Hello, Not sure if is the right place to ask but I'll try. We raised the structure of our farmhouse using Amvic 3.30 this year and now we are looking for next step. We would like to have a backup on our radiant flooring and we are thinking to use an air forced furnace using propane. Do you guys have any recommendation in term of model, brand, etc.? Thank you.

Very few furnaces go that low, and you don't want to oversize by more than 1.4x if you don't have to.

What fuel?

What location (or 99% outside design temp)?

There are hydronic air handlers with output that low that can run off a water heater. There are also ducted and ductless air source heat pump solutions in that range.

Fuel propane. Outside design temp -4F The lowest BTU for an air forced furnace which I found was 38K. Just wondering from where 1.4X oversize factor coming?

The 1.4x oversize factor is straight from ASHRAE, as the compromise between recovery from setback, comfort, and efficiency. Though fossil burner hot air furnaces efficiency isn't much affected by even fairly large oversizing, comfort and noise is improved with the lower output, lower cfm, and longer burn times of a smaller unit.

Propane is a pricey fuel, and with a 99% outside design temp of -4F/-20C there are plenty of modulating mini-split heat pump (ducted or ductless) that would have half the operating cost of condensing propane in most markets. To make reasonable recommendations would require knowing a bit more about your floor layout, etc. You're looking at about 2, at most 2.5tons of mini-split/muli-split to cover it. Most cool climate multi-split offerings in that range can support up to three zones of ductless, ducted, or a combination thereof.

http://meus1.mylinkdrive.com/files/MXZ-3C24NAHZ_Submittal.pdf (3-zone)

http://meus1.mylinkdrive.com/files/MXZ-3C30NAHZ_Submittal.pdf (3-zone)

For a single zone traditional ducted solution, a 2-ton Mitsubishi MVZ air handler married to either of the above compressors would get you there: http://meus1.mylinkdrive.com/files/MVZ-A24AA4_For_MXZ_MULTI-ZONE_SYSTEMS_Product_Data_Sheet.pdf So would a 2-ton Carrier Greenspeed 25VNA024 compressor with some selection of air handlers. To play around with the capacity curves across temperature with the different air handler options, click on the "HEATING CAPACITIES" tab: http://www.tools.carrier.com/greenspeed/ Most are a bit marginal for 18K @ -4F, but unlike mini-splits, they have auxiliary heating strip options to cover the shortfall. As long as you're covered by the heat pump at the 97th percentile temperature bin the heat strips will have essentially no effect on seasonal efficiency. Even if the heat pump only covers to the 95th percentile the hit is still pretty low (but more easily measured in the power bill.) There are others: http://www.fujitsu-general.com/us/resources/pdf/support/downloads/submittal-sheets/24RLXFWH.pdf (single head ductless)

http://portal.fujitsugeneral.com/files/catalog/files/24RLXFZH3.pdf (3-zone)

In fossil-burner land, the 2-stage Goodman GMVC960403BNA operates at ~27K at low fire, which isn't ridiculous oversizing for an 18K design load.

http://www.goodmanmfg.com/pdfviewer.aspx?pdfurl=docs/librariesprovider6/default-document-library/ss-gmvc961a6d390022fa6258827eff0a00754798.pdf?sfvrsn=cf975bc0_0

There are number of FirstCo hydronic air handlers that can deliver the heat at domestic hot water temps, using a condensing propane water heater as the source. eg, the 18HBQBR can deliver 19-20K with 120F water, ~28K with 140F water.

https://www.firstco.com/documents/ProductDocuments/hbqbr217.pdf

There are others in their lineup, both with or without cooling coils, multi-speed, single speed ECM drive (or not), etc.

If you already have a HR heated floor and are just looking for a backup heat source, why are you thinking of using a forced air furnace? A masonry heater or soapstone faced stove would simply get the job done without creating any drafts and would be truly backup in that electrical power wouldn’t be needed. You could just cleanly and quickly burn the small quantity of wood required and slowly radiate the heat over the course of the day. You would also avoid the unsightly look of duct grills or mini split heads. Anyhow, maybe something else to consider.

What's the fuel source and equipment used for the radiant floor?

I'm amazed day by day with the value of information which this forum gave. Thank you guys for this.

So, just a little bit of info where we are with our project and plans which we have:

- because of the price involve to bring hydro (electricity) on the property we are planning to stay off grid. Just as a note, the local company quoted 40K to bring electricity on site
- for electricity, we are planning to go solar. We are thinking for a 10KW system
- for RH we are planning to use the NextGen 4 (4Kw) system which is an electric system. Also, I'm planning to have solar thermal vacuum tubes with a water tank inside the house just to still some heat from the the "mother nature"
- as I have 3 bedrooms on one level, not sure if a central stove/fireplace will be enough to heat properly the rooms, so that was the reason why I was thinking at the air forced furnace.
- as I mentioned, these are the plans so far but you guys gave me info to think. Appreciate any other suggestion, comment.

Thank you

Your place sounds very similar to our place. We have a 2400 sf single level ICF building that uses passive solar, HR floor heating for primary heating and a masonry heater for backup heating and baking. The masonry heater is located in center of house which is an open floor plan allowing it to heat the entire house. If you don’t know about masonry heaters, I suggest that you research this subject. Masonry heaters are way better than fireplaces or wood stoves in terms of efficiency and pollution. They can be fired for a short period time once or twice a day and then radiate the precise amount of daily heat you need in an energy efficient home. We also use a NextGen-4 for our HR floor heating and we have active solar, hydro and grid power. While the NextGen doesn't have the high COP of a mini split or GSHP system, this not an issue when you generate your own power. Perhaps more importantly, you don’t have the failure and maintenance issues of these more complicated and expensive heat pump systems which can be the most critical design element when you live in a remote area.

Note that while design temp around here is 6F, it's not that uncommon to have -15F for extended periods. Also note that a very strong wind on a cold day can create a huge heat loss. So there are rare but important comfort arguments for including the extra 40% over actual average wind design day loads. On the other hand, the issue can be addressed with backup heat (fireplace, portable electric heaters, etc).

You could look at two speed and variable speed furnaces.

If you are going to produce hot water with solar, think about how much water storage you need.

You will probably want a propane generator.

Is there not a propane fired boiler that could heat the radiant using much less electricity then an electric boiler? Wouldn't that be better then 2 systems, 1 air & 1 water.

Long term money spent on off grid batteries and generator ect.. might make grid connection a wash.

Are there still indoor wood boilers? Whatever uses electricity just for circulators.

The ASHRAE 1.4 factor is for generating the higher heat gain that is needed to reheat interior thermal mass (e.g., furniture) when recovering from using indoor temperature setback. Use of indoor temperature setback is most beneficial for less energy efficient buildings and trades less night time temperature comfort for less night time heating energy usage. The 1.4 factor was first introduced back when temperature setback was first introduced to reduce the morning recovery time. If you have a high thermal mass slab HR emitter, you don’t ever use temperature setback and you don’t need to apply the 1.4 factor on top of your ASHRAE or ACCA Manual J8 heat loss analysis. Even though the 1.4 factor is code, you can and will receive a deviation when you have an energy efficient building that incorporates high interior thermal mass for passive solar and HR floor heating. At least that is how this is handled in green and progressive Oregon. However, I can’t say if this intelligence and understanding is common place in other States. Some people make lots of money selling more expensive heat sources that are NOT really needed and politics tend to favor the people who have the money.

I passed these options of backup heating to my wife: masonry heater and mini split heat pump. For me the masonry heater is a great idea in special we have free wood on the property. Unfortunately, source of smoke is a no no for my wife. Also, the house is not designed for extra mass to build the masonry heater, but the heat pump is a go from both of us.
We saw these system all over the place in Europe, one condenser outside and each room with own evaporator. Still not have a feel about the cost of those systems. Looking over Fujitsu web site I couldn't find the maximum length for tubes between the condenser and evaporator. Anybody?

Maximum length between condensing unit and evaporator unit depends on the actual brand/model and the number of evaporator units and is listed in the installation instructions. Typical maximum length and lift for a couple heads is 100’ and 100’. I have seen some less expensive brands/models where maximum length and lift is only 50’ and 25’. I think mini splits are a way better option than a forced air furnace.

Masonry heaters hardly smoke at all and don't even exhaust much heat out the chimney. They extract nearly every available wood Btu by burning at about 2000F and then circulate the heat through the masonry before going up chimney. So in addition to a lack of smoke, there also isn’t any significant ash or creosote to deal with either. As I said, they are not anything like fireplaces or wood stoves which only burn at about 600F and some research is needed to gain a full appreciation/understanding. It is true that the building is designed/built around the masonry heater and masonry heaters can't usually be added later as an afterthought. Masonry heaters are also relatively expensive.