Is in-floor radiant heating more efficient than forced air?
Last Post 08 May 2014 12:47 PM by MickeyM. 18 Replies.
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MickeyMUser is Offline
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05 May 2014 12:56 PM
We’re looking at heating options for our new build in Canada, North of Toronto. We don’t have access to natural gas, so are thinking of using propane which is more expensive. Question is, will having in-floor radiant heating in the basement and forced air in the main floor save us energy costs vs. going forced air the entire house? The hydronic floor installer certainly made it sound that it would be, something about efficiencies from heating the water heater at the same time, and keeping the cold out from the slabs. We would be insulating underneath our floor slab in either case, since it can get really cold up here in the winter (< -4 F).
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05 May 2014 01:28 PM
This is a very common practice here in Minnesota -29°C. The reasons are two-fold. First, a properly sized water heater can be used to heat the basement slab and domestic hot water, with the benefit of a heat exchanger to separate one from the other. This can lower installation cost and requisite real estate. Second, it is not easy to make sub-grade spaces comfortable without radiating the slab. Such spaces have relatively low loads so the operating "savings" can generally be counted in small percentages.

Since the slab is a nearly perfect radiator, already built into the cost of most houses, it always make me sad to see one with out PEX installed.
MA<br>www.badgerboilerservice.com
jonrUser is Offline
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05 May 2014 02:15 PM
save us energy costs vs. going forced air the entire house


Heating up external surfaces causes more heat loss. Fortunately, you can add just a little more insulation to make up for it. And the radiant floor will be more comfortable.
Dana1User is Offline
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05 May 2014 02:50 PM
In some locations propane is more expensive per BTU or Joule than electricity.  In those cases going with propane may turn out to be a bad investment, in that electric boilers are cheaper and easier to deal with up-front, and electricity pricing is less volatile.   If you can get time-of-use (TOU) rates for electricity heating the slab with an electric boiler could easily be half the cost of doing it with propane over the life span of a propane boiler.

Propane prices track that of crude oil, despite only about half of the propane being derived from crude, the other half from natural gas processing, which doesn't bode well for the a stable or falling future price.  Electricity has a much more stable retail price, and the coming very low priced renewables in the next decade (particularly when grid tied rooftop solar hits the $1-1.50 price point, which is widely anticipated to be less than 5 years away) will put a damper on runaway electricity price inflation.

Heat pumps are more expensive than electric boilers, but can cut the amount of power used by half or more in a southern Ontario type climate. (Ductless heat pumps or "mini-splits" even more so than ducted).  Hydronic output air source heat pumps are expensive, and have limited output below -15C, though with the very low water temps required for a slab you'd still have something left at -20C.  There are mini-splits out there that have pretty good output even at -25C, and even at that low temp would only be using half the amount of power of an electric boiler for the heat deliver, and over the course of a season would average about 1/3 of the cost of heating solely with an electric boiler.  But a mini-split is a point-source heater- no ducts for distributing that heat around, making them more appropriate for open floor-plan spaces.   That said, a combination of electric boiler for the slab and panel radiators in any doored-off rooms, with most of the main areas heated with ductless can work pretty well. If you slightly over-heat the common areas to say 22C with a "set and forget" approach, and keep the doored off rooms at 20C (or use setback strategies for those rooms) with the hydronic system a good fraction of the heat in the doored off areas can still come via the more efficient source.

But it still comes down to:

What are your all-in (distribution included) electric rates?

What is the recent 5 year average price of residential delivered propane in your area?

MickeyMUser is Offline
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05 May 2014 02:57 PM
@BadgerBoilerMN So to sum up your points. Instead of the hot water tank sitting in the utility room and leaking heat into the air there. Some of that heat can be transferred to the water used for heating the slab? is that the idea behind the savings? But I shouldn't expect too much savings because it cost less to heat the basement anyways, so it is more about comfort.

@Jonr I read online that 20% of the heat in the slab can be lost into the ground below. So how much insulation do I need, and how much heat would still be lost?

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05 May 2014 03:14 PM
Thanks Dana for bringing up the question of propane vs electricity. The impression I got from heating contractors is that propane is still cheaper than straight resistance heat here. But, yes electricity is more stable and it is difficult to judge where propane rates will go.

I have been looking at ground source heat pump too. I have not yet come across mini-split, so will do some research on that. The problem is my heat loss currently for the house is very high based on standard building code materials (75,000 BTU), so I'm not sure if heat pumps would keep up (such as a 6 ton geothermal furnance). We will be looking into increasing insulation.

All-in electricity cost is 0.1377 $/kWh
Propane price was 1.00 $/L this Feb, 5 yr average was $0.64/L
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05 May 2014 08:16 PM
I am paying 4 cents here, so I have an electric boiler. It has been better to buy LP in most places but as Dana suggests (gospel) the future looks bright for electricity. I also us a mini-split, primarily for AC. For DHW I use a super-insulated Marathon with an option for a second fuel--maybe wood boiler--in future. If properly sized and installed an electric boiler is usually a low-maintenance appliance. We will not install on without a low-water cutout.

Insulate the attic, foam the rim joist and look at thermal pane storms.

I am a condensing boiler zealot, but propane is dirty and annual maintenance by a competent technician is a must. Natural gas condensing boilers or water heaters are hard to beat for combi-space and DHW performance and economy.

One of the best things about hydronic heating is that you can change your mind. If electricity skyrockets your contractor will be more than happy to pop in a new propane boiler.
MA<br>www.badgerboilerservice.com
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05 May 2014 08:20 PM
“I read online that 20% of the heat in the slab can be lost into the ground below. So how much insulation do I need, and how much heat would still be lost?”

A competent HR floor designer would never allow the downward floor heat loss to exceed even 10% of the upward heat gain. You can use the free DIY heat loss software on our website to determine the floor heat loss for the insulation R-value you are considering. When doing this floor heat loss analysis, I would recommend increasing the software design indoor temp about 5-7F from your actual design indoor temp so as to better capture that a HR heated floor is higher than the room temp by that range. Our free DIY HR floor design software outputs the downward floor heat loss in percent of the generated upward heat gain.

The overall efficiency of your heating system and your operating cost is largely governed by the heat source and fuel that you select as Dana eloquently explained. There are some who believe that a HR floor heating system uses less heating energy because it better places the heat where people are actually located and you don’t need to heat the entire living space like you do with a forced air heating system.
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
jonrUser is Offline
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05 May 2014 10:02 PM
In floor radiant (or even any type of hydronic system) is more compatible with storage and therefor time of use electricity rates, wood boilers, PV solar + air to water heat pumps, etc.
Eric AndersonUser is Offline
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06 May 2014 08:41 PM
Mickey
Propane info.
I am a propane user, and proponent in the right situation. Energy prices are quite regional, I live in the land of ~0.18$ kwh electricity rates, which makes resistance heat uncompetitive, and natural gas is not available. Oil is less expensive than electric resistance heat, but oil is a poor bet going forward. The other huge problem with oil is there is a lower limit on the size of the furnace/boiler you can get based on the minimum nozzle size. These are way oversized for any reasonably efficient house.
My house has a 14 kbtu heat loss and the smallest oil boilers would be 5 times oversized~ 75 KBTU.
You can buy modulating propane boilers that go down to ~12-14KBTU which is a lot better. The other advantage to propane is that it does not degrade. The downside is that the price of propane roughly tracks that of oil, but is much more seasonal and volatile. This is also the upside, because it means propane is cheap in the summer. The other point is that there is usually a much bigger difference between whole and retail costs for propane.

How do you make this work to your advantage? Number one, you have to buy your own tank, second it has to be big enough to last an entire year, 2-3 years is better. This means that you can negotiate to find the best deal, and fill the tank in the summer when propane is cheap. Many companies will give you a big discount if you are buying offseason and getting hundreds of gallons. I can squeak 4 years out of my tank if I had to, but fill up whenever propane gets down under 1.80 per gallon. The other advantage is power generation as a backup. Propane generators are pretty reliable and you don’t have to have a separate fuel for it. You need a much larger generator if you are using electricity for heat. All that being said, for most people, in most locations a heat pump is a better deal if natural gas is not available.
Cheers,
Eric
Think Energy CT, LLC Comprehensive Home Performance Energy Auditing
sailawayrbUser is Offline
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06 May 2014 09:10 PM
We are also propane users and it is great fuel when used appropriately. Eric is dead on with regard to how to procure and use propane economically. We live in a remote area and we don’t have natural gas. I like cooking gourmet home meals, so that means I need a propane stove. [Note: I have a masonry heater with an oven, so I already have that covered…] We have a hybrid electric/propane boiler for our HR radiant floor heating. We normally use electricity generated using a cross flow turbine on one of the streams on our property, but the propane provides a backup fuel source for this boiler…and the masonry heater provides backup and supplemental heating for the HR floor heating system…and passive solar heating provides about 70% of our heating needs many days. We also have a diesel generator that normally uses vegetable oil for fuel, but the propane provides a backup fuel source for this engine as well. This engine/generator setup is quite unusual and was designed/built by my husband. The starting point was a 1920s Lister style single piston diesel engine. You can see photos and read about it here if interested:

http://mbryner.fatcow.com/listerenginegallery/main.php?g2_itemId=351
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
agagent3User is Offline
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07 May 2014 10:22 AM
A very interesting discussion. The objective seems to be one of optimizing the home for a heating system, i.e. balancing operating costs with fixed costs. The apparent heat loss of 14,000 BTU is relatively low, so does it pay to install several different appliances? I like the KISS principle, so install the Pex, put in a boiler of your choice and be done with it? You have already sunk the money in to the fixed asset of the slab. Has anyone created an Excel spreadsheet that can come up with the solution?
MickeyMUser is Offline
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07 May 2014 11:58 AM
Thanks for all the replies. I've been also posting on the geothermal heat pump section of this forum, and I'm starting to lean towards that as the heat source. My heat loss is actually quite high (75,000 BTUs), so although Eric raised a great point about buying propane in bulk in the summer and storing, I'm not sure if I can buy a tank big enough to cover usage for the year? (Is it possible?)

Regardless of heat source, I guess I still have to decide on the method of heat delivery (ie. radiant heat (basement) and forced air (main floor), or forced air (entire house). So the feedback I've been getting appears to be that there is not a whole lot of difference in terms of efficiency (especially since radiant heat would only be for the basement). Radiant heat could be a tad more efficient since I could couple it with the water heater, and I may not need as much heat to feel the same comfort. Some of that efficiency would be lost by way of slightly more heat escaping through the slab since I'm heating the slab directly. But it makes below-grade living more enjoyable without ice-cold floors. Finally, if I have for pay of time of use electricity, I could generate the heat during non-peak hours at night and it will store better through peak-hours.

Sailawayrb, thanks for pointing me to the DIY heat loss software, I'll check it out to determine insulation requirements for the slab.

Has anyone used the Waterfurnace Synergy 3D for in-floor radiant heat? It's a geothermal heat pump furnace that can be use to supply forced air and radiant floor heat. Website says it can "help achieve ultimate comfort, heating priority may be given to forced air zones or radiant floor heat." I'm sure it has a nice big price tag, but has there been positive experiences? Also, if I went with a regular geothermal furnace, would I still be able to use that source to supply heat for in-floor radiant heat?
Eric AndersonUser is Offline
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07 May 2014 01:17 PM
Mickey, If your design heat loss is 75,000 btu’s/hour, your first order of business is reducing that to a manageable number, via increased insulation, better windows and decreased infiltration. In new construction, there is no reason for those kinds of numbers unless you are building a Philip Johnson, glass house knock off. I think you need to work this out first before settling on an energy source and heating type. The lower the load, the more options you have for heating. More than likely the cheapest overall strategy for you long term is to decrease your load, rather than installing a large geothermal, or other system. IF you have heat loss calcs for the house as well as construction assumptions, post them, and we can give you a more nuanced view. If we assume that the 75,000 btu’s/hour is at a 75°F temp differential and you have ~7,000 HDD ( wild guess, probably an underestimate) then you are looking at 170 million btu’s/year (ballpark, not including solar gain) This would be ~50,000 KWH so at 0.15$/kwh = 7400$ year in heating costs for resistance heating with electrical. If propane is 0.64$/L, that is 2.43$/gallon assume a 93% efficiency boiler and 92,000 btu’s/gallon and you get 170,000,000 btu’s/85560 btu’s/gallon* 2.43$;/gallon = 4850$ per year Propane is a clear winner, Vs resistance heat, but a Heat pump averaging over COP of 1.5 would be cheaper still. To answer your question about storing a full year of propane, at your consumption level you would need a 2500 gallon tank – not practical in a residential setting. Cheers, Eric
Think Energy CT, LLC Comprehensive Home Performance Energy Auditing
MickeyMUser is Offline
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07 May 2014 01:47 PM
You're right Eric, I should look to reducing that heat loss number. The house is large because there is an in-law suite, it's 3,700 sqft on the main level, and the walk-out basement is the same size and includes a large garage. Window size is pretty standard, with only a few over-sized.

The house was designed to only building code standards, so we are definitely looking to increase insulation. 33% of the heat loss was due to infiltration, 19% due to windows, 13% to walls, 12% to floor and 8.5% to ceilings. Calcs were based on 76°F temperature difference.

Should I post the detailed heat loss calcs and construction assumptions on this tread or start a new one in a different section of the forum? Which section is best for this sort of thing?
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07 May 2014 03:51 PM
Correction. 75 kBTU vs. 14 kBTU is a big difference. Have you calculated how many million BTU that is every year after every rear? I'm thinking I would much rather put my money in new, emerging companies i.e. solar, foam, PV, heat pumps and the like rather than fat cat oilmen or Middle east terrorists. Isn't there a moral issue to address in light of the new report on global climate change?
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07 May 2014 09:05 PM
It is nearly impossible to lose even 10% to the ground unless you have high ground water. Don't believe everything you read.

Start with a proper ACCA Manual 'J' heat load to model the heat loads and help decide where hour comfort dollars are best spent.
MA<br>www.badgerboilerservice.com
Eric AndersonUser is Offline
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08 May 2014 08:37 AM
Mickey,
Start a new thread in the design and planning thread with as much detail as you can for the heat loss numbers and construction details. Include your postal code.
Cheers,
Eric
Think Energy CT, LLC Comprehensive Home Performance Energy Auditing
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08 May 2014 12:47 PM
Thanks Eric. I've started a new thread in the design and planning section.

http://www.greenbuildingtalk.com/tabid/53/afv/topic/aff/21/aft/82418/Default.aspx
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