I have been talking/working with Rob/NRT and feel I have already taken up enough of his time trying to decide. I would like to ask, with the follow thoughts below, what is the best/cheapest way to heat water for my Radiant system.


What I have:
-1600sq ft house, Aprox 26k/BTU max design load
-Electric only service
-Seisco 11kw Electric on Demand (Which gets expensive to run, $.55/hr to $1/hr)
-14 year old Trane Heat pump, R-22 (For forced Air heating/cooling)


Issues/What sucks:
-No Gas Service
-Dedicated Seisco for Radiant
-Very Limited space in closet
-Looked into poor mans Geo,(Ebay unit+dig my own 6' hole with backhoe) but I don't have yard space for enough coil length for 2.5t unit

Trying to Accomplish
-Cheapest way to Heat DHW and Radiant Heat.


Thoughts
-Virgina has granted me up to $12k reimbursement ($2/KW AC PV, & $1/kw Solar Hot Water)
-- Install a 4.1 KW AC PV system - and use the remaining for Solar Hot Water(4.1 because thats all the southern facing roof space.
-Install an 80 Gallon Dual Coil Storage tank with in tank Electric Backup
--Bottom coil/loop for solar (W/Glycol)
--Top coil/loop for Radiant (Tho in Winter, the solar will not produce enough heat load alone I believe..)
--Tank Mass for DHW.
-to use or not to use Electric coil backup on tank?


-Or keep the DHW solar system completely separate.. (but will not have a buffer tank for radiant)


-Install a new low temp Heat pump/chiller (air to water) (IE, unichiller,aermec) Daikin is pricey.. .. (I got msrp of 6k for unichiller) --Have this heat storage tank via water/glycol from outside unit(Via bottom coil with solar, or between hot/cold DHW), or heat Radiant system Directly?
--If new pump, install new chiller air/water exchanger for AC inside existing duct work. ---Meaning Radiant is only heat, and AC is for forced air.

I Also thought of getting a 2nd old condenser, running a line set from outside directly to an r-22/water heat exchanger inside. Tho I feel the pump may short cycle way to much.


I am at the end of my rope, and have only a week to decide how to use this Virginia Money before I loose it. The Solar Guy says to loose the radiant completely and go forced air, but I love the feel of radiant & Already have a bit of money into the retrofit. (Roth board on semi-uninsulated slab). I have read as much as I can take for today and yield to the advice of those much more knowlegable then I. Any Suggestions are GREATLY appreciated.

Thanks,
Tom
Sterling, VA

With limited roof area you can max out the solar thermal while keeping the PV with something like this:

http://www.sundrumsolar.com/files/documents/SunDrum_Total_Energy_Data_Sheet_Rev_C.pdf

You can still do radiant off the solar, but control the room temp with an appropriate 2-stage thermostat using your existing heat pump as the second stage, and finish the DHW heat with your electric tankless rather than maintaining the solar storage at a higher temp. Even with the heat pump carrying more than half the load, the warmer floors will add significantly to the comfort factor, even at lower room temps. The lower temp you can run the solar, the higher the collection efficiency and better benefit overall. The incoming water seen by the tankless shouldn't go much below room temp that way, which means it'll take a big fraction off the hot water heating even when the space heating has depleted the solar to the low-limit of useful space heating temps.

Heating design temps for most of VA are in the mid to high teens, making a 26K heat load "feel" a bit high for a well insulated & tight 1600' house. (I'd guess this is 2x4 16" on center stick built, with a heat load that high.) If you haven't already, look into air sealing, see if you can't get the infiltration rates down to at least 3 ACH/50 or lower, then boost the insulation where you can. The lower you can make the heat load, more solar fraction you'll get out of it, the better the overall efficiency.

A Well Insulated house this is not. Its a 70's steel frame House. I've been adding 1/2" rigid foam taped at all seems to help with cold and air infiltration.

Thanks very much Dana1. Let me see if I understand this correctly. Basically, use the storage tank heat to heat the radiant as much as it can until the tank temp match's the room temp. And if at Any time there is a call for DHW, the on demand will make up the temp difference between tank temp and output temp of on demand?

This actually makes sense. Now how I get my heatpump to work 2nd stage as I have it zoned (3 Zones) via a RobertShaw Slim Zone Deluxe is another fun adventure.


Now what if I could increase my Tank Capacity? With a lot of work.. I can move my hot water tank to the garage (Which will be converted to living space). Would Going to a single 120g dual coil make a huge difference? And would I keep the same 2nd stage heatpump setup?


Thanks so much for your input
Tom

You've pretty much got it- the on-demand makes up the difference whenever the solar is coming up short, but it's always getting warmer water than you'd be getting out of the well or water mains. When heating loads are low the on-demand would use very little power as the solar would be picking up the lions share of the DHW heating rather than just half or so.

If your solar is undersized for the average space heating load there's not a huge amount to be gained by adding more storage, but some. On colder days 100% of the solar is going into space heating, so there's really nothing to store, but your floors will be pretty comfortable (more so in the evening hours than first thing in the AM.) During lighter heat loads you may have sufficient heat to store for warmer floors in the AM, and the bigger the tank, the greater number of days that this would be true. Just as nobody ever complains about having too much R-value, nobody ever complains about having too much thermal storage. If you're using it for both space heat & DHW, 120gallons isn't overkill by any means.

If you're looking to offset significant heating loads heat pump like that will be far more effective than solar, I have to say.

Based on your 26kBTU's/hour heat load and Richmond design temps/heating degree days, it looks like you need about 47 million Btu's per year, or 14 megawatt-hours, if using electric resistive heat, just to heat your house. I.e., you shut off your heat pump and run the radiant exclusively with the Seisco. (Check my math! I'm not an HVAC professional, just like to play with numbers!)

For a 4kW PV system around here in Colorado, it would cost me about $13k out of pocket after all rebates/tax credits (my rebates aren't that different than yours). Here in CO, a 4kW PV system is claimed by the local PV guys to deliver about 6 megawatt-hours/year. Your PV annual output may be more or less than that. Assuming the same, your heating bill goes down to 14 - 6 = 8 megawatt-hours per year.

Rob has said (in another thread) that his Daikin COP is better than 3. If you went with the Daikin, your heating bill goes down to 14 / 3 , or less than 5 megawatt hours per year.

Note that all of the above is in megawatt hours, so it doesn't matter what your electric rate is... Your bill (and CO2 produced, if you are interested in that) will be lower with the Daikin than with PV unless your heating requirement is less than about 30 million Btus/year (the breakeven point between the two, again assuming 6 MWh/year PV production), as the Heatpump divides by about 3 your electric use for heating while the solar PV might subtract 6 megawatt-hours/year from your electric use. The Daikin will probably cost you less out of pocket than PV too, even with all of the PV rebates.

Solar hot water for space heating has the issue of storage... even if you install really big tanks, they will carry you for a day (or two, maybe), not months. Your highest solar output is in the summer, when you need it the least, lowest output when you need it the most. At least PV uses the grid (through credits on your electric bill) as an infinitely large and 100% efficient battery for the times that you are generating more than you're using.

If you really want to go solar PV in the future when budget allows, you'll have a smaller electric consumption to offset with a heat pump in place.

My $0.02,

Jeff

Posted By NRT.Rob on 22 Feb 2011 06:16 PM


If you're looking to offset significant heating loads heat pump like that will be far more effective than solar, I have to say.

Yabbut, that wasn't what he was askin', eh? ;-) 

Clearly the heat pump will be carrying the bulk of the space-heating load until/unless he got serious about upgrading the building envelope.

Whether the PV & thermal solar investment (with or without the subsidy applied) is the most cost-effective way to meet the various goals is also highly unlikely, but again, that wasn't the question.  Active solar heating or PV on a low-performance building envelope is rarely the first-best approach to reducing power consumption or increasing comfort.  You can buy a HELUVA lot of retrofit insulation and air-sealing for the same money- maybe even "chainsaw retrofit" that includes 4" of exterior rigid foam an new siding & windows. ( google the phrase  "deep energy retrofit"  for some good examples and case studies.)

I actually qualified for a VA $2/watt rebate for installed PV panels. So $13k for system, $8k from Va + $3900 federal credit = $11,900 Rebate. That means out of pocket is $1100. Producing about 6MegaWatts/YR. Which gets me 6 REC's which are selling for about 220/each. Meaning my First year, I will pocket $220 + What ever monthly electric savings. Hard to Beat that.

I also qualified for a VA $1/KW for Solar Thermal. Which is free after VA rebate.

Using free stuff is what had me trying to think way way out of the box.


You are right on the money about Heat Pump. If I Could afford the Daikin, its probably the preferred choice.. but trying to source a unit, even if I want to install it myself.. is tricky and still 10k+.

But after lots of talks and math, There is no way to use the free solar thermal cost effectively to justify it.

With that Said, my best bet is to just keep DWH completely separate.

Now I am currently looking into so called "low temp" heat pumps for heating the radiant, and cooling via force air. The only one that ive been able to research is the UniChiller RC. Says it will run to 10F with cop jut over 2. The outside unit I think is $6500 MSRP, plus whatever air handler for AC. The smallest unit is 3Ton, which is oversized I belive as I'm currently running a 2.5t Heat pump.

Any idea's for air/water heat pumps that does what the daikin does but for a whole lot cheaper?


Tom

Wow... you can get 4kW of PV for $13k pre-rebates? The $13k that I mentioned in my post was after all rebates and tax credits.

Are you installing it yourself? I would think that just the panels, inverter, and other miscellaneous components would run around that price, not to mention labor and permit fees.

Also, watch out on the tax credit. My understanding is that you cannot double-dip; i.e., the federal credit has to be taken on your net cost after state rebates. I.e., 30% of ($13k-$8k) = $1500 in federal credits. Check with your favorite tax advisor, of course.

Jeff

in america, there are no heat pumps that do what the daikin does (i.e. none that go to as low of a temp or retain as high of a COP at lower temps). It's pretty much alone in its class.

If the unichiller does what it says it will though, and I have no reason to think it doesn't, it may be a good match for you. You don't get that cold. I have no idea if a DIY install makes any sense though, since I don't know what's involved with the installation of that unit.