent freezing in case the system ever went down in the winter. Is that common? Any risks/downsides with closed loops? Minimal maintenance and simplicity are key to me, since it will be operating mostly when I'm not there.

2. I'd like to heat the fluid in the loops with a tankless on-demand heater. Do those work with a water/glycol mixture?

3. I would then use three low-watt circulation pumps (Grundfos?)

4. I live in the high deserts of Southern Utah with plentifully sun. Would it work to send the fluid (after return, but before the propane heater) up onto the roof through black solar tubes to preheat before entering the propane heater (no preheating tank, etc). I assume the more sophisticated on-demand water heaters (Rinnai etc) can sense the incoming temperature and reduce the amount of heat given, thus saving propane?

5. What are the cons/risks to the setup in number 4 above? In thinking about it, it would only be using solar to heat the water for a few hours a day during direct sun. Would that benefit be outweighed by the other 18-20 hrs a day when it is cold and no sun, thus chilling the water going up on the roof and being exposed to those colder temps than in the house? I'm wrestling with whether it would be more simple (and more efficient) to use only the tankless water heater, and not send it up on the roof for solar heating.

Thanks for your input-
Mike

westbranch · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

Well our in-progress build is very similar to your place size wise, 3 floors ~ 1000ft ². We have hydronic tubes in the concrete basement floor only.
It will be a 2 loop system, #1 for hydronic heating (winter), #2 for DHW in summer.
I want to keep the basement cool in summer.
We are at 3000 ft and it 'cooks' in summer. Daytime breezes are 'hot'.
We have to work at keeping the interior cool, keep doors and windows closed, etc.

My position is redundancy is your friend when off grid.

If it freezes where you are, you must protect with some type of antifreeze solution. Just make sure the heater unit will accept it. Our present house uses an instantaneous heater but it does not accept glycol solutions...

ADD: Depending on the hardness of your water this can be a bad thing!

I have evacuated tubes ready to install, far more efficient than the black plastic ones. Others will argue the reverse, but when you get minimum hours of sunlight, efficiency matters... also matters when you don't have space for more collectors...

hth

BB. · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

Here are some hot water/domestic heating links. A bit older, but still seem to be good information:

BB. wrote: »

If you want a do-it-yourself kit... This one appears to be hard to beat:

www.solarroofs.com

Solar Guppy has many years experience with a system from them and has been pretty happy. It does require proper maintenance to keep running well and to prevent problems (like freeze damage).

There have been a few threads here that link back to several extensive home projects--right down to installation photos, and documentation of mistakes and corrections...

Link 1
Link 2

Fpllow the off-forum links. The several projects/websites highlighted are very educational.

This is an interesting project for a solar heated shed and a control the guy now sells to support a pure DC off-grid solar heating system.

And this one is a bit more low tech home made heating system. Also very interesting and informative.

Between the two above links, they probably give the best detailed explanations of how to do a major home heating/domestic hot water project that I have seen.

The second one is, by itself, probably not practical for a city home system--but both give great ideas of the scope of such projects.

-Bill

Dave Angelini · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

mjp24coho wrote: »

I'm working on some plans for adding an additional cabin to my offgrid property, and am considering doing hydroponic radiant floor heating. I'd like to get some thoughts on pros and cons, as well as answers to a few questions. A bit of background first. Cabin will be three levels: full basement and main floor, with a loft; 3000 sq feet total. It will be all solar, with backup generator and propane for water heat and stove, and wood stoves for main heating. It's a weekend use cabin, plus weeks at a time thought the year, but I hope to eventually turn it into a retirement home down the road. It freezes in the winter, so I'd like to use the radiant floor heating to keep the temperature above 40 degrees on all three floors when we're not there. Here are my thoughts/questions:

1. My initial thought was to use a closed loop system, with a separate loop on each floor (one zone per floor). I'd like to use a water/glycol mix, to prevent freezing in case the system ever went down in the winter. Is that common? Any risks/downsides with closed loops? Minimal maintenance and simplicity are key to me, since it will be operating mostly when I'm not there.

2. I'd like to heat the fluid in the loops with a tankless on-demand heater. Do those work with a water/glycol mixture?

3. I would then use three low-watt circulation pumps (Grundfos?)

4. I live in the high deserts of Southern Utah with plentifully sun. Would it work to send the fluid (after return, but before the propane heater) up onto the roof through black solar tubes to preheat before entering the propane heater (no preheating tank, etc). I assume the more sophisticated on-demand water heaters (Rinnai etc) can sense the incoming temperature and reduce the amount of heat given, thus saving propane?

5. What are the cons/risks to the setup in number 4 above? In thinking about it, it would only be using solar to heat the water for a few hours a day during direct sun. Would that benefit be outweighed by the other 18-20 hrs a day when it is cold and no sun, thus chilling the water going up on the roof and being exposed to those colder temps than in the house? I'm wrestling with whether it would be more simple (and more efficient) to use only the tankless water heater, and not send it up on the roof for solar heating.

Thanks for your input-
Mike

You really need a budget to answer. The other thing is to do this right, you need a tank. Solar Pre-heating a tankless in your climate would be more of a hobby project than something you could live off in good winter solar weather. Good Luck!

Graham Parkinson · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

Hi Mike

1) All sounds good. Closed loop systems can have issues with glycol overheating in summer and degrading. Maybe they would work if you provided collector shading in summer?

2) Heating glycol/water mixes in tankless might cause the same glycol degradation issues. The concept might work better with a heat exchanger system (gylcol loop in coil within the top of a stratifying tank water heater setup). However in Utah you might have sub-freezing temps in the house if not heated all winter.

3) There are many European water circulating systems / controls that have gotten quite sophisticated that are low power - I have lurked on several radiant heating forums, will try and dredge up info / contacts for you.

4) I believe that the better tankless heaters modulate, however the cheaper ones just manually throttle flow to achieve set temperature rise.

Have you looked into the "Warmboard" in floor system and other similar products? Warmboard consists of a grooved plywood panel with an aluminum heat transfer plate pressed into the grooves. Although somewhat costly it allows using "low grade" (ie lower temperature) solar hot water for heating compared to the lower efficiency under floor staple up systems etc. The low operating temperature required with Warmboard also greatly improves efficiency of heat transfer into your house when used with solar collectors.

On our building site we have a south facing black rock bluff below our cabin - it really soaks up the heat in the sunshine. I've been trying to figure out if our budget can be stretched to incorporate a Warmboard system with vacuum solar collectors on the bluff below floor level with and convective assisted glycol circulation ..... just enough heat to keep a vacation place a few degrees above ambient all the time to keep everything dry inside would be great. It should be possible to build a homebrew system with all the performance of Warmboard by placing PEX loops on the subfloor between spacer boards of same thickness as the PEX, overlaying with a flat Al sheet to spread the heat and laying flooring directly over all this (carefully to avoid perforating PEX).

PS - How do you like your Magnum system? I'm trying to decide between an Outback system: VFX3648 or Radian 4048a (Outback- seemingly better supported, more reliable? / vs Magnasine: MS4448PAE - better feature list, better gen support, but no users forum, not quite as bullet proof build quality?) Can you PM on any advice to avoid thread grabbing?

- Cheers, Graham

gww1 · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

Graham p
I really like my outback but have never used anything else. I would say that the outback forum is dead much of the time. It has some knowladgable posters but not enough to garrantee good answers. I have had good luck with there tech line though and that is with 9 year old inverters and no warrentee. The fx series also due to not many changes through its history is easy though not cheep to get parts for if you have issues.
cheers
gww

jonr · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

I would put my effort into making it safe for the place to freeze and not worry about the heating system failing. For example, an air compressor and fittings to make blowing out the water pipes quick and easy.

If you want radiant floor heat, consider a low temp, air to water heat pump plus a storage tank (so it doesn't make much use of batteries).

icarus · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

There are a number of hot water heating heat pumps that are very effective for hydronic systems. With in floor radiant, the design temp is very much lower than that of conventional radiant, in e neighborhood of ~100f, ideal for heqt pump. Consider solar collectors for the hydronic (Pre heat) and then top up if needed with a heat pump. The Daiken Altherma units come highly praised.

http://www.daikinac.com/content/assets/DOC/ALTH-ENWP11-03%20-%20Daikin%20Altherma%20Overview%20Paper.pdf

Rinnai makes a series of Combi boilers that do both DHW and hydronic heating. You can dial down the output temp to suit the proper heqting regimen. They are also condensing units, with out door resets so they are very efficient. I just instqlled on in a grid tie house and it is magnificent. A conventional can be used to supplimental Pre heated loops, as you can adjust the output temp of the water heater, but they are not as efficient as the combi boilers (96.5%)

http://www.rinnai.us/boiler/products/e-series

I would contact a local hydronic designer/installer. There are many pitfalls with hydronic, getting the advice of someone who is al familiar with both your local climate, but also the solar aspects of it as well.

Tony

PS, there are a number Of pitfalls to be aware of with hydronic systems, most a good installer can watch for, including ensuring O2 is kept out of the system, using the right PEX tubing, the proper air venting regimen, proper and improp use of iron pipe, fittings and pumps etc, as well as using components that are properly sized for the loads. A mistake that many folks make is designing too big a system for the load, such that it short cycles or runs inefficiently.

t

mjp24coho · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

All - thanks for all the input. This type of feedback is exactly what I was looking for. I definitely plan on utilizing a local radiant floor heating installer to benefit from their expertise. Graham - I'll send you a PM with my feedback on my Magnum system, which is great.

BB. · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

westbranch wrote: »

I have evacuated tubes ready to install, far more efficient than the black plastic ones. Others will argue the reverse, but when you get minimum hours of sunlight, efficiency matters... also matters when you don't have space for more collectors...

Just some warnings... Evacuated tubes are fragile--So be careful (and of falling objects too like pine cones, blowing debris, etc.).

They run Hot, especially in summer (duh...)--So watch the temperatures in the hot side of the panel/piping. May have issues with breakdown of anti-freeze and choices of types of antifreeze too.

Lastly, interestingly, evacuated tube collectors do not shed/melt snow nearly as well as various flat plat collectors (yep, they "leak" more heat, and can self clear of snow better). If you can, make sure you have a good way of keeping snow off the collectors in winter (--of course, what do I know of "winter" living near San Francisco California:roll:).

-Bill "My two Cents" B.

westbranch · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

Good point Bill, about shedding snow. Most (all) pics of installs I have seen North of the ''49 th'' have been vertical, on the side of a structure...:cool:

BB. · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

Vertical mount may be the best way to go... Get the most sun in the winter, and much less in the summer (when solar thermal can overheat).

Using PV Watts for Prince George, BC, 90 degree vertical mount:

Month

Solar Radiation
(kWh/m²/day)

1
1.59

2
2.69

3
3.22

4
3.84

5
3.24

6
3.18

7
3.11

8
3.47

9
3.36

10
2.68

11
1.68

12
1.29

Year
2.78

Vs a 53 degree mount:

Month

Solar Radiation
(kWh/m²/day)

1
1.55

2
2.74

3
3.78

4
5.16

5
4.94

6
5.35

7
5.02

8
5.11

9
4.23

10
2.97

11
1.69

12
1.26

Year
3.65

Don't know what would be best... Not much winter improvement, but "lots" more summer sun.

A shallow summer tilt (something like 30 degrees), gives a lot of sun in the summer months (if you need a lot of hot water).

-Bill

inetdog · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

Note that PVWatts does not (AFAIK) take into account reflected or scattered sunlight off snow cover or water, so for some folks the winter output will actually be higher than the PVWatts value.

BB. · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

True Inetdog... As far as I know, it does not model ground reflections.

-Bill

Organic Farmer · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

We have Radiant Heat in our flooring and we love it.

We heat with a woodstove. We have two basic loops. One loop circulates water through our woodstove and to a thermal-bank. The second loop circulates water through the thermal-bank and to the floor loop.

Two loops, two pumps controlled by two switches. No computer.

Both of our loops are closed loops, we only use water in our loops.

We plan to add a Solar-Thermal array in a year or two. That will be a third loop. That loop will likely be a water/glycol mix.

We are doing it this way, to keep things as simple as possible. If something fails, you know right where to look.

This is a very cheap system to install.

techntrek · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

Here is a great system if you want to see what someone else has done - PV and thermal (domestic HW and radiant floor). In Maine!

http://www.solarhouse.com/

fowlerrudi · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

Hi,
I'm heating 5200 sqft with most of it warmboard. 100% Off grid in a log home in New Brunswick, Canada with a 4.6Kw Xantrex 6048, a wood boiler as well as a propane boiler. I run grundfos alphas. No glycol all water - 8 zones. I am working on 1000 gallons of pressurized water storage and also use an air to water heat pump as an 'opportunity load' for surplus solar power. Still a work in progress of course - many refinements. It is -15C right now with 60 km/hr winds and I'm warm. Also have one wood stove. Probably a little more complicated than you are looking for - however if I can be of any assistance - give me a call: 506-261-3535. -Rudi

jtdiesel65 · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

Off grid with Rehau radiant. It's a walk in basement with one story. The basement has Rauboard over concrete and the first floor has staple up Rauplate. Boiler is a propane Bosch combi (heat and domestic) on demand. We have two wood stoves and we use one almost exclusively. The heat was put in so we can leave the house and for when we're old and can't manage wood. We've considered adding an outdoor wood boiler and solar water with indoor heat exchanger (a tank). Currently don't use glycol, but I've been thinking about adding it. We have the newer grundfos adaptive pumps. They use very little juice.

The thing with on demand boilers from the UK is that some are designed to be the boiler and recirculator for a single zone. This is the way the Bosch is designed. In the US, these boilers don't get plumbed like that. The boiler is used with a zone control with recirculator pumps and with a manifold to control flow to zones. Unfortunately in the case of Bosch, the engineers don't recognize this use case. In the Bosch units, they hardcoded a temperature setpoint range for feeze protection that doesn't go below something like 38 degreees F, which means the boiler's recirculator will run if outside temp is 38 or less. The idea being that the unit is hooked up to a single zone and when the recirculator comes on, it will pump warm water in the piping to prevent the pipes from freezing. However when using external zone controls, all this does is waste electricity pumping water that doesn't go into the piping unless a particular zone calls for heat. And if a zone is calling for heat, the external recirculators would be on anyway. You can't shut this off on the Bosch units. In winter, the unit's recirculator will run constantly even when there is no call for heat. We worked around this by installing a three way switch and adding another temp sensor inside the building. The temp sensor is used to regulate the condensing boiler operation to improve efficiency. However, a condensing boiler only operates in condensing mode when supplying heat. It's just a regular boiler when there is a call for domestic hot water. So when heat is off, we switch to the internal sensor and when heat is on, we switch to the external sensor. Not ideal, but Bosch documents didn't say anything with regard to this feature. Idle consumption is advertised at 6w, but that is only true if the temp is above 38 degrees. So in a nutshell, watch out for things like this. I think you can shut this off on the THI units.

Anyway despite that problem, heat and DHW work well. The system is low mass, so it doesn't take long to get some heat versus in-slab installations. Looks like about 100w usage when DHW is on.

BTW, we also have central air. I had the installer put an intake above the wood stove in the basement. In winter with the wood stove running, we can turn on the exchanger's fan and circulate warm air throughout the house. I didnt put in a humidifier as an addon to the exchanger, but I've been thinking about doing that.

jtdiesel65 · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

BTW, my uncle installs heat systems (unfortunately in a different state than I live) and I asked him about different wall units back when I was planning. Here are his answers to a couple of my questions. Maybe it will help you too.

Decisions, decisions, decisions... One can get bogged down with all the propaganda the manufactures put out. The truth is they all are vying for the same market shares, so it's a matter of one-up-man ship, one coping another and another. The end result; not much difference in any of them.

The only real difference is the product's popularity, is there more than one supply house carrying the unit, are there many companies in your area selling them? The most important aspect of the whole deal is the company (man) doing the installation . Are they qualified to: 1) install it properly? 2) are the certified to service it? (Most manufactures offer qualifying seminars, some require it)
Here's how the three stack up with each other:

• NTI Trinity TI: uses the most popular heat exchanger in the world today (tried and true technology, nothing special about them)
• Bosch Greenstar Combi: The oldest boiler manufacture in the world (they require certification of the servicemen to keep warranty intact)
• Rinnai: Bestselling unit, cost more, easy to install, but pop the front cover off and most servicemen are in tears... How do they pack so much stuff in there? You really need to be certified to work on this unit. (but they have a great technical help line, I've had trouble with their heat exchangers, had the parts the next day...)

The issue of the DHW; they all use a flat plate heat exchangers and will perform just fine for your needs (even two shower heads). However, the problem comes with the minerals in your water supply... Got soft water and there will be no problems. Got hard water and you'll wish you never heard of a flat plate heat exchanger (only cure is to replace it (not a big deal)). An indirect fired DHW tank can be added to any system at any time and they can be remote from the boiler too.

If I were to be buying a boiler with today's fuel cost, I'd want the highest efficiency I could get. Have a look at the HTP Elite Plus and its companion SuperStor Ultra DHW tank. This system delivers a consistent 98% efficiency.

I'd stay away from creating extra zones, it will work against you in the long run. Radiant works best set at one constant temperature and constant circulation. Cycling it defeats the design purpose and produces undesirable lag in recovery. If you're planning on deep cycling (turn it off when you leave and back on when come home) expect long delays in getting sensible heat (2 hrs. or more) and costly to operate. Do not use energy saving thermostats (set back style), let the boiler's weather responsive control feature do the controlling of the system.

All new gas boilers are just about silent, just a slight hum of the motors.

techntrek · January 2015

Re: Design of Radiant Floor Heat - Solar Option?

jtdiesel65 wrote: »

I didnt put in a humidifier as an addon to the exchanger, but I've been thinking about doing that.

You may not want to. Adding humidity will make it feel colder. We discovered this with our own wood stove when I got lazy one year and decided not to put a pot of water on it like I did for years. We are now comfortable at temps 5-8 degrees lower than before.

Design of Radiant Floor Heat - Solar Option?

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