i have a new ICF house with radiant floors on the main level and basement, no heat source yet. I need to get heat on in the house but for reasons i wont go into, cannot install the ground loop for about another 4 months or so.

I have not had the heat loss calcs done to size the heat pump so do not know what i will be using yet but i think i read watersource does a combination unit that does water and air. Whether it will do domestic, radiant and air cooling i dont know.

Someone suggested to me that i can still install the heat pump and make an artificial core loop using a small electric water heater running at around 60 degs until i can install my ground loop. Is that feasible?

Does anyone have any other suggestions? i dont want to invest a lot of $$ in something that will just be temporary.

Thanks

Peter

My first reaction is that it probably won't work - most water heater elements are 4500W, which is about 15,000btu. With a relatively high "loop" temp you'll get a fairly high COP, so figure the compressor will kick in another 20-25%. So if your load is less than 18k or so it might work. If the heat pump is rated for more than this you'd have to have enough tank capacity to last through a cycle. Even if the numbers worked out, in terms of simplicity you'd be far better off just driving the radiant with the hot water heater directly. It would cost exactly the same to run. The only advantage I can see to doing a fake loop would be that you'd have the 4 months to make sure everything was working perfectly on the house side of the geo unit and you could test the efficiency at different loop temps, flows, etc. If you're looking for harebrained schemes, why not rig a temporary air-water heat exchanger outside? If the weather is mild it might work, but I'd worry about it freezing if you got a cold snap. Where are you located?

I would think that the heat pump would extract heat from the hot water heater faster then it could warm the water, unless you have a very small heating load.  It wouldn't take very long before the water in the hot water is freezing.  Not to mention this is horribly inefficient, you would be better off with radiant electric heaters in the rooms of the house. I guess it could work if you had a series of hot water heaters chained together to create your artificial loop, but that a lot of cost for a temp setup, not to mention the operational costs.   

I wouldn't suggest using city water either, I figured it out, it's something like a $300 a month water bill.

Bad idea for reasons mentioned above. the geo unit would quickly overwhelm a small electric water heater

You might look at plumbing in a standard storage electric water heater. They are cheap. With some minor modifications you could temporarily double the capacity of such a heater by simultaneously energizing both elements using separate 30 amp circuits - be sure to have separate high temp cutouts for each element should you try this.

You would NOT run the geo system under this scenario, just the radiant water circulation system

Such an arrangement could have a capacity of 37,500 btuh and consume 11 kw of electric power

If you have a place to discharge the water you could temporarily run your heat pump as open loop.
Joe

And the water would come form where?  As I said before, it's at least a $300 a month water bill alone for city water.  Electric resistance heat is by far the cheapest solution, unless you want to get involved with a Propane space heaters.... Not recommended.

A gas (propane) water heater may have a better output and recovery rate than an electric tank, but there would probably be additional expenses to have one (venting, temporary gas tank, gas line installation, etc.) However, if gas rates are much lower than electricity in your area, the total cost may offset to a point depending on how much heat is needed.

The propane heaters I had in mind were the kind they use for construction in an unheated house. It's just a big propane tank with a hose connected to a stand alone heater.   They can be quite dangerous if the house isn't properly vented or someone knocks over the tank.

TG, I agree with you on safety concerns of running these (forced air propane) type of heaters in the house. I was just proposing an alternative with the propane water heater since no one else mentioned it. Difficult to know what suggestions would work well since the OP can't go into more details, so I thought I'd throw in another alternative so he can decide for himself.

I'd have to agree with Techgromit that the electric heaters would minimize your added expense.

How did your radiant floor get designed without heat loss calcs?

I was curious and did a quick calculation to see if running city water through a heat pump is less expensive than electric resistance heat.

At $0.10 per kwh electric resistance heat costs about $29 per million btu.

At $0.10 per kwh and $15 per thousand cu. ft. of water I'm coming up with a cost of about $22 per million btu.

This was based on a 4 ton Climate Master TT 049 running 6 gpm of 50 degree water and 46,400 btu/hr output. The cost of electricity was $6.68 per million and the water was $15.56.

Thanks all for the suggestions so far. water/air exchanger wont work, to cold in MN right now, using city water wont work either, our water comes out of the mississippi so runs as low as 34 degs in winter, too expensive and wasteful too. Direct fired propane not suitable for safety venting and condensation issues.

I take the point that the HP would draw too much heat from the electrical artificial loop and it would be tough to get a fast enough recovery but as far as being horribly inefficient, if it were possible, (or by using a gas fired HW storage heater), what im struggling to get my head around is that HP's get 3 units of energy from the ground loop for every unit of energy used to run the HP so wouldnt it get get more out of an electrically heated artificial loop that was used to generate the loop? But i also understand that energy can neither be created nor destroyed, just converted from one form to another. So if you used 1kw on the artificial loop generation and 1kw running the HP wouldnt that create 2kw of energy, most of which would go into my radient heat? I know im missing something simple here.

I can put in a gas fired water heater for my radiant heat but id need a seperate one for domestic water or a combination storage water heater or an instantaneous combination water heater to do both but thats rather costly if im ultimately going to use a HP. Hooking up the gas and venting isnt an issue.

As far as installing my radiant without a ht loss calc, i've kept my spacing and loop lengths constant so should be able to adjust with water temp if necessary when balancing the system to avoid cycling.

Thanks so far for the comments.

Geothermal heat pumps function on the simple basis of moving heat from one area to another.  The only heat they are generating is from their consumption of electricity   Heat is moved from the ground as BTU's in a sensible exchange as the fluid changes temperature.  In your application, the only source of BTU's to change the temperature of the water would have to come from the water heater itself and is thus limited to the capacity of the water heater. 

-Adam

How do you know the spacing will work with low temp geo? Are you sure 120* (Max Temperature) water will heat the space?

Bergy

Me personally I use about 4,000 gallons of water a month, which runs me roughly 25 bucks a month.  Based on your 46,400 btu, you would require 360 gallons of water to run your system for 1 hour, so even if your system ran only 3 hours a day, it would use 1,000 gallons, multiply that by 30 and you get figure of 30,000 gallons of water used per month, So if it cost 25 bucks to buy 4,000 gallons of water, logically it would cost me $187.50 to buy 30k of water and this is at a very conservative operating schedule for the system.  Keep in mind this is just the cost of the water, not the cost of operating the system.  Also remember that sewer bills are based on water usage, if you have city sewer, that bill will be magnified as well.  

You can't heat the water 1 btu and expect to extract 3 or 4 btu's out of it with a heat pump, it's impossible, the water would freeze solid in no time at all.

This is my understanding of how a heat pump works as applied to your situation. This is expanding a bit on what Adam and others have said.

Let's say a heat pump can move heat 3 times as efficiently as making the heat. So, the cost of running the heat pump is 1/3 the cost of making heat. If an electric water heater would work in your situation (I'm not saying it would) and it is 100% efficient, and you connect it directly to the radiant system, it would be 100% efficient (assuming no heat losses). It would need to be capable of supplying all of the heating (BTU's) that you need.

If you run the same electric water heater through the geothermal system, and then to the radiant floor, the cost would increase due to the operating cost of the geothermal system, and as a result, overall efficiency would drop below 100%. (Maybe reducing the overall efficiency to 67% - I don't think I can correctly subtract percentages in this case, but you see where I am going with this.) Why move the heat from the water heater if it is already available to use? If the heat is in the ground outside, it must be moved into the house to make use of it. Moving the heat from the water heater into the geothermal system cannot generate more heat than is already in the water heater since the geothermal system only moves heat, it doesn't make heat.

You have to pay to make all the the heat that you need in a water heater either way you go.  With a ground loop, the heat is free, so you just pay to move it at 1/3 the cost (of generating it) with the geothermal system.  This is why ground loops are needed to make the cost of heating lower.

We used to have a 70,000 or so btu 75 gallon propane water heater (capable of space heating) from Bradford White. The recovery rate was amazing. I'm sure it was expensive (it was in the house when we moved in), but if you decide to go this route, you may be able to recover some of your costs by either selling it or by using it as a finishing tank used in conjunction with a buffer tank (if you plan to keep gas in the house).

If the entire system resides in the house, you will get 100% efficiency - no more, no less, whether you use a heater and heat pump or just a heater.

Interesting that city water pays off - and of course it includes the right to dump it down the sewer.

How is 100% efficiency maintained if you need to use more electricity (at an increased cost) to run both the water heater and the heat pump as opposed to just the water heater?

Regarding my previously mentioned buffer tank idea:
Could the tank be flushed adequately to provide domestic hot water later?  You don't want any health issues if this cannot be done safely.

Also, if converting a radiant tank to a domestic hot water tank is safe, another option may be to use 2 electric water heaters (if this would supply your heating needs).  Then you could later use one for a buffer tank and one for a finishing tank for domestic hot water.  Just turn off the breaker on the buffer tank when using it for domestic hot water.