Hi all.. I'm in the process of planning a small ICF house with about 800 ft2 plus a 400 ft2 basement. I'm wondering what kind of/how large a system will be required. Would like to do radiant heating (under-floor, will install myself if I can). I'd like to add solar down the road, but not sure if it makes sense when you start adding collectors and storage tanks etc. I could probably get by with a large water heater with flat plate exchanger...or am I wrong on that? Otherwise I was thinking just go with a small tankless boiler for now... Any thoughts appreciated! -michael in WI

In Wisconsin it could go a lot of ways. If the total house is 1200 square feet and your glass is modest, You could be around 15kBTU load here or so. But, you have a serious number of degree days a year. So the answer is not totally clear cut... with less degree days I'd say get a tank water heater with a plate HE, but you might be on the cusp of where a Munchkin T50 or a Prestige Solo 60 with a small buffer tank makes some sense... or skip the tank if you are doing slab floors (over 2" thick, say).

If you can get the load down enough, say you max out your windows and beef up the foam in the walls, solar might not be crazy for a small, very well insulated house. It wouldn't be cheap but it might be achievable.

With all due respect to my illustrious colleague and friend Rob.

A proper Manual ‘J’ heat load analysis is the first step in any heating design (radiant floor, geo-thermal, ModCon boiler, wood boiler, ICF, SIP, log home, or wickiup). Degree-days are really passé.

Tankless water heaters are not designed for heating space (and thus don't perform to the task). Buffer tanks and SS heat exchangers (with necessary trim) cost more than a properly sized condensing boiler with indirect. With a slab, a buffer tank is unnecessary.

Solar is great if you live south of the Mason/Dixon and have more dollars than sense.

I design these systems with a water heater made for heating domestic hot water AND space heating. In more modest, well-insulated homes, the domestic hot water demand often exceeds the space-heating load, and the combi-water heater is really the only logical and cost efficient choice.

Thanks for the replies to both of you...

So what kind of combi water heater product are you referring to? Can you name a product? I assume you're talking about a gas-fired tank with a built-in heat exchanger...? I suppose electric ones (like a Rheem Solaraide) are out of the question. My load should be quite low, with 800 ft2 of space + basement. Walls are effectively R-40, with ceiling R-60....

thanks again!
-m

I just ran into a small house plan that has 877 sq ft of living space, and the customer was told the house had to have at least 900 sq ft. Is this possible?

With all due respect back, degree days analysis is a necessary component of determining what heat source and level of expense is necessary for a given building, along with a heat loss calculation. a 15k load in socal does not use nearly as much energy on a yearly basis as a 15k load in wisconsin. In socal, the efficiency increase may be negligible in terms of energy usage, to trade up to a mod con and indirect makes no financial sense. In wisconsin, not so cut and dry. To say "degree days are Passe" misses a very, very important part of the overall analysis. Fact is, a basic tank heater and heat exchanger is much cheaper than a mod/con and indirect. A big, expensive tank heater maybe not, but one that can meet a 15kBTU load, definitely. You only trade up if you want to "be green" and don't have other upgrades to make or if it makes sense to do so.

Also, in very small heat load situations it is completely possible to offset a significant fraction of your heating load with solar. It is not cost effective at this point without tax credits, but it is possible. And tax credits do exist.

Good points all, though I have trouble getting people to tell me what the cost of available fuels are. Sizing the equipment for the coldest few days of the year (without doubling the number) is more important than "degree days" where system efficiency is concerned. Degree days do not speak to this most important issue and often add to the consumer confusion.

BTUs consumed in a season is certainly a factor in choosing equipment but as you point out, being Green and even comfort have to be taken into consideration. It is really a finer point, best left to experts like yourself.

Still stuck on ModCons, but we agree (as is often the case) a CombiCor is just the ticket for our new friend.

degree days do not affect the efficiency of the unit (though you could look at bin data for that). They affect how much you should care about the efficiency of the unit. You don't buy a race car to go to the grocery store, and you shouldn't feel like you need to buy a mod/con just because it has a certain output rating if your load is small in terms of yearly energy usage. Though it is USUALLY the right choice, we're in agreement there, but in a tiny well insulated home, maybe not.

but maybe. I'm not a fan of the combicor myself, I prefer separate flat plate heat exchangers, but the concept is the same and dropping a pump is nice, I can't argue with that. whether or not it's a good choice, load calc and DD analysis for this home in this climate would answer for sure... or as sure as sure can be, given fluctuation in fuel cost, of course.

To "Smartwall:" ..told by whom...?? The ICF mfg? Local code? I don't have any restrictions like that, as far as I know. I ran the plan past the local inspector...

You are right sir, and astute as ever. But at some point one has to weigh just how much time should be devoted to saving $10/month on the fuel bill.

I love the New Combi-Cor, spec and even deliver many where they are shunned or otherwise unavailable.

Proper system design is still the critical element missing in most DIY radiant heating jobs.

If you don't know the question, you can't get the answer.

I don't think people can fully appreciate the service you provide here and elsewhere. Rest assured, I am your devoted fan (not just for your technical prowess-which is considerable, but for your patience and benevolence which I often lack).

To NRT.Rob and BadgerboilerMN: Great discussion, but I'm low on the learning curve here. Could you dumb it down for me a bit? For ex, what products are you talking about when you say "modcon" and "combicor?" I like the idea of simple (like a big water heater with a flat plate exchanger), but would also like to know what "upgrade" it wouild take to plan ahead for adding a solar panel or two down the road, if not sooner. Again, I plan to do radiant under-floor (not in cement), with maybe a simple radiator or two in the basement (to save money). My builder will be sending the plans to an energy expert for "modeling" so I should have some good numbers for you in a few weeks. Thanks! -m

I design and install systems for living (like NRT Rob). You are at a point where you should have someone do-the-math. You can follow my link for more specific design suggestions, unless Rob has time to walk you through it here.

www.badgerboilerservice.com

Being careful not to "Astroturf". Is that the phrase Rob?

Ok, here is the deal:

you require a heat load calculation on this house to figure out the answers to your questions. This will help figure out if you even want radiant, as well as what class of heat source makes sense for you and your wallet, and how best to work your water temperatures to be "future proof" with low water temperature requirements to allow for stuff like solar.

No radiator is cheaper than some tubing in already insulated concrete though..

your energy modeller will do whole house loads, which is good for yearly energy usage but not so good for zoning, water temps, and installation method decisions. that requires room-by-room calcs.

way to not astroturf morgan ;) isn't it nice not being a "guerilla marketing" slimeball?

I think that's the nicest thing anyone ever said to me Rob... brings a tear.

heheehee

Nice debate gentlemen,
Now for another opinion, Mod con or combi core in a small tight home,
the fellow has indicated a desire for solar, super efficient home (icf standard). I would lean toward a combi core, money saved on the condensing boiler could be applied to a drain down solar system one or 2 panels drain tank and an 72 gallon combi core as storage, seems this would then need a couple 3 way mix valves in the design to prevent over heating on floor and domestic h20, the advantage is the tax rebate on the panel and drain down and labor portion, the simple function of the combi core from the mechanical view will make it possible for the basic plumber to install with design.
The solar will have positive effect even in the worst of climates during many days in the year, certainly domestic in late spring-early fall.
I would be curious from a material view how the 2 systems would price out, with out doing the math and given the tax credit I am guessing they are close. the solar approach might run a couple thousand more but could this not be of set by 20 years of fuel savings?
Dan

not necessarily. the heating system racks up its gains when solar is weakest, so a 20% efficiency jump might make more sense than a solar add. But depending on his DHW usage, you might be right. If it's fairly high the solar might be money better spent.

in the end it's all in the math.

Rob,
as usual you are right However I might add, it is Math plus the unknown of where our energy dollars will be in 10 years, and last and perhaps most important for the current generation to embrace alternate energy forms and display them for out youngster mechanics to ponder.
This is the reality I see, that are we to survive as a whole win or loose there needs to be examples of these different systems presented as normal everywhere.
$.02
Dan

In tiny tight houses with more than one showering occupant the DHW peak load can be several times the design-day heat load, which if un-buffered means you're stuck with an oversized burner and higher cycling losses. Mod-con sans-tank (even with slab buffering on the heating end) will have to be way oversized for the heating load to provide mid-winter showering peak-loads, and you'll have traded in condensing efficiency for cycling losses, and you'll be lucky to get more than 88% average efficiency out of it.

But you can cut the showering peak in half (or more) using drainwater heat recovery heat exchangers, which can reduce the size of the burner required to support the load by ~25-30kbtu/h, even un-buffered, which MIGHT boost you over 90% (or not, depending what the real numbers for the heat load are.)

The size of a solar setup required to support WI heating loads is huge compared to what it would take to get a decent annual fraction of DHW (with or without shower heat recovery), but the DHW load can easily represent more than a third of annual (but not winter) fuel use in a very well insulated very tight house. Integrating the thermal systems to the extent possible tends to increase net operating efficiencies overall. Designing the systems around a multi-heat-exchanger buffer tank to allow both fossil & solar thermal inputs would give you greatest flexibility on boiler size/fuel/efficiency and allow you to max out the solar fraction out of whatever your solar array puts out. Just about any solar tank designed with boiler backup + space heating in mind works. Hacking solar onto a fossil DHW/heating combi tank like a Combicor, not so much. (With the former you can even opt for a small woodboiler, if you so choose.)

If you're serious about integrating solar, it'll be cheaper & more effective to do some of that design work up front, even if you start out with an oversized overpriced hot water tank in the beginning before the full extent of the solar is specified or installed. A small "right-sized" mid-efficiency boiler with a big buffer like a solar tank is going to run only a few percent lower average efficiency than an unbuffered mod-con sized for handling peak winter DHW loads.

Many solar designers seem to prefer using tankless water heaters for backup, (an approach that makes both Rob & Morgan cringe somewhat.) With a big enough buffer and very light heating loads a tankless burner is overkill, but they can be throttled-up or down, tweaking boiler loop flow & output temps such that they modulate around the upper 1/3 of the design day heat load. Since the low end of their modulation range is quite low they can be made to work reasonably with low/moderate heat loads- lower than most mod-cons (or any cast iron) boilers can go. The smallest mod-cons can get you there too, with ~15kbtu/hour outputs on the low end. If used to maintain a tank rather than than heat a slab with 90F water you'll be giving up ~5% of the condensing efficiency, putting them in the same efficiency range as an oversized un-buffered mod-con only a few percent better than a mid-efficiency tankless + buffer, dialed-in to the heat load. In many instances the difference in cost of the stainless steel vent stack vs. a PVC vent will make the tiny mod-con or a condensing tankless a cheaper solution. But a tiny mod-con running a low-temp slab with good condensing efficiency can also be used to back up the tank (at lower efficiency for the DHW) as a "priority zone". The bigger the buffer, the smaller the burner you can get away with- don't rule out a 50kbtu or smaller mod-con if you go with a slab. There are a few out there now lowest modulation under 20kbtu/h (Peerless Pinnacle PI-T50, MUNCHKIN MC-50, Triangle Tube Prestige Solo 60.) But that's still 2x the lowest-output of some of the tankless heaters. If you're not going with low-temp high-mass radiation to get the condensing benefit they'll do little better than a tankess on efficinecy (but will probably have better longevity.)

Tankless hot water heaters are a lot LESS efficient than their EF or steady-state efficiencies lead you to believe when used as hot water heaters. When less than 2 gallons are drawn at a time (hand washing, many/most kitchen draws) their true efficiency is ~50% due to cycling losses. At draws over 5 gallons (like showers/baths) they perform near their steady-state thermal efficiency numbers. EF tests are done with 10.2 gallon draws, which somewhat inflates their numbers. In space heating apps non-condensing versions run near their raw combustion-efficiency numbers, but condensing versions run well below their stated thermal efficiencies (since those hot-water heating derived efficiencies require the heating water to be well below room temp, which would be a helluva neat trick if you could pull it off.) Assume in a well designed system you'd get low-mid 80s efficiencies for a non-condensing tankless, high-80s for a condensing version, when used as solar backup. Used unbuffered as a hot water heater/combi system, a non-condensing tankless will have summertime efficiencies in the mid-to-high 70s, and a condensing version might hit 85, but never 90 no matter what they use for an EF number. Assuming you use at least 30 gallons/day, supporting a low-loss buffer like a solar tank the standard efficiency tankless' efficiency will be somewhat improved, while the condensing version's efficiency will remain about the same.

Efficiency issues aside, don't expect to get the service life out of a tankless that you get out of a mod-con- and DO be prepared to be your own tech (take their technician certification courses if you can, or make sure your heating system installer/designer does.)

In the end Rob is right- it's all in the math. A solar tank is a chunk of change up front, but lowers the size of the burner needed and makes a competent solar retrofit dead-easy. If you're really not sure about the solar committment, there are cheaper ways to go. Design-by web-forum isn't likely to result the the best most-perfect solution, no matter how competent & well intentioned the advice, but it might refine your questions, step up the level of technical discussion with whomever ends up designing the the thing in detail. Setting it up with integrated heat/DWH and set up for solar DHW from the get-go will work better than pieced together Rube Goldberg contraptions & drain-backs (and trust me, I'm the king of Rube Goldberg contraption hackery on this board- just ask Morgan & Rob! :-) There's a reason I know WAY too much about the efficiencies of tankless HW heaters in different operating modes, eh? ;-) )

And for payback on purchased-efficiency on the hot-water heating end, you'll never go wrong on new construction to include a drainwater heat recovery heat exchanger (GFX, PowerPipe, Retherm, etc.), the bigger/fatter/longer the better. If it reduces the size of the boiler so that it runs a higher duty cycle(=higher efficiency), and/or buys you the same solar fraction with less collector area, it pays for itself up front. (They work for shower-flows though, not tub filling, since counterflow heat exchangers need both the drain and the potable water to flow simultaneously.) IIRC Wisconsin has hefty subsidy on 'em, but only for those who heat their water with electricity.

Dana is right as usual, but the DHW can be (effectively and relatively) stored in an indirect water heater matched with a small ModCon, practically GREEN and ...sings like a song!