Dual fuel system

johnnyd

I'm a little hestitant to post here, but let me say that I once was a plumber by trade for about 2 years(but 26 years ago) and am now an "advanced" DIY building a 1600 sq foot house that will be heated via radiant floor. Please excuse some of the gaps in my knowledge. Two reasons for extensive DIY on this project...#1 is enjoyment #2 is accumulaitng sweat equity.

My radiant floor experience so far is limited to a garage/workshop slab, for which the tubing and manifold were installed by the concrete guy and designed by his brother, a plumber. I installed the pump, expansion tank, valves, HWH, etc myself (supplied via one of the Vermont companies)and it works great.

Now I'm involved in designing (with some professional consulting) a low-mass sandwich system with AL plates. Sizing, spacing, etc will be determined via a professionally done room by room heat loss calc. This is a very well insulated and sealed house...R44 ceiling, R23+walls, sealing around electrical boxes, low E window glass, etc.

Here's the deal: My electric utility offers a winter dual fuel rate of 3.5 cents per KWH, but you need an automatic backup. Here's my thought:

Install a properly sized electric boiler as the primary radiant floor heat (RFH) source, and circulate its output through a heat exchanger in a propane water heater (like the Bradford-White Combi-Core). Under normal winter circumstances, the electric boiler would be consuming 3.5 cent/KWH electricity, heating the heating fluid to ~120 degrees for the radiant floor, and also indirectly heating domestic hot water. When the demand for both domestic HW and RFH hot water is great, the propane would kick in to pick up the slack. It also would automatically kick in if peak demand shut the current to to the boiler, and satisfy the requirement for an automatic back up.

So I would be able to heat both domestic and RFH water with relatively cheap BTUs during the winter and also take advantage of quick recovery during a long hot shower...and just shut off the electric boiler (or flow to the RFH system) during the summer and use propane when the off peak rate goes up to 5 cents/KWH. Depending of course on what the price of propane is at any given time.

This would also leave me open to potentially heating domestic hot water with solar panels during the summer.

The questions are:

Is this plan flawed in your estimation?

What would be likely brands of heat exchanger type HWHs?

Would a heat exchanger work in this application? I don't really understand the concept of a "flat plate" heat exchanger.

Thanks very much for your help.

Weezbo

Dude!

your dollar dont speak too loud. bill clintons dollar dont speak to loud dan hoolihans dollar dont speak to loud my dollar dont speak too loud,...however....put them all together hey ! whada ya know!? the electrical co op buys oil and fuel with buncha peoples bucks hence lower rates. you or i migtht be paying 1.56$ a gallon for fuel oil electric co op paying 80 cents. so despite the claim that electric is not competitive with fuel oil or gas W to btu to 1000 ' the little known truth is the flaw is in pricing of the fuel...it blows by the casual observer and they buy into having had some one else do thier thinking for them. i think you think the king thinks....here is where some use of the noggin for something besides a hat rack pays off.supply me with accurate information, i can divine the answer:) You are thinking straight. so here is a clue.

johnnyd

This infomation is not accurate? What else do you need to know? Climate is SE MN. Worst case would be -30 F, desire indoor air temp of ~68 on that day.

Weezbo

hello again *~/:)

like i was saying i dont see any Flaws in your thinking. .. plate exchangers are like seperators seperating a heat source from the"Field" or "demand".unless you have a need for one dont use it... from what you are saying i dont see a need for it. some of this opinion is based on piping some on control. you are going to use some relay to turn off the electric and turn on the combi core right? no problem. to me ,that is an intelligent thing to do. when the relay closes the one and opens the other it acts the same in the field depending on the piping ararangement. i made the inference that is all correct.... the field is on a secondary loop and the boiler and combi core share the primary supplying the heat source.are you concerned at the diffrence in temps from the heat sources? the variable speed injector is smarter than a box of rocks:) it clearly will preform its duties unerringly with proper controls.

Weezbo

oops.

oh ! does the electric boiler have a side arm? oh now i see where you are going with your thinking you have an open system being designed and you dont have the potable water seperated from the heating in the electric side of this equasion. .oops now that Does need some means of delineation.....cut a high temp zone to a plate exchanger to the cold side of the domestic on the combi core coldside of the domestic check against cold potable supply ...now you have seperated potable from boiler water ,you have storage of hot water in the combi core coil,your recircu (instantaneous hot water return line ) could then be given a connection to the cold after the check and it checked against flow also.

Joe F

ETS?

Our company makes a thermal storage furnace that works on a Duel Fuel strategy in a lot of electric Co-ops. Ask your utlitiy if ETS qualifies. If it does, you can use our hydronic unit and indirect for DHW. We also offer an air handler with comfort modulation via hydronic duct coil and variable speed pump that is excellent for air source heat pump applications if you want air conditioning. Might be worth checking out. Propane = BOOM!!!

johnnyd

Thanks for your input.

Actually much simpler than what you're suggesting:

The PoCo shuts down just the designated heating circuit(s) via radio signal whenever they sense "peak demand". The usage on that circuit(s) is metered separately and at the much lower (3.5 cents compared to 8.7 cents)rate. The shut down usually occurs right during and after a cold snap in winter or during extended hot summer weather, when the peak rate is 5.0 cents a KWH. They supply the equipment and metering stuff for a nominal fee. The consumer either gets an electrician to hook it up or (in my case) wires it themselves. In order to put this system in you must have an equivalent back up system, with non-electric fuel that will fire up when they shut you down.

So what I'm thinking (and I ran this by thier engineers)is that I will normally perk along, heating my floor with the electric boiler at 3.5 cents a KWH, and my DHW with a propane fired HWH, with the initial output of the electric boiler routed through the heat exchanger coil in the HWH. Would also incidentally heat alot of DHW with the probably cheaper PoCo btus. So the DHW and RFH WILL BE SEPARATE. When the PoCo reaches peak demand and shuts down my boiler circuit, my circulator(s) will still be satisfying thermostats' call for heat, and begin to ciculate cooler and cooler water through the HWH, until the HWH fires up and brings everything back to proper temps.

So I would get:
>qualification for the dual fuel rate
>redundancy in case either source failed
>extra heating capacity in case (lord forbid!) the electric boiler ends up being under-sized for some -40 degree day
>options to use wood-fired boiler or solar panels if and when appropriate...probably pie in the sky but thought I would throw it in.

Now how does that sound?

So really the only difference between this and separated RFH and DHW syatems is that the output from the RFH syatem goes through the HWH via a heat exchanged coil.

johnnyd

Thanks, I'll do that.

The guy at the utility mentioned some storage device that uses ceramic bricks to get you through a peak demand period..is that what yours is?

Weezbo

I like it!

really simple is the way to cruise.let me ask why propane? it is les amp draw on back up system electric?

Joe F

Yep

That's us. Check it out.

johnnyd

Maybe I've just got an attachment to propane/natural gas HWH..much faster recovery less problem with liming up, need propane for cooking anyway, and who knows? Maybe propane will someday be cheaper than electric. Guess I want to keep my options open if possible. Don't want to be hostage to any one source if not necessary.

scrook_2

If you can...

get electricity for $0.035/kW-Hr it is like burning propane at $1.17/gal at 80% or at $1.04 at 90% eff. or oil at $1.68/gal at 85% so it's a deal. At $0.05/kW-Hr the prces are 43% higher, at $0.087/kW-Hr, you'll go broke. The question is how often in a typical winter will you need to switch to fossil fuels, and does the extra cost of service enterance, wiring and electric boiler get offset by the savings in fuel? It might be an attractive deal, even if you use fossil fuel for HW recovery. If the electricity is mostly fossel fuel generated the pricing should tend to follow oil or LP prices, the bigger risk is that the utility changes their pricing plan for interruptable/non peak service, but that seems to me to be low risk -- it's peak demand that kills them, they have to bring inefficient secondary generation on line or buy on the open market.

Assumptions: oil at 140,000 BTU/gal, LP at 91,500 BTU/gal, compustion efficiencies as noted, and 3413 BTU/kW-Hr.

In any case, you can switch to fossil fuel any time or to electricity any time except peak demand (cold snaps etc.)

A flat plate HX (the blue thing) is just a stack of flat water flow paths alternating between the heat source water and the heat sink (load) water, smaller than a coil in a tank or a tube bundle in a bigger tube (grayed out). Either way there is a sourse water inlet and outlet and a seperate load water inlet and outlet. It looks like a metal brick w/ 2 threaded fittings on one side, sort of.

johnnyd

Hmmmm

When I figured it, I came up with more like 3.5 cents was equivalent to 70 - 80 cent a gallon propane. I contracted for propane last fall @ 97 cents a gallon. Do you have a link to a real accurate BTU/fuel type conversion site?
That will make a difference in which way I go.

For a flat plate HX to work, doesn't there have to be flow both ways...from both the primary and secondary? Don't know if I used those terms right, but know what I mean?

Joe F