pbrane
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| 09/29/2009 8:33 AM |
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| I have to make heating decisions soon on my small ICF house. The builder is pushing me toward gas forced air to keep within tight budget. I was thinking I wanted radiant under-floor heat, although I've never had any experience living with it. I thought that I could do some of the installation to save money and come in within budget.
My heat load calc's are in the 10k to 12k btu's per hour, so I don't need much, either way. And I'm not all that interested in air conditioning, but then, I may need HRV (so may need the vents anyway).
I also wonder about efficiency in the sense of not being able to turn off the heat when you leave the house...like I do now with a regular furnace. Seems like with radiant, you'd just leave it on all the time because of the delay...
Any comments or experiences with radiant appreciated!
Thanks,
-michael |
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jonr
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| 09/29/2009 9:09 AM |
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| Floor radiant has a very nice feel to it. True, it creates a slower reacting, higher mass system that reduces the benefits of turning down the heat at night and when not at home. |
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Blueridge company
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| 09/29/2009 10:24 AM |
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I have in floor radiant, install it and sell systems. Would not build a house with out it. Your heat load is light, you could run it of a TACO x block attached to a 50 gallon water heater. isolated system. as to the HRV, these are a separate issue, easy install. In our home we have programable t state, set back in the night 10-6 am 5-8 degrees, and in the mid day same. house recovers in about an hour.
If you are lab on grade insulate under w/ 2 inch rigid.
Good luck,
Dan |
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Dan
BlueRidgeCompany.com |
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Dana1
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| 09/29/2009 11:21 AM |
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With heat loads that low the floor temps you may not get that cushy "cruisin' in yer socks" appeal associated with homes of higher heat loads. High mass low temp radiation from radiant floors are still pretty comfortable, but so are low-temp Euro-panel radiators, which would likely be cheaper, more responsive to quick changes in heat load, and you could run THEM off your HW heater as well. But how small is "small", in "my small ICF house"? How many BTU per square foot of floor are we talking? With small enough floor area, the floor temps & cush-factor will be higher, and cost of installation modest.
The ducts associated with an HRV are pretty tiny, but would have to be configured differently to use in recirculation mode rather than strictly as outdoor air exchange ventilation. At peak heat loads that low you could probably get by with a 3.5kw resistance element for a "furnace", or hydronic coil off your HW heater. But panel radiators are much cushier than any hot air system- it's a decent option. For even less money you could go with fin-tube baseboard convectors (you can get 150-200BTU/ft out of fin-tube @ DHW temps), but cozying up next to a panel on the coldest morning of the year just plain feels better.
Odds are pretty good that your hot water heating will be a larger average load than space heating, and a combined system is likely to be cheaper to buy & operate than separate systems. Nighttime setbacks aren't going to buy you much with peak heat loads that low- there's probably sufficient thermal mass in the house that you'd lose less than 5 degrees overnight even on design-day, far less on most days, making even a 5F setback rather meaningless.
How are you heating hot water? To run the heating system off it you'll need ~3.5kw of output to match your peak heating load, which would be above the output of some electric tanks, but many are 4.5kw, which should work. Gas/propane fired tanks all have burner to spare, with 8kw+ of thermal output.
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pbrane
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| 09/29/2009 7:10 PM |
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Thanks to all who responded. Great thoughts.
I already have a hot water heater planned and is included in the plumber's quote. I could maybe get a slightly bigger one and add an external flat plate exchanger. But then what do i need for the heating system? I assume at least one pump, a manifold, etc. How simple can I make this? Can I control the "zones" by adjusting valves? That seems a bit crude. So would I need expensive contollers? I was already leaning on doing europanels or some other radiator, but might still do one or two rooms with under-floor radiant.
The heat load calc's are in my original post (10k - 12k btu's per hour) for an ICF house with 976 ft2 in main house, plus basement plus future bonus room above garage (total maybe 1450 ft2)...in upper midwest.
Thanks again!
-michael |
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cfoster
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| 10/01/2009 2:01 AM |
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We're two weeks into our place, and our radiant system itself is absolutely fantastic. (Just based on the somewhat chill days we've had.) A guy I work with made the terrible mistake of adding radiant to TWO rooms in his house (kitchen, bath). It was a 'mistake' because now he's aware that those two rooms are the most comfortable. When it was all forced air he acclimatized to how that 'feels', not realizing it wasn't as comfortable until he had the direct comparison.
Here's the most important thing I can tell you about radiant:
Be sure to have your concrete/gypcrete LEVELED by the guys who pour the floor. Our contractor said "No problem; It will self-level."
Well, no. It didn't. (Which is probably why the flooring company offers the option of leveling at an extra cost.)
Then we were faced with having to pour floor leveling compound everywhere but that didn't work as well as we had hoped (and it's expensive over such a large area -- much better to do it at the time). The floors are in and they aren't terrible or anything but I can feel the occasional dip and/or rise here and there.
If you're adding wood floor, you'll need to add "sleepers" (2x2s) to your floor before you pour to make sure you have something to nail the boards into (assuming they are nailed and not a floating floor). Make sure the sleepers don't touch the tubing. You want to make sure the flooring guys can put a nail into the sleeper and not worry about going through end or side of the sleeper and hitting a tube.
Since radiant heats you, and not the air so much (only 15% of the energy is wasted heating the air) you don't need radiant floors anywhere you aren't standing (e.g., under the stairs, a closet). And you'll want to make sure the tubes aren't 1' apart or you'll be aware of 'banding' across the floor. I think 9" apart is the maximum recommended distance. This allows overlap in the heat pattern.
Hmmm... I think that's about it for lessons we learned our first time out. Hope it helps!
-Colin.
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Dana1
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| 10/01/2009 3:56 PM |
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Posted By pbrane on 09/29/2009 7:10 PM
Thanks to all who responded. Great thoughts.
I already have a hot water heater planned and is included in the plumber's quote. I could maybe get a slightly bigger one and add an external flat plate exchanger. But then what do i need for the heating system? I assume at least one pump, a manifold, etc. How simple can I make this? Can I control the "zones" by adjusting valves? That seems a bit crude. So would I need expensive contollers? I was already leaning on doing europanels or some other radiator, but might still do one or two rooms with under-floor radiant.
The heat load calc's are in my original post (10k - 12k btu's per hour) for an ICF house with 976 ft2 in main house, plus basement plus future bonus room above garage (total maybe 1450 ft2)...in upper midwest.
Thanks again!
-michael
As for the water heater- it's the size of the burner, not the size of the tank that you'd have to verify. There are 60KBTU burners on 40 & 50 gallon tanks, and there are 35KBTU burners on 40 & 50 gallon tanks. Anything north of 45KBTU would likely work just fine. And yes, an external plate HX is a reasonable approach.
A single ECM pump with zone valves controlled by the zone thermostats with separate zone flows tweakable with ball valves would be about it. Any slab radiation would benefit from thermostats with PID algorithms to keep over/undershoots in check. If you're trying to run it all as a single zone, single thermostat and adjust the flow with ball valves, that can work (easier if it's a fairly open floor plan), but not if you have a mix of both high mass (slab) and low mass (panel/baseboard) radiation in different rooms. Separate thermostats controlling zone valves is probably better in most cases.
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dmaceld
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| 10/06/2009 4:45 PM |
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What about the efficiency of hot water heaters? When I was considering what to do for a heating source, and particularly what to do for a hot water heater, I came to the conclusion that the efficiency of water heaters stinks!
I went to a couple of water heater mfg sites and what I found showed that the overall energy efficiency of even direct vented high efficiency gas water heaters was at best about 70%. This must be because of the design of the tank, annulus design with heat pipe up the center. I concluded that an electric water heater with a water heater heat pump running at a COP of 2 was cheaper to run than a natural gas high efficiency water heater.
Did I miss something along the way?
Wouldn't a high efficiency boiler with a DHW loop be a better choice, or is this a case where the operating costs will be so low it's difficult to justify the additional capital expenditure?
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Building house - what a way to spend retirement! It's done! We're living in it! |
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jonr
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| 10/06/2009 6:06 PM |
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A water heater heat pump is taking heat from the inside air - great in the summer, not so good during the heating season.
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dmaceld
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| 10/06/2009 11:42 PM |
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Posted By jonr on 10/06/2009 6:06 PM
A water heater heat pump is taking heat from the inside air - great in the summer, not so good during the heating season.
That's right. Not so good in winter unless you have an efficient way to heat the air the HP is using for its heat source. I thought I was going to have that with my Daikin heat pump system w/ a 9000 Btuh wall unit in the garage until I figured out the overall efficiency w/ the 3 ton outdoor unit running (albeit at a slow speed) and only the 9000 Btuh indoor unit running was only about 100%. I'm considering ducting air from inside the house to the WH HP because the air inside the house is heated with a 3 ton indoor unit and the COP is 3+ even at outdoor temps down to 10°F.
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Building house - what a way to spend retirement! It's done! We're living in it! |
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NRT.Rob
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| 10/07/2009 7:49 AM |
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in a heating application, tank heaters run somewhere over 75% efficient typically. the regular ratings include standby losses and low level usage (DHW only) so the standby loss is a larger percentage of the overall usage. as you use more, standby loss diminishes as a percentage of overall use.
for very low loads, tank heaters are not bad choices. |
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-=Northeast Radiant Technology=-
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Dana1
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| 10/07/2009 3:14 PM |
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What Rob said- with very-low heating loads like that the difference in operating expense between a tank and a mod-con will pay off approximately never, in a net-present-value analysis, or 50-80 years after the anticipated lifetime of a mod-con in simple-return terms. With peak heat loads of ~12K, they'll likely use less than 250-300 therms/year for heat, maybe another 150-200 for hot water with a 75-80% efficient burner. How much capital are you going to plunk down to save $40-50/year
The EF rated efficiency is based on delivery of 62 gallons of hot water per day, and it's only measure is the heat delivered to the water, ignoring heat lost to the conditioned space, or heat wasted by taking combustion air from conditioned space. If the tank is located within conditioned space, a fraction of the standby loss ends up in the space too, lowering the heating load. A direct-vented version that doesn't draw combustion air from conditioned space also lowers the heat load since it isn't driving infiltration by stealing combustion air from conditioned space. The net heat delivered is way higher than an EF test measures. Conventional tanks are pretty much limited to raw combustion-efficiencies of ~80%. But since they're self-buffered, cycling losses are low. With a design day heat load of 12K, even the smallest ~35-37K ~80% atmospheric cast iron boilers are by-definition 3x oversized, and un-buffered would deliver only 65-70% as-used AFUE (and that's before discounting another 3-5% in net effectiveness if it isn't sealed combustion and thus driving infiltration rates.) You could bring it back to the mid-high 70s with a buffer tank or reverse-indirect tank/DHW heat-exchanger, but that too is a hunk o' change. A tank HW heater is not only cheaper, it can be as-efficient (or more) than middle of the road conventional goods.
Sure, a mod-con that goes down as low as 15KBTU/h might save you $50/year, but for the extra few grand you could enhance it with a drainwater heat exchanger &/or a bit of solar, pocketing ~3x the ROI. |
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dmaceld
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| 10/07/2009 4:01 PM |
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Good information Dana & Rob. Makes a lot of sense to me. Doesn't do me any good since I'm not in the boiler buying mode, but I'm sure others will profit.
BTW, we had our first cold snap of the season these past four days with outdoor temps dropping to low 30's. Put my Daikin air to air heat pump into heat mode. As you will recall, I'm using the crawl space for the supply plenum. The set point is 74°. The crawl space temp is staying around 78 - 79°. The floor, all hardwood and tile, is comfortable under foot. I haven't put a temp probe on it yet but it sure feels at this point that I have the quasi radiant heated floor I was hoping for!
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Building house - what a way to spend retirement! It's done! We're living in it! |
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jonr
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| 10/08/2009 9:02 AM |
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I believe in the PassiveHaus principle that HRV delivered to every room is best. So this means that air ducts will always be required. So air based radiant systems that use surfaces (floors, internal walls) as ducts make sense to me. But as discussed before, code doesn't like this.
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NRT.Rob
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| 10/08/2009 9:06 AM |
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with passivehouse, you don't use radiant at all, you just put a small air handler in your HRV ductwork to reheat the incoming air a little, if you need it.
in a regular house, an air system like that is not "radiant", and the CFMs required for indoor air would never deliver useful amounts of heat to the home. |
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-=Northeast Radiant Technology=-
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jonr
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| 10/08/2009 12:32 PM |
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Passivehouse (an energy usage standard) allows radiant (and lots of other things) - but most examples don't use it. The concept of heating the inhabitants vs the air is valid, although with the very low amounts of heat it can't be worth much.
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NRT.Rob
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| 10/08/2009 12:36 PM |
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it allows radiant sure, but it's not used because in a passivehouse radiant isn't of much benefit... you don't need enough heat to notice that it's radiant, so they just reheat the incoming HRV air to avoid that part of their load and let the rest of the house passively maintain temperature.
european passivehouses are primarily using this technique for whatever portion of their heat load they cannot meet passively. |
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-=Northeast Radiant Technology=-
NRTradiant.com |
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pbrane
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| 10/08/2009 9:03 PM |
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I'm also concerned with noisey pex as it expands and contracts. With new construction, is it better to set the tubing in gypcrete (or ?) on the upper floors, rather than do underfloor with transfer plates? I'm planning on doing full 3/4" hardwood. Which way is cheaper? I assume there'd be no noise if it's set in a slab....
Thanks!
-m |
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Dana1
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| 10/09/2009 1:38 PM |
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Thin sheet metal plates will give you some amount of tick & creak on startup, but extruded plates will have less (maybe none in your case). I'm not sure how much noise to expect out of Warmboard, etc.
I don't find the thin-plate stuff too annoying through 1.5" of subfloor + hardwood. The noise is typically short-lived, and only really noticable when coming off of dead-cold. When there's a sustained heat load it's not noisy at all.
You'll likely be running water 20-25F colder than the 140F I need even if you went with the cheap plates, and with extruded goods it could be cooler still. A lower difference in starting vs. operating temp means less expansion, less noise, quicker settling-in time. Don't sweat it too much unless you're very sensitive to noise- it's not exactly loud ever, but it's more than a whisper.
Gypcrete etc will add some thermal mass to the house which will even out the swings in load, but slab floors have very little flex & give, which some find uncomfortable. Everything's a tradeoff, eh? |
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Jimmery
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| 10/18/2009 3:34 PM |
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Posted By Blueridge Company on 09/29/2009 11:21 AM
I have in floor radiant, install it and sell systems. Would not build a house with out it. Your heat load is light, you could run it of a TACO x block attached to a 50 gallon water heater. isolated system. as to the HRV, these are a separate issue, easy install. In our home we have programable t state, set back in the night 10-6 am 5-8 degrees, and in the mid day same. house recovers in about an hour.
If you are lab on grade insulate under w/ 2 inch rigid.
Good luck,
Dan
[/quote]
Dan, what thermostat do you use with the Taco X block pump? Could you use the same thermostat to control both this device (and the radiant floor) and a back-up/second floor air handler? |
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