Hi All, This forum is full of people far more knowledgeable than me, so I have a question. Is it feasible that a solar hot water system, using a tankless gas water heater for backup (or something else), could provide all of the domestic hot water [u]and[/u] radiant floor heating needs (radiant in 1620 sq ft slab only) for a home in Nebraska? I'm sure there are some important variables I haven't addressed here, so feel free to fire back some questions.

Thanks

Mike S

It's been done, many times, but it takes a good solar designer to come up with the most appropriate price/performance for the situation. If it's new construction, you're generally better off improving the thermal envelope and managing passive gain than going with full-on active systems sized for the larger heat load of a lossy house. Peak heat loads in Nebraska can be fairly high during cold snaps but the total annual heating-degree-days aren't huge. For any reasonably insulated 1600' home the peak output of a whole-house tankless will be several times the peak heat load, and probably 5-10x the mid-winter average (even in North Platte/Valentine.)

If it's existing construction and the slab doesn't already have at least R10 of insulation under it, unless you have the headroom to add that much and are willing to raise the floor, cabinets, plumbing, etc by ~3", to accomodate the insulation, radiant floor may not be your best bet, since your losses to the ground will be large when you need to keep the slab at 75F instead of 60F (or whatever it normally falls to, mid-winter. But air-handler coils or hydronic baseboards can work well at domestic hot water temps too, when properly designed. (Slabs generally run at temps much lower than DHW temps, and the dual temperature requirements adds a bit of complexity to solar DHW/slab-heating combi systems.)

There's very little difference in cost (if any) between the smallest condensing boilers and a condensing tankless, and in a dual-temp system with ultra-low temp radiation like slabs it's probably both easier & more efficient to use a boiler in parallel with the solar space-heating system, since both will work significantly more efficiently at lower-than-DHW temps. Condensing boilers come with controls (like outdoor reset) that the tankless heaters won't have, which will make it easier to tweak the efficiency out of it. Solar is expensive, and running the space heating at DHW temps will be a 15-20% performance hit (meaning, you'll have to add at least 15% more panel area to pull in the equivalent BTUs.) Since space heating is typically more than 4x the total thermal load of a house in 6000+ heating degree-day climates, that can add up to a $ignificantly up-$ized $olar $ystem, that is only used 6-8 months of the year, and in summer you'd need to dump heat to keep it from boiling over.

At current natural gas prices it's hard to make an economic rationale for active-solar space heating without significant subsidy. But if you're talking about going with a standard solar package sized primarily for DHW and using it's back up system for space heating, it can be more reasonable. A typical DHW system will have 50-100' of solar panel, whereas in most of Nebraska in non-superinsulated houses you'd be looking at a minimum of 250' of panel to get significant space heating out of it. Unless your walls have have a clear-wall R-value of R25+ it could take a larger panel array than the size of your S-facing roof pitch to get as much as 80% solar-fraction on space-heating.

Bottom line: Don't just hack at it, design it (or hire a solar expert to design it for you). I'ts feasible, yes, but only if you do your homework. Unless it's extremely well insulated you'll get more fuel savings out of improving the thermal envelope than going with bigger solar. If the projected gas use for the house pre-solar is already under 400 therms/year you can put a big dent in that with reasonable-sized solar. If it's well over 1000 therms, fuggedaboudit- the system required wouldn't even fit on your roof, (and for the diner it would set you back you could probably super-insulate the place, heat it with a cheap gas-fired hot water tank and a heat exchanger.)

Dana1,

Great advice. Your expertise is truly appreciated. Here are some basic specs on the home:

• New construction
  
• 1-story plus basement with three bedrooms
• Super-insulated
• Passive solar
• Want radiant for IAQ (family with severe dust allergies) and comfort (4-1 female-to-male ratio)

I agree with not hacking it. I'm working with someone here, but trying to educate myself along the way. Do you consult?

Thanks again

Mike S

Hope you have 5 bathrooms with that ratio. I have a 2:1 and my current 2 baths is not enough. My home under construction has more than enough.

DO look at it from a PassiveHouse perspective, calculate the relative cost of more insulation vs. more solar, and I think you'll find the right balance. see: http://www.passivehouse.us/passiveHouse/PHIUSHome.html In general, superinsulation is much more cost-effective on new construction, and tends to be more cost-effective than active solar. Pay particular attention to foundation & basement slab insulation, which so often gets ignored, and becomes a major fraction of the heat load when the rest of the structure is highly insulated.

If you go superinsulated, you may find that complex heating systems like radiant floors go away, and that money can be applied to the building envelope. Indoor air quality is controlled by active heat recovery ventilation (in much of Nebraska you may be able to utilize earth-tempering of ventilation air to good effect unlike more humid parts of the US.) When super-insulated, your largest thermal load will be the domestic hot water, and you may be able to use it for the now very modest heating loads as-needed. The PassiveHouse approach tends to be electric resistance elements to elevate the temp of the ventilation air, but that could as-easily be a small hydronic coil running off the thermal storage for the hot water with a sub-100W pump instead of a 1000-2000w resistance element. When the loads are low enough and the storage large enough the size of the tankless required to back it all up shrinks dramatically too- instead of a 150-200KBTU "whole house" tankless, you could run it all off a tiny sub-100KBTU point-of-use models at a fraction of the cost, and if you're burning less than 100 therms/year there's little point in going with 90%+ condensing efficiency vs. a cheap 80% model. Just be sure to avoid standing-pilot ignition, which would use as much or more fuel than just heating the water in a DHW backup situation. (There are several models out there that use water flow powered magnetos for ignition, which requires no wiring to the unit itself. A bronze pump driving the loop under aquastat control provides the flow. )

Air flows on heat recovery ventilation are a tiny fraction of typical forced hot air or conditioning- it won't be kicking up any dust, and it has to be very quiet to even hear them cycle on in most cases. By building the house extremely tight and actively ventilating, you're guaranteeing that the vast majority of air entering the house has passed through a filter on it's way in, and only the amount air necessary to control indoor humidity & cooking exhuast, material outgassing, etc is allowed in. It's often touted as "the cure" for allergies. If you keep it under 50% relative humidity dust mite populations die off. Keeping it over 30% most of the time is important for comfort & virus-transmission issues, etc. For most of the year you can control the indoor RH by controlling the ventilation rate of the HRV using de-humidistat control alone, but you may still need compressor-based dehumidification/air conditioning on some days in the summer.

To further downscale the size of the solar, look into using as large a drainwater heat recovery heat exchanger. With a full basement you probably have the headroom for at least a 4"x48" unit on the main drain to the house, and if located next to the hot water system and water feed to the house, it's dead-easy to plumb. (It's often messy/impossible as a retrofit.) With 5 people taking daily showers, the payback will be good any way you slice it. The hardware cost for even the largest tends to be less than a single 32' solar flat panel, and it'll recover ~50% of the heat used in showers (which tends to be ~40% of DHW use, but could be more in your case). If it then means you can scale back the size/cost solar array by 25% (likely) you're money ahead up front, and the operating costs of the backup systems are the same or lower. Most solar designers are on top of this, but I'm throwing it out there just in case...

I consult, but not on this stuff (I'm an electrical engineer- I design electronics for a living.)