I'm trying to come up with a strategy for integrating radiant heat in slabs and domestic hot water for a new house in Rhode Island. Total conductive (excluding ventilation and infiltration losses) heat loss on a 5* design temp is 18,000 Btu. This is so low I'm struggling with how to handle it. Before everyone says mini-splits let me say my power costs $0.58/kWh so I'm not going to anything with a compressor. I am also considering some thermal solar so and wondering how to tie together a propane or oil source and radiant and domestic demand. Would using an Ergomax as a buffer tank be a direction to consider out of a short cycling concern? How about a Polaris water heater as the sole source? How can the solar be integrated into the system? My fuel options are propane and oil and per 100,000 Btu propane is about double the cost of oil (elaborate reason as to why but trust me - it is). Propane has lots of advantages but it's hard to get past that cost difference. I'd like to get ideas for an overall approach.

A water tank allows low btu output. At those prices, I'd use a diesel generator (running on oil) and recover the heat from it.

if your pricing is that far apart, then probably a small oil boiler with proper control and a buffer tank is called for. or perhaps a tank water heater with an oil burner like the Bock.

I'm thinking about using as small an oil boiler as I can find and that would heat water in a buffer tank (like an Ergomax). The radiant would run off the boiler water that is in the Ergomax tank and domestic HW would come from the heat exchanger coil within the Ergomax (sort of the reverse of a typical indirect DWH). That part works - what I can't figure out is how to tie some solar into this system. Perhaps a second tank dedicated to solar? This could be heated with a coil within it as part of the glycol loop from the panels and the water in the solar tank could be preheated there before going through the Ergomax coil. That takes potable water from the well, through the solar tank, through the Ergomax coil to the faucet. That could work, too. But then how can I extract heat from this setup for the radiant heating? And how do I dump excess heat in the summer?

you would want a second tank, otherwise you have to maintain the tank temp at a temperature that would basically kill your solar contribution utterly.

for this load I'd strongly consider the bock with heat exchanger, and use a solar tank separately. tempering valves can be used to "choose" solar water, oil water, or a blend for DHW. Solar water for space heat is typically not worth thinking about unless someone is giving you free panels or something.

I'm confused as to how all that goes together. I understand why having a high temp in the solar tank kills the solar production - that's why I was thinking of the solar as a "preheat" to get well water for somewhere around 50 up to say, 80 or so. How would the oil water heater be piped in with a heat exchanger? Would the water in the water heater tank be DHW or would it circulate through the radiant? Thanks for sticking with me on this...

the water in both tanks would be domestic with the solar tank either preheating the bock and/or allowing for summer bypass of the bock entirely. that can be automated with appropriate tempering/diverting valve placement so the solar meets DHW by itself whenever it can.

the heat exchanger splits the DHW water from the heating water.

IF you are going with a small oil boiler, get a very efficient double HX tank like the Viessmann tanks which have larger HX than most. Boiler pump into the top HX, solar to the bottom and add a "fresh water" station to the top of the tank (see below). The tank is full of dead boiler water and the reason to keep the boiler separated from the floor heat loops is to, as discussed, allow the solar to do its job seeing as much cold water as possible. Not having the upper HX (and just having ports in the tank) will allow the water to mix too much and destroy the stratification. I can make a drawing if you PM me. This can all be done with one 100-120gal tank.

http://www.paw.eu/en/products/solar-thermal-engineering/frischwasserstation.php

you CAN do that, but you are still killing your solar input for about half of your tank volume. I really disagree with maintaining even portions of solar tanks with other heat sources, unless you really aren't doing much solar in the first place.

Not in this case because you put in a bigger tank than you would with just solar and because the HX are separate, one doesn't affect the other nearly as much. This kind of tank is a standard in Europe and they work very well.

But two tanks provides better isolation and solar efficiency.

you put in smaller than 100 gallons tanks with solar typically? have to be a pretty small DHW requirement to make that make sense, wouldn't it?

"you really aren't doing much solar in the first place" A thread of truth is sticking out of this thread...

Posted By BadgerBoilerMN on 14 Dec 2012 09:17 AM
"you really aren't doing much solar in the first place" A thread of truth is sticking out of this thread...

who are you referring to?

Few can seriously consider space heating with solar, especially if they live in R.I. But you knew that.

We design multi-fuel hydronic heating systems all over N.America and recognize that the local energy market must dictate the fuel source for all but the wealthiest of our clients. Oil is the obvious answer here. Pursue it without guilt. Economy is the essence of GREEN living, since waste is the antithesis of sustainable energy.

I'm all for getting off oil. And I don't understand the fuel pricing issues here, sounds like an island or something but then oil should be just as expensive as propane.

I think Badger's off a bit on what is "Green"... there is nothing green about oil, and there is nothing inherently green about cheap, that's "greenwashing" of the worst kind. but propane isn't so much better that it should be considered worth doubling your costs, and if electricity is that expensive it's probably diesel or some other dirty source as well, so "green" here would be either solar ONLY or something like wood.

to really heat with solar... which, maybe at these prices would be worth pursuing after all... you would need very large tanks and very low temperature heating. Birdman, how much is oil for you?

Green is as Green does.

If fuel oil is the cheapest fuel and not GREEN enough for you than insulation is the only reasonable alternative.

If you must get GREENER still, grab your sweater and stocking cap, communal bathes, no wait forget the bathes...

A condensing propane boiler lower NOx and SOx and be as GREEN as you can get on fossil fuel, but the market is driven by desire not economy. People generally want to be comfortable without severe sacrifice. There are resources invested in every technology but the cost/benefit from raw materials is rarely accounted for.

he's got an 18k design load. I'd say he did alright on the insulation.

no doubt you can reduce your impact by making lifestyle changes. I don't see the point in bringing up straw men though and acting like if you ever take showers you shouldn't consider what's green... if birdman lives in an area with 0.58/kwh electricity then perhaps simple price isn't everything to the fellow. If it was, he'd live somewhere else.

fact is if fuel costs are really high on a really low load like that solar may actually make a dent in an economical way. maybe not. but it doesn't sound like a standard analysis which would just say "no, don't think about it". if oil is double normal cost it probably is worth seeing what we can do with the sun here.

PV and heat pump MAY be better if there is net metering, but pricing like that would indicate, probably not, if he is indeed in diesel generator territory. at 0.58/kwh electricity PV probably makes incredibly good economic sense anyway just for the plug loads. We are meeting a heat load almost twice the size of birdman's, plus significant cooling and office computer/light usage, with a heat pump and a 14kw array... that's big, true. but again, depending on actual fuel costs and whether net metering is an option it may make some sense here. If his house is passive solar oriented, doesn't need cooling, etc... a much smaller array could suffice. If he has concrete mass, so he can time shift demands, that would be even better. PV plus air to water heat pump with oil backup may in fact make the most sense.

lots of possibility. it'd take a little real digging to see. first point of info to get is oil pricing though. if it's basically standard NE fuel pricing then yeah, probably sticking with oil is the way to go. if it's double, maybe not.

The answer is clearly a properly sized oil-fired 'combi' water heater with sub-system to separate potable from space heating. If you want to pile in solar after-the-fact, it's all good.

Doing economically inefficient green things leaves you that much less money to do efficient green things. So I believe in carefully looking at ROI (in terms of dollars vs benefits).