Wood Boiler - Radiant Floor Heating Questions

mjp24coho
mjp24coho Solar Expert Posts: 104 ✭✭✭
I'm exploring doing radiant floor heating w/ a wood boiler for an off-grid cabin. I've been reading a lot of the posts here to get as much information here, to decide whether it is feasible. If any of you have experience, I have a few specific questions I'm hoping to get answered. A bit of background first: I'm looking at using three zones - basement, main floor, and top floor. The basement will be conrete floor with carpet, while the main floor will be a wood floor and the top floor will be carpet. I'll probably have the basement be one zone, the main floor split into 2 zones, and the upstairs one zone (4 zones in total). I'll use a large outdoor wood boiler (possibly with LP option to start the wood) ~ 20' from the cabin.

Now for my questions:

1. I've heard a lot about the benefits of the Grundfos Alpha pump - can this system be configured with only one pump (and pressure valves for each zone, controlled by thermostats), or do I need a pump at the boiler, and one for each zone?

2. I assume the boiler draws some electricity (to run it's own thermostat?) - anyone have an idea as to the wattage consumption for a typical wood boiler? I've looked on the websites of a few of the major manufacturers, but couldn't find any details (and emails to their tech support went unanswered).

3. In the winter, the cabin may go a few weeks without being occupied, and may get down to 50 degrees inside while I'm not there. How slow is radiant floor heating? I'm trying to figure out how long it will take to bring the cabin up to 70 degrees (I assume each floor would be different). 3-6 hours would be one thing, but 12-24 hrs. would be a problem.

As you all know, every watt counts, so I'm trying to get all the details.

Thanks!

Comments

  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Wood Boiler - Radiant Floor Heating Questions

    Don't have in floor heat myself, but friends do.
    One circulator pump will do the job. Zone valves (roughly 12 watts each when on) control which zone gets flow. One zone valve for each zone. Circulator only runs when one or more valve is open. Infloor heat uses only warm, NOT hot water in the infloor piping, so usually some of the cooler return water is mixed with water from the "boiler" (that should never boil, if it does, you'd better head for the hills) There will also have to be an extra zone and control valve, refered to as the "dump" zone, and it's purpose is to dump heat if the wood fire is supplying more heat than the infloor can use, to keep the "boiler" from boiling.
    Not too sure about infloor heating with wooden floors, the heat tends to dry it out terribly, and cause it to warp, crack and split.
    Speed of heating? It's VERY slow compared to normal hot water baseboard, hot air furnaces, or wood stoves. 10 to 20 hours is common, so usually the thermostats are not turned back at night, it just takes too long to warn back up.
    To be honest, when you are in a position where every watt counts, I wouldn't go this way. Due to the long heat recovery time, the zone valves and circulator will be putting in long hours + overtime with infloor heat.
    Also the top floor with carpet will be hard to heat. the carpet will be an insulator, holding back the transfer of heat to the room, so the circulator will keep on pumping, trying to get heat through the carper and draining your batteries in the process.
  • wrdaigle
    wrdaigle Solar Expert Posts: 65 ✭✭✭✭
    Re: Wood Boiler - Radiant Floor Heating Questions

    I just installed an Alpha in our off-grid home and I couldn't be happier.

    http://bluecloudpowerco.blogspot.com/2012/01/toasty-warm-feet-with-energy-to-spare.html

    It is certainly possible to run a system off a single pump and maintain multiple zones. You will need to use zone valves instead of individual circulating pumps. Whether or not it will work for your particular situation will depend on your flow requirements. You need to calculate the volume of water you need to move and the pressure (i.e. head) that the pump will need to push against and double check to make sure it fits within the Alpha's specifications. If not, Grundfos does make higher flow variable speed pumps, as does Wilos. If energy consumption is your main concern, you just need to figure out what your flow requirements are and decide which single pump or combination of pumps will get you the most bang for the watt. In my case, I could have also gone with 3 Ivan Labs El-Sid pumps (one on the boiler and one for each zone). This would have used about the same amount of electricity and would have just barely met my flow requirements. I went with the Alpha because it was significantly cheaper than 3 El-Sids and met my flow requirements with room to spare.

    If you do go with zone valves, make sure do your research and find a reliable model. From my understanding, zone valves can be the weak link in the system. For simplicity sake, I ended up eliminating my zones, so I don't have any suggestions.

    I don't have an outdoor wood boiler, so I can't speak to speak to its electricity consumption outside of the water pump, but I assume it would be pretty low. It will probably have an electronic damper and possibly a fan. The thermostat itself should be negligible.

    Your third question is a tough one because it depends on so many factors. It all about heat transfer rate. Radiant heat is not fast and is not really designed to be. Since floors tend to have a lot of thermal mass, they warm up slowly. The benefit is, they also hold that heat for a long time so they cool down slowly as well. If you want quick heat, install baseboard heaters or use some other method of heat transfer. All of the following factors can greatly affect the heat transfer rate in a radiant system:
    • In-concrete versus staple up
    • Use of heat transfer plates in staple up
    • Water temperature
    • Water velocity
    • Water vs. Glycol
    • Boiler Capacity

    ...and a whole host of other factors that I'm probably not considering.

    That being said, I have relatively poor heat transfer (staple-up PEX without heat transfer plates) and a lot of mass in my floor. If I turn my boiler on, the thermostat doesn't move at all for about 2 hours. After the first 2 hours, the temp goes up about 2-3 degrees per hour.

    Cheers,
    Bill
  • mjp24coho
    mjp24coho Solar Expert Posts: 104 ✭✭✭
    Re: Wood Boiler - Radiant Floor Heating Questions

    Thanks - those are all good points to consider. If this were the Best heating option, I could always do wood floors throughout instead of carpet. I've heard varying opinions on whether a closed loop radiant floor system would be harmful to wood floors or not.

    This got me thinking of a potential way to configure this. I wonder if it would be possible to have a dual heating option - the outdoor wood boiler for when I'm there, and an indoor Rinnai tankless water heater to heat the water when I'm not there, to keep the temperature in the cabin from dropping below 50 degrees. Any thoughts on the feasability of that option? Could those two heating sources be plumbed in parallel? I assume the thermostat could be set to run the pump, only pumping when the temperature is below 50 degrees inside, and the pump turning on would trigger the Rinnai to turn on? Then when I'm there and heating with the wood boiler, the water running through the Rinnai would already be hot enough so the Rinnai wouldn't activate to try to heat it?

    Another question - how do the outdoor wood boilers regulate water temperature. For instance, how do they regulate the water so it doesn't get too hot when the boiler is running, but the thermostat has turned the pump off?
  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Wood Boiler - Radiant Floor Heating Questions

    Re the water heater for infloor heat, I know one guy from Germany who set up several cabins for rent here in Nova Scotia, and for wintertime use, he used a 20 gal domestic electric water heater to supply the infloor heat, one for each cabin. Of course the hot water was mixed down to a cooler temp before being sent to the floor coils. The cabins were about 20X20 feet, and the water heaters if I remember right had just one element, rated 1500 watts, which is why I argued with him that it wouldn't supply enough heat. He informed me that it's done that way in Europe and with his experience with it, it would indeed work. I assume it did, but never did have a chance to verify that.
    Re maintaining outdoor furnace water temperature, when the water is hot enough, the draft fan, if the unit has one, is shut down, and the draft entry door is closed, to smother down the fire. There are times however, when if there's been a good hot fire, and the circulator etc shuts down, closing the draft etc will not cool the fire in time, and the water will over heat, thus the necessity of a dump zone to get rid of that extra heat for safety reasons.
    If your off grid power supply is limited, you should definitely confirm the consumption of any draft fans, zone valves, circulators etc. While their combined consumption may be considered nil for anyone living on grid, it could be shockingly high for someone living off grid. You may need power for things other than just the furnace/heating system.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Wood Boiler - Radiant Floor Heating Questions
    Re the water heater for infloor heat, I know one guy from Germany who set up several cabins for rent here in Nova Scotia, and for wintertime use, he used a 20 gal domestic electric water heater to supply the infloor heat, one for each cabin. Of course the hot water was mixed down to a cooler temp before being sent to the floor coils. The cabins were about 20X20 feet, and the water heaters if I remember right had just one element, rated 1500 watts, which is why I argued with him that it wouldn't supply enough heat. He informed me that it's done that way in Europe and with his experience with it, it would indeed work. I assume it did, but never did have a chance to verify that.

    I just have to comment on this. ;)
    If you dig in to the specifications on water heaters, 90% of them say somewhere in the literature that they specifically are not to be used in this manner. Frankly, a 1500 Watt radiant heater would have been a lot cheaper, easier to install, and more efficient. For that big a room in Canada, it would also be working almost all the time. Normally we'd have 3kW of heater (or more) in a room that size, even with good insulation.

    I guess Europe has to maintain its reputation for bloody cold bedrooms. ;)
  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Wood Boiler - Radiant Floor Heating Questions

    Totally agree with you Coot! :D
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    Re: Wood Boiler - Radiant Floor Heating Questions

    Wayne. read about lowering the temp from waterheater before it went to heating coils. When I installed my woodheater boiler they told me to set the water temp to 180f. so there would be good heat transfer thru heat exchanger. I know of a couple of people that use waterheaters for homeheating but never asked them about the results. :Dsolarvic:D
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Wood Boiler - Radiant Floor Heating Questions

    I have built a couple units out if 10 gallon hot water heaters that performed well. I add a small 1/35 hp circulator pump and build a "A" coil out of a couple copper Transmission coolers ( 8 x 12 ) and put them on a manifold in the output duct. I used a 50/50 antifreeze mix as the fluid medium . I left the thermostat and overload breaker in place, but used a relay to override it so it starts heating as soon as the heat pump comes on. I set the thermostat on the water heater at 140 deg's and that's about it. I left it as a open loop so pressure is not a issue and it remains fail safe by removing the pop off valve.

    It's amazing how well they work. The antifreeze is 140 deg coming out and the return fluid is 95 deg or so. I have never saw the heater element run a few minutes at the end of a heating cycle. The trade off is the 1,500 element, not energy efficient.

    These units were used on Boats and the antifreeze was used so the units would not freeze during shutdown. The 10 gallon size was used because of space constraints. I would think a larger one would give more reserve capacity. They were used in conjunction with small Water to Air Heat pumps once the output air dropped below 95 or so degrees. You can see I was getting a 40 degree rise in air temps from the in/out fluid temperatures.