Air to water heat pump

September 21, 2019

Starting a new build in spring and will be having in-floor heat throughout house.
Main floor 2400sq and upper 900 sq with a 900sq bonus over the garage which will also be heated.
I have 2 quotes for geothermal and its a lot more than our budget
Looking into air to water heat pump instead.
Looking for anyone that has tried these in a cold climate. Zone 6 Minneapolis.
Both quotes show a 9-10 ton unit for geo is this equivalent to the tonnage of a regular unit? Seems high.
House will be 2x6 walls with 3" of exterior insulation so i thought the heating demand would be lower.
These systems intrigue me as i could offest with solar if needed and use my propane as a backup.
Any thoughts on these?

Comments (25)

  • sktn77a

    Well, if you really must have in-floor heating as your primary heating source then what you are suggesting would work (other than you can't really cool the house in the summer with in-floor water pipes). Personally, I would just go with forced air and an air-to-air heat pump with supplemental electric in floor heating in the bathrooms to take the chill off.

  • fsq4cw

    Geothermal hydronic systems are the most efficient method of space conditioning with zoning. The quotes you’ve been given sound too large for the info provided. I recently visited a country cottage located in the mountains of eastern Canada that is 8000sq ft. of conditioned space that requires only 10-tons of Geo HP (triple function). A smaller system will save you ‘tons’ on drilling and ground loop installation.

    You could install a ‘Triple Function’ geothermal HP (liquid-to-water & liquid-to-air) that will provide full capacity HW for hydronic heating, DHW and forced air heating and cooling for rapid space heating and air-conditioning or a full function liquid-to-water geothermal HP that can reverse the water process to either water hot water for heating or chilled water for hydronic cooling, no duct work (necessarily) required – very efficient!

    These are low maintenance long life cycle systems that will cost a premium to design and install properly but should save energy to run and money on maintenance. You could size it for air-conditioning and make up any heating shortfall with electric or gas backup boiler. Geo only would likely still provide about 90% of all heating.

    Air-to-liquid systems do exist however they are not as efficient as geothermal, require more maintenance and do not have the same long life cycle as full geothermal.

    Keep in mind that in-floor pipe spacing has to be designed for low water temperature that requires (much) less energy to generate. Heating the entire floor surface with radiant heat means that the primary concern is the temperature of the air from the floor up to about 6-feet, forced air requires conditioning the entire volume of space within the envelope.

    In-floor radiant also means a lower temperature differential between the outdoor air and the indoor air at the ceiling beneath the roof, that translates to less or lower heat loss.

    Only you can decide what you’d like to spend on HVAC and what’s right for you. The above-mentioned system was well over $100k, however money was not a factor; being efficient was.

    The future is – Water!

    Probably more than you ever wanted to know:



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  • Matt M

    sktn77a's answer is it.

    In floor heating just isn't worth it in a high efficiency home.

    Your house just wont lose enough heat to make those floors noticeably warm. A good tradeoff, I think.

    I'm building a similar sq.ft. home, climate zone 5 with 2x6 and 1.5" ext. insulation. My HVAC is all air source heat pumps, air-air.

    All off the shelf common equipment, so it's inexpensive and reliable.

  • nexp

    I would be hesitant to do air-to-water in zone 6. If you plan on being in this house a long time, geothermal will likely cost half to run. With a house that compact and with that much insulation, 10 tons seems extraordinarily high - that’s about 90,000 btu/h of heating depending on the actual Heat pump. My (pre-Reno) poorly-insulated 1970s 2x4 r-11 2700 square foot ranch with barely any attic insulation in zone 4a needed only 50,000 btu/h at design temp (17 degrees).

    I would hire an engineer to do a proper, independent Manual J heat gain/loss calculation. For cost, are you factoring in the 30% (going down to 26% next year) federal tax credit? It applies to all aspects of the system - wells, equipment, labor. I don’t recall if it applies to the distribution system (radiant floors, etc). You state May also have tax credits or rebates. I agree with reconsidering the radiant floors. In a super-insulated house, they will never be “warm.” They will just feel neutral. It’s a huge expense without much benefit. I wouldn’t necessarily go with forced-air heat. Consider European-style radiators, such as Myson, which is readily available here. They can be sized large enough so that they will work with the low-temperature water of a GHP. They each have a built-in thermostatic valve that allows room-by-room zoning without expensive thermostats, wiring and electric pumps and valves. They can be inexpensively plumbed home-run to a manifold normally used for raising heat using PEX. I have 16 of them, all running off of one constantly-on circulator and each room stays within a half an degree of the thermostatic valve setting. The radiators were between $90 and $200 each. Radiant floors would have been $20,000 at least.

  • seabornman

    We have 2400 sf house and 4 tons of geothermal gives us great heat and air conditioning when we need it Zone 5 We ditched propane and went all electric with solar to help out.

  • PRO
    Springtime Builders

    An air-to-water heat pump is not a Ground Source Heat Pump GSHP (Geothermal). It uses the ambient air temperature, leverages it with refrigerant like a normal heat pump but then transfers the energy to water, which is used in radiant floors or radiators. To sum up from that blog link and concur with some posters above, GSHP makes the most sense in large, energy intensive structures. Building efficiently with modest size floorplans brings simple paybacks into question.

    Air-to-water technology is even more prohibitive than typical GSHP in climates that require air-conditioning or dehumidification. Among GSHP and typical air-source heat pumps best attributes is they heat AND cool AND provide dehumidification. Radiant floors, radiators and air-to-water heat pumps don't usually combine all those functions.

    Agree that ditching propane or going all-electric is almost always a wise choice.

  • redbird2010

    I was looking at both the chiltrix or arctic.

    I would have air handlers for ac and would use chilled water. Just not sure how to proceed. I would like to do all

    The in floor and any other supply piping myself to save. Would love geo just dont know if I can do enough myself to bring the cost down.

  • PRO
    Springtime Builders

    Including personal labor for circulation and distribution tubing is one way to keep costs down but its the fittings, controls, manifolds, zoning and pumps that require experience and cause the most headaches. If upfront costs are important, why not use mini-splits? The best brands are very reliable and proven, especially when it comes to dehumidification. Chilled water for AC purposes sounds like premature levels of early adoption.

  • redbird2010

    Mini splits are not an option.

    They are unsightly in a new home.

    Especially a larger 1 with multiple rooms. Around here the only places they seam to use them are commercial, and older homes.

    My only concern with diy geo would be the design and the programming if thats a thing. Piping is what i do for work so that part is easy.

    But i still have to have someone run ductwork for air.

  • mtvhike

    I'm also in Zone 6 and love my in floor radiant heating. 8 zones on two floors and I'm always comfortable. Electric resistance heat, however. No AC.

  • fsq4cw


    "I'm also in Zone 6 and love my in floor radiant heating. 8 zones on two floors and I'm always comfortable. Electric resistance heat, however. No AC."

    Is that electric resistance 'cables' in-floor or hydronic, electric boiler?


  • redbird2010

    If i end up going this route I'll have a electric or propane boiler for the super cold days.

    Hopefully with a super insulated home i would limit my need for supplemental heating.

  • mtvhike

    fsq4cw, hydronic, with an Argo electric boiler with four 40A double pole breakers.

  • nexp

    You can hire an engineer to design the system and provide drawings. This may be helpful regardless even if you're doing air-to-water. There shouldn't be any need for a back-up boiler. Most geo that supplies in-floor heat is a combined water and air system. If the floor can't provide enough heat for material or occupant safety, supplemental heat will come out of the ducts. If the geo doesn't have enough capacity, electric resistance strips will come on.

    I wouldn't necessarily discount mini-split out of hand. They don't necessarily mean having an unsightly unit hanging on the wall. They are available with small air-handlers that connect to short duct-runs. In Europe this is often used in super-insulated houses as the heat requirements are so low that a separate heating plant makes little financial sense.

  • Dick Russell

    There is something terribly wrong with your description of what the house will be vs the heating load of 9-10 tons. A "ton" of heating or cooling load is 12,000 BTU/hr. The house you describe is not what you'd call "huge" anyway, at just 3300 sq ft main plus another 900 over the garage. Finally, such a high heat load clearly is inconsistent with any house that is "superinsulated." As others have pointed out, you need to have someone competent do a detailed heat loss calculation for the house, using all details of construction, U values and sizes of all windows and doors, and anything else that affects heat loss. Most HVAC contractors either don't know how to a really good calculation or won't take the time to do it, relying instead on rules of thumb.

    For comparison, my superinsulated house in central NH (climate zone 6), finished in 2011, has a footprint of about 2000 sqft, with two levels, for a total gross square footage of about 4000 sqft, close to what you propose, but with a different configuration of course. It is heated by GSHP (standing column well design). The regional distributor of the unit installed and two of their "approved installers" all came in with around 4.5 tons of heat load, which they proposed satisfying with a 5-ton unit, even though I had given them all details of the house and my own spreadsheet showing a heat loss of somewhat less than two tons. In the end, I bought the two-ton unit. The tables for the unit showed that at the air and water flows to be used, it would deliver over 25,000 BTU/hr of heat in second stage, or about 75% of that in just first stage. As it turned out, the unit keeps the house at 70 F throughout the winter, in just first stage. My best calculation of heat load when it's 0 F outside is about 19,000 BTU/hr, somewhat better than my last run of the spreadsheet prediction. In summer, cooling mode provides AC; the unit is really loafing then, as the cooling load, worst case, is less than one ton.

    Your first money ought to go toward making the shell of the house truly "superinsulated." There is a wealth of information on how to achieve this, but still relatively few builders with experience beyond building house just "to code." I followed the subject of building science and superinsulation over a span of about 30 years, leading to my specifying the construction of my own house. I found a local builder interested in doing something new and different, and we worked well together. I didn't tell him how to frame, and he followed the design and my input on air sealing and other details.

    The house has a double exterior wall frame, typical of a great many superinsulated houses. The outer wall is 2x6, inner 2x4, with a 3" gap between. The insulation cavity is 12" thick, filled with dense pack cellulose. Windows are a mixture of triple pane casement and fixed glass. The foundation walls have 2" of rigid foam inside, another 2" outside. If I had to do it all over, I'd just put it all on the inside, as I had to cover that with 2x4s flat-wise and sheetrock over that for thermal and ignition protection, as required by code. The lower level slab is poured over a 4" layer of foam. The house is air-sealed to a high degree of tightness; the final blower door test came in at about 0.8 ACH50, vs Energy Star requiring 3.0 or better. Fresh air for human health and to keep interior humidity from getting too high in winter is provided by an HRV (heat recovery ventilator).

    The incremental cost of making your new house truly superinsulated class isn't all that much. I'd give serious thought to skipping all that expensive radiant floor heating. As someone else pointed out, in a highly insulated house the floor surface temperature won't get warm enough for it to feel warm underfoot. That wouldn't happen until the floor got into the upper 70s, and with little place for the heat to go the whole house would warm up to close that temperature, which most people would find stifling. Save the plumbing cost of radiant floor heating, and put it into the building shell and windows. Do your reading, get someone competent to design the shell and do a really good heat loss calculation, and you likely will not exceed the capacity of a three-ton unit.

    One final thought - give careful thought to isolating the living space over the garage. Such rooms typically are hard to heat and leak too much air from the garage, something you really don't want.

  • fsq4cw

    Re: Dick Russell

    Great story with a wonderful result, I would be interested in hearing as much detail as you can provide about the design of the standing column well.


  • Elmer J Fudd

    "The incremental cost of making your new house truly superinsulated class isn't all that much. "

    It added how much, to a total of how much? (excluding the cost of the HVAC equip)

  • barrybud

    Please check out Nordic Geothermal as they have been keeping the Canadians warm for a long time in colder climates than yours. They have traditional geo systems as well as air to water solutions.

  • PRO
    CoolAir Inc.

    The company is giving you a quote of much larger capacity space. Why you are going to need such strong heating equipment even for the main floor area that is 2400 square. Geo thermal is good option but it is also heavy on the pocket. Try to take different quotes from other companies. Do your research in the market and contractors then decide what is appropriate.

  • redbird2010

    I have two quotes for geo and both came out to over a 100k for everything. Contractors are super busy here but I'm still going to find another option for heat and ac.

    I was quoted a 7 year payback and a Rebate of 32k.

    I'm pretty sure i cant write off all the infloor piping and ductwork so i feel like this # is skewed a lot.

    Still looking at the cold weather heat pump as they are designed for just as cold of weather as the geo was designed and i dont think its going to cost me an extra 62k.

    I have looked at nordic they look promising too.

  • Elmer J Fudd

    100K minus rebates of 32K = 70K

    To recover 70K in 7 years, it needs to save you nearly $1,000 each and every month. To be fair, lets assume the equipment otherwise is $10K and costs $3K/year to operate (which is high). Then you'd need to save $3,000 per year, $300 each month, to recover the added cost.

    Is that reasonable to assume for this house? Properly done radiant can have many zones and you can keep unoccupied spaces turned down.

  • mike_home

    The federal tax credit for a geothermal system in 2019 is 30%. In 2020 it drops to 26%. You must close on your house by December 31 2019 to qualify for the 30% credit. The duct work and electric heat backup costs do not qualify for the credit. This is a credit so you have to pay the money when the work is completed, and then get the credit while you file your 2019 federal tax return. It may take two or more tax years to get all of your credits.

    A new build house that is approximately 4200 square feet with the amount of insulation you describes does not need 9-10 tons of cooling. That is probably double the size you need. Even though you live in a cold climate, the system has to be sized for the cooling load. Sizing it for the heating load will lead to problems in the summer.

  • Dick Russell

    I posted my comments above because of that initial comment that the two GSHP quotes called for 9-10 tons for a home of 4200 sqft. That immediately smacks of having used an old but still common rule of thumb calling for a ton of AC per 450-500 sqft. Either that or the GSHP system proposed was simply one to match the output of a typical fired boiler. Here is a good starting point for reading about the need to avoid rules of thumb: As the article notes, even Manual J calculations typically oversize a system significantly (20-40%). Particularly for GSHP, where cost scales largely with size, oversizing should be avoided. An HVAC contractor typically would rather oversize, even grossly, because he makes more on the contract and certainly won't be called back for the system failing to keep the house warm in winter.

    There are other tools, some free yet quite good, for doing detailed heating and cooling load calculations. Here is one, BEOpt, said to be quite good: There are others, too.

    GSHP isn't necessarily the most sensible choice for heating and cooling, mainly due to high initial cost. But there are times when that can indeed make sense, as was the case with my home. The house was to be superinsulated, so right off the loads for both heating and cooling would be very low, and the size of the unit would be small and the cost thus quite reasonable. Ground water in the northeast generally is of good quality, so that the Standing Column Well (SCW) design would work. A new well would be required anyway for the new house, so that right away minimized any extra drilling cost, a large chunk of the cost of GSHP.

    A decent number to use for water column depth is 80 ft per ton of load. When the well was drilled, it wasn't known if I could get by with a two-ton unit or if I would be somewhat short and need the three-ton unit. Good water flow was achieved at around 270 ft down, but I had them go deeper. When they reached 330 ft, I called a halt to the drilling, as that would cover three tons of load easily. It wasn't until the house was closed in and insulated that the first of the blower door tests was done. Putting that result (0.65 ACH50) into my spreadsheet model told me that the two-ton unit would be a comfortable fit. There is a 20-ft dead leg of pipe at the bottom, then a 20-ft perforated section where the pump is located, and the return line from the heat pump is about 50 ft down from the top, so that the outlet would be under water at any drawdown rate likely to happen. This minimizes pumping power for the 6 GPM flow through the heat pump when the unit is running. Water for domestic use is a branch line from the pipe to the heat pump.

    As to cost of making the house truly superinsulated and very tight, numbers of course vary a lot, depending on the house. But I've seen the figure 5% extra given to ballpark it. I don't have exact numbers, although I tried to estimate the bump, and that gave me something in that range. At one point, after framing was done, I asked the lead framer how much longer he figured it took his crew of four to do the inner wall framing for all the exterior walls. He thought perhaps a day and a half total. That was only a couple thousand, and the additional lumber cost was easy enough to estimate. There was extra insulation, of course. I did not factor in any cost for GSHP, as the house was going to be superinsulated anyway, no matter how it would be heated and cooled.

  • fsq4cw

    Re: Dick Russell

    What is the diameter of the borehole? I would assume that there might be steel casing down to solid rock; any idea of how many feet of casing if this is the case?


  • Dick Russell

    There is 8" steel casing at the top, to accommodate the second pitless adapter for the return flow. Below that, the bore hole through rock is the nominal 6". I don't know the exact depth of the casing, but a quick search gives this from one source: "Casing must be set 10 -15′ into the bedrock by code depending on the state and 18″ above grade. .... The typical average of casing depth is 20-40 feet in Massachusetts and New Hampshire but its important to understand that it is possible for the bedrock to be located deeper." In my case, there isn't much soil over the bedrock at the well location; blasting was needed before foundation footings could be poured, over perhaps 2/3 the house footprint.

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