I am slowly converting my old 1940 colonial to radiant heat, ( hope to be done this fall )and got to thinking about solar panels as a fuel instead of gas. My roof could accommodate about thirty panels for 8400 watts at about $6500 for just the panels. It would be setup as a grid tie. If I did that I could use an electric boiler at 100% efficiency? If my calc's are close would need about 34K boiler. Minnesota pays back at the user rate but I,m still not sure I would break even on energy use. It's a 2400 sf with soon to be new windows and the exterior will be foamed from the outside as well. Most of my appliances are new and I have slowly been converting my lighting to LED. Haven't looked into incentives yet just thought I would throw it out there and see what you think. Tom

If I did that I could use an electric boiler at 100% efficiency?

It's just not a good match to turn PV into resistance heat. What about a heat pump boiler like ground source if you want to do it all-electric?

Would that work when it gets to 25 below like it did a few times here in Minneapolis this winter? also no ducting in this house and geothermal is out of the question. Tom

If properly designed and installed, it could. Keep in mind that unseasonal lows can go outside of the capabilities of any system.

You say it's not a good match but doesn't resistance operate at 100% ? How would a heat pump beat that?

Instead of generating heat with the electricity, heat pumps use the energy to MOVE heat from outside (usually the ground or the air as a source) to inside. In this manner, they actually have "efficiencies" above 100%, such as 300% or even 500%.

Wow. Will that provide me with the water temps I'm going to need for my radiant?

It depends on what kind of "radiant" you are talking about. Old steam and hot water radiators operated at very high temperatures and it is not feasible to think of supplying them using a heat pump. However, modern methods such as radiant slabs, underfloor tubing and newer radiators and baseboards can work well. Generally, the lower the supply temperature, the more efficiently the heat pump will run.

Posted By layth on 15 Mar 2014 11:53 AM
I am slowly converting my old 1940 colonial to radiant heat, ( hope to be done this fall )and got to thinking about solar panels as a fuel instead of gas. My roof could accommodate about thirty panels for 8400 watts at about $6500 for just the panels. It would be setup as a grid tie. If I did that I could use an electric boiler at 100% efficiency? If my calc's are close would need about 34K boiler. Minnesota pays back at the user rate but I,m still not sure I would break even on energy use. It's a 2400 sf with soon to be new windows and the exterior will be foamed from the outside as well. Most of my appliances are new and I have slowly been converting my lighting to LED. Haven't looked into incentives yet just thought I would throw it out there and see what you think. Tom

The value of solar deals going forward just changed- odds are pretty good that under the value of solar tariff (VOST) approved just this week would pay be BETTER than residential retail:

http://www.ilsr.org/initiatives/energy/

Air source heat pumps with hot-water output are expensive (you'd be looking at 20 grand or so fo a Daikin Altherma, the only one out there with a sub-0F output rating, and nowhere near 34KBTU/hr), but they would use less than half the amount of power as a $1.5-2K electric boiler.

If you're looking for the cushy comfort of radiant floors, an electric boiler keeping the floor at ~72-75F with a floor-thermostat and using cold-climate ductless mini-splits would get you the best of both worlds. The Fujitsu 1.25 tonner (AOU-15RLS2-H) puts out about 15,000BTU/hr @ -15F, as does the comparable and slightly more efficient Mitsubishi MSZ-RH15NA. Either of those would come in at around $4K per, and would give you a seasonal average efficiency of about 3:1 over the electric boiler (1/3 the power use!), and even at -15F would still be giving a 2:1 advantage (half the power use of an electric boiler.) During the shoulder seasons the electric boiler would be delivering a larger share of the load and your efficeincy would dive, but you could also just lower the floor temp setpoint or turn it off.

Dana when you say better than retail are speaking energy prices?

We would agree with Dana’s perspective on this. With regard to electric heating efficiency, YES it is 100% efficient in the sense that all the electrical watts get transformed into heat BTUs with zero transformation loss. However, you also have to consider the efficiency of creating those watts (from burning coal, hydro, PV, etc) and the cost to you in getting them to your house.

Posted By layth on 15 Mar 2014 07:40 PM
Dana when you say better than retail are speaking energy prices?

I'm talking about the residential retail price per kwh that people pay for electricity, which is lower than the actual value of the electricity put onto the grid by the rooftop solar. The new rules mean the grid operator will have pay for the actual value, which on average is likely to be higher than the residentialretail rate. Most of the PV power is being put onto the grid during demand hours, when the wholesale price of electricity is high. By putting that power onto the grid very near the where it is used also reducing the longer haul grid capacity requirements for peak demand. That reduces capital costs for the utility that would otherwise have to upgrade grid capacity, a benefit to the utility that they previously hadn't had to pay for.

With old-school net metering deals the meter just runs backwards when the rooftop PV array is delivering more to the grid than the house is drawing. In that scenario the grid operator is paying the PV operator only the residential retail rate for that power, without regard to the actual spot-market price of electricity during the minutes that the power is being sourced(which is often higher than the fixed residential rate- during peak demand periods can be several times that rate). In the case of the Minnesota VOST there is even some amount of payment to the PV operator for the avoided social costs of the carbon that wasn't emitted in to the atmosphere for those PV-kilowatt-hours that would otherwise have come at least in part from fossil combustion sources.

It's a complicated and constantly evolving calculation that can change to pay day to day, year to year as the generating mix and the amount of distributed generators like rooftop solar goes up, but at least for the next decade or two the value of that PV power to the grid operator is likely to be more than the fixed-rate residential retail price. That means with the utility paying the true value for that power, the financial return to the PV owner will be quicker than under the rough-justice "run the meter backwards" net metering approach previously in use.

This approach to paying for small-scale PV power is going to cut into the profit margins for some utilities, who have been taking a free ride on the backs of the distributed operator, but at some level of saturation of local generation the value begins to diminish, and in super-saturated situations (as with some neighborhoods in Hawaii where the local PV output exceeds the mid-day load for the very-local grid) there is even some additional infrastructure cost to adding more. But that's unlikely to happen anywhere in Minnesota for at least a decade, and by then the cost of grid-storage for managing intermittent renewables should make those capital costs very low. We'll see how it shakes out, but Minnesota is the first state to take this approach on a state-wide basis, which should be a net-boon to solar operators, despite the very low standard electricity prices in that state that might not make financial sense for small scale PV if compensated at only a net-metered rate.

The ink is still drying on the signed documents, to it'll take awhile to see just how rich the VOST is in different locations & utilities, but I'd be surprised if there are ANY locations in MN where it's a worse deal for the PV owner than net-metering.

Layth,
You might want to do a little research on heat pumps. Heat pumps do not need ducts, they do work when it is cold out and they are as ICF said 3-5 times the efficiency of an electric heater.

I think we should be careful when comparing devices that must be quantified in terms of efficiency (e.g., boilers) and devices that cannot be quantified in terms of efficiency (e.g., heat pumps). Heat pumps are quantified in terms of COP since they MOVE existing heat energy and do not CONVERT energy (e.g., gas, electric energy) to heat. So a better statement might be that heat pumps have the potential to cost the consumer significantly less in utility bills than an electric heater. There is nothing more efficient than an electric heater.

Thanks for the detailed reply Dana. I missed the link you gave me. After reading your response and the link I see I would get a better return on metering. As for the heat pumps and mini splits I think it adds to much complication and expense for what I want to do ( in floor radiant ) It still intrigues me that I could heat my house without paying an electric bill. I know I have to factor in investment of the solar and all that goes with that ( grid tie inverter, installation ect ) My best choice I guess would be to stick with a gas modcon boiler and maybe add solar for the rest of my electrical usage. Not so many panels needed.

There's no way you'd be able to cover the heating energy use of a house with 34K of heat load with 8.4kw of rooftop PV without leveraging it with heat pump technology. But with an electric boiler & floor thermostats for the radiant floor plus a couple of mini-splits you'd be able to cover a large fraction of your heating cost with it for an up-front installed cost of about $6-$12K depending on how much micro-zoning you're doing on the radiant, and how much of it you can do DIY.

The installed cost of modulating condensing boiler done right is going to run ~$10-15K or more, so it's not as if you'll be saving any up-front money. (And you won't get any air-conditioning benefit out of the mod-con.)

The annual output of 8.4kw of PV in a MN climate given reasonable shading factors is going to be a bit north of 8,500 kwh (more than 25% greater than the power used by my current family of 3.) Leveraged at 3:1 with minisplits 8,500 kwh is about the same as 900 therms of natural gas burned in a mod-con (which is probably the range you'd be looking at, if your design heat load really is 34,000 BTU/hr).

The panel cost is less than half the total per watt cost of rooftop PV. The Q4 2013 all-in cost pre-subsidy cost in the US averaged $2.59/watt, but it will vary quite a bit by how "hot" the local market is. In Germany (where the market is very hot) it's averaging about $2. There are some markets in TX that are expected to hit $1.75/watt by Q3 2014. But the smaller the array, the higher the $/watt ratio.

WITH subsidy there are many places in the US where the post-subsidy costs are under $1.50, and the downward trends would indicate that even un-subsidised we could be looking at buck-a-watt grid tied PV shortly after (and maybe even before) 2020. The solar market in MN has been pretty tepid until last week, but under the higher return scenario of the VOST 2014 may double or even quadruple the 2012-2013 numbers, which will bring the local prices down dramatically. MN may have some of the crummiest output number per watt of PV in the lower 48 (see: http://www.solarreviews.com/solar-power/how-much-electricity-does-a-solar-power-system-generate/), but at some $/w + subsidy, and with the VOST getting paid back more for your output during the daylight hours when the clearing prices of wholesale electricity are high (and the heating load is low) than they charge you at night when the clearing price of electricity is low (and your heat loads are high) means it's better deal than a simple net-metering situation.

Whether heating with mini-splits is cheaper than gas depends on your actual rate structures for both gas and electricity. I live in an expensive-gas expensive electricity market, and for me it's pretty much a wash. But MN electric rates are about half what mine are, so it could be cheaper to take the mini-split + electric boiler route.

I have been doing most of my own remodeling with some help with demo and I think I can handle most of the boiler install. I can get a Triangle Prestige for $3130 and a boiler panel with expansion tank for $2000 ( two pumps for a two zone. system )probably two manifolds six branch $380. This doesn't include fed or state credits. All my water temps will hopefully be about 100 . I might hire to setup the system but I can handle the plumbing and wiring and intake and exhaust ducting. So hopefully less than ten to fifteen grand. Definitely would like to go for electric bill canceling solar.

Most subsidies for boiler installations requires licensed certified installation to collect. (In some states DIY installation of gas heating equipment is expressly prohibited, but in those same states DIY heat pump installation has similar issues.)

Getting design-day (or even winter-average) water temps down to 100F for a house with a 34K load is going to be tough for a retrofit. Even with extruded aluminum heat spreaders (also not cheap) or and above-the-subfloor approach such as WarmBoard (also not cheap) it could be pretty tough. (With an electric boiler and floor thermostat approach to the radiant you could go suspended tube or cheap sheet metal under-subfloor heat spreaders and a higher water temp with no serious efficiency hit to the radiatn, which would save on installed cost, and assume that the bulk of the load would be covered by high-efficiency heat pumps.)

How are your radiant design numbers shaping up? How many BTU/hr per square foot of radiant are you looking at in your worst-case BTU/ft room, and what are your indoor & outdoor design temps?

I'm working on it, I'm working on it. About halfway through my Beopt calc's inputs. I am using Blue ridge companies above floor plates and plywood rips for my radiant conversion from my old gravity water boiler ( 74 years old ),so I'am hoping for close to Warmboard performance.