We are planning a new construction (3000+ square feet, two story) in Silicon Valley that will have radiant heating.
We are looking at ductless mini-splits. Do you have any comments on these or other options?
Winters are mild. Lows in high 20's. Summer's are mild too. Highs in 90's.

Thanks.

Look at Fujitsu, Mitsubishi, Daikin and LG in that order.  I believe Fujitsu makes a lot of the parts for the other brands.  I believe Daikin is the only company that I know of that will quote their failure rate.  With most companies this is a closely guarded secret.

The part-load efficiency of mini-splits are extremely high in both heating & cooling mode. In silicon valley it's likely that heating with the mini-split will be more cost effective (if not as cushy) as radiant heat, and comparable to (or higher efficiency than) heating with ground sourced (geothermal) heat pumps if you oversize it by 1.2-2x for the peak loads.

In a 2 story you may need a 2-head multi-split or 2 mini-splits to get reasonable balance between upper & lower floors. In heating mode convection will distribute some of the heat from the lower floor to the upper, and conversely in the cooling season. The cooling load of the upper floor will generally be higher, and if it's used only for cooling you may be able to get away with a single-head strategically placed in the upper floors. A room-by-room heat loss/gain calculations that include solar gains will be key to getting it right.

You may want to look at using the Daikin Altherma air source heat pump for doing the radiant, using it for cooling and heating your hot water too. It uses the same compressor technology as their mini-splits, and can be cheaper to operate than condensing natural gas in a climate like yours. It's more money than a mini-split or a boiler, but is likely competitive with the combined up-front costs of multiple systems, and cheaper to operate overall. The very low latent cooling loads (humidity) of Silicon Valley means that you may be able to use the radiant floor for sensible-only cooling, without needing to add a mini-split type blower head in multiple zones. Summertime dew points only rarely break out of the low-60s F, and a single 70-pint standalone dehumidifier would lbe able to keep up with the peak latent loads on that day or so per year when the dew point breaks 70F. This all needs to be designed for the actual loads though- there's no substitute for doing a Manual-J type analysis up front before committing to any heating & cooling system.

I am building what sounds to be a similar thing, but up here in WA state, North of Seattle.

We are doing a passive solar using the Daikin Altherma for radiant heating of the main floor and bathrooms and cooling of the solar areas. We went long on the passive solar, so the chilling function of the Daikin is important in keeping the slab temperatures down during Summer.

We are using mini-splits on the second floor, where the bedrooms can have individual control over heating and cooling. As Dana mentioned, strategic placement of the minisplit heads can help. For example, you could place a head in an anteroom for the Master Bedroom where opening one door could send the cool air into the BR for the night, or another to send it into the home for the daytime.

Thanks for the comments. Are you (Dana1) saying that if we have mini-splits (cooling & heating) then we may not need radiant heating.

Just yesterday, someone told me that during the summer, when the radiant heat is not needed, they hook up their radiant heating system to the water source and run cold water through their slabs on the way to the garden or the lawn. They called it poor man's slab chilling and they claim it works wonderfully to cool the place down.

Perhaps it is ok if your tubing goes through the slab. We are thinking of using wooden sleepers and aluminum fins on top of the raised subfloor. Humidity/condensation could be a problem. Talked to various people. But we are not getting any definitive answers about using the radiant heating system for cooling.

Posted By lm on 12 Jan 2012 01:48 PM
Thanks for the comments. Are you (Dana1) saying that if we have mini-splits (cooling & heating) then we may not need radiant heating.

Yes. 

Mini/multi splits aren't as cushy as radiant floors when padding around in bare feet on cool foggy (or even frosty) morning, but they're quiet and efficient, and modest in cost.  If you're willing to pay for that extra bit of comfort, the Daikin Altherma air-source heat pump built on some of the same technology may prove cost-competitive as a total solution for heating, cooling & hot water too.

Many people in cooler foggy-dew western WA (US climate zone 4) are heating exclusively with mini-splits and achieving geothermal-like performance.  Even in zone 5  (colder still) mini-splits are often able to handle all but the 95th percentile extremes, with average coefficients of performance  (COP) of ~2.5 or better.

Silicon Valley is US zone 3, and if you build a reasonably tight house even at code-min CA Title 24 insulation levels your peak heat loads will be will be within mini-split range, and if you slightly oversize it for the peak your average COP will  be in the mid to high 3s, maybe even 4 during the heating season.  Oversizing a continuously-variable mini-split by 1.5-2x for the peak load ensures that it always operate in the lower-speed end of it's range, adding more than 0.5 to the average COP, which is a big uptick in efficiency.  It's likely that sizing it perfectly for the peak air conditioning load would put into something 1.5x the design condition heat load.  When in doubt, upsize it a half-ton or more.  If your heat loss calc comes up with 20,000-24,000BTU/hr , get a 30,000-36,000 unit.  The extra grand or two in upfront cost will be paid back in higher efficiency in short years.   Being fully modulating systems with at least a 3:1 turndown ratio they won't short-cycle at part-load even if oversized by 3x for the peak, but there are rapidly diminishing returns on anything more than 2x oversizing.

Posted By lm on 13 Jan 2012 06:12 AM
Perhaps it is ok if your tubing goes through the slab. We are thinking of using wooden sleepers and aluminum fins on top of the raised subfloor. Humidity/condensation could be a problem. Talked to various people. But we are not getting any definitive answers about using the radiant heating system for cooling.

The condensation potential is something that can be designed out in your climate, due to the relatively modest summertime humidity.  In order to have condensation the surface of the floor needs to be below the dew point of the air in the room.  If you use heat recovery ventilation, the indoor air's dew points will track that of the outdoors.  The mean July-August outdoor dew points for Santa Clara are in the mid-50sF, and only occasionally rise to the mid-60s or higher.  If the floors need to be below 60F to keep it under 80F indoors you've mis-designed the building envelope for R values, mass, and summertime solar gains.  I suppose that's possible at Title 24 minimums, but you don't HAVE to screw it up, en? ;-) 

Avoiding big unshaded windows on the SW and W sides, and paying attention to roof angle/orientation/colors and you should be able to get your floor-temp requirements well bounded.  Going with a combination of radiant ceiling & floor would be more effective for both heating & cooling, but probably isn't necessary if you just tweaked the house design relative to the site & orientation.

In a tight house a single $250 70-pint room dehumidifer is enough to handle the peak latent-loads for the whole house in Silicon Valley if you dial back the ventiltion rates when the outdoor dew points are in the high-60s.  A tiny 3/4 ton minisplit as supplemental cooling to the radiant would also take the edge off if it starts to get sticky inside, and may well be worth having, even if you use the radiant floor for cooling as well as heating.  Many have a "dehumidify" mode,  to run for optimal latent-cooling efficiency with minimal sensible cooling, if for some reason it isn't condensing enough in standard cooling mode.

Posted By Dana1 on 13 Jan 2012 10:34 AM

. . .

Silicon Valley is US zone 3, and if you build a reasonably tight house even at code-min CA Title 24 insulation levels your peak heat loads will be will be within mini-split range, and if you slightly oversize it for the peak your average COP will  be in the mid to high 3s, maybe even 4 during the heating season.  Oversizing a continuously-variable mini-split by 1.5-2x for the peak load ensures that it always operate in the lower-speed end of it's range, adding more than 0.5 to the average COP, which is a big uptick in efficiency.  It's likely that sizing it perfectly for the peak air conditioning load would put into something 1.5x the design condition heat load.  When in doubt, upsize it a half-ton or more.  If your heat loss calc comes up with 20,000-24,000BTU/hr , get a 30,000-36,000 unit.  The extra grand or two in upfront cost will be paid back in higher efficiency in short years.   Being fully modulating systems with at least a 3:1 turndown ratio they won't short-cycle at part-load even if oversized by 3x for the peak, but there are rapidly diminishing returns on anything more than 2x oversizing.

Dana1,

I hear you on increasing the COP during the heating season but how efficient will the Daikin Altherma be while it is in the cooling mode?  I ask because if the cooling mode is nearly as efficient as a water-to-air heat pump, then I may have had my last well drilled.  Will the Altherma short cycle in cooling mode and leave my clients feeling sticky from all the humidity still in the conditioned air?

Posted By Dana1 on 13 Jan 2012 10:58 AM

Posted By lm on 13 Jan 2012 06:12 AM
Perhaps it is ok if your tubing goes through the slab. We are thinking of using wooden sleepers and aluminum fins on top of the raised subfloor. Humidity/condensation could be a problem. Talked to various people. But we are not getting any definitive answers about using the radiant heating system for cooling.

The condensation potential is something that can be designed out in your climate, due to the relatively modest summertime humidity.  In order to have condensation the surface of the floor needs to be below the dew point of the air in the room.  If you use heat recovery ventilation, the indoor air's dew points will track that of the outdoors.  The mean July-August outdoor dew points for Santa Clara are in the mid-50sF, and only occasionally rise to the mid-60s or higher.  If the floors need to be below 60F to keep it under 80F indoors you've mis-designed the building envelope for R values, mass, and summertime solar gains.  I suppose that's possible at Title 24 minimums, but you don't HAVE to screw it up, en? ;-) 

Avoiding big unshaded windows on the SW and W sides, and paying attention to roof angle/orientation/colors and you should be able to get your floor-temp requirements well bounded.  Going with a combination of radiant ceiling & floor would be more effective for both heating & cooling, but probably isn't necessary if you just tweaked the house design relative to the site & orientation.

In a tight house a single $250 70-pint room dehumidifer is enough to handle the peak latent-loads for the whole house in Silicon Valley if you dial back the ventiltion rates when the outdoor dew points are in the high-60s.  A tiny 3/4 ton minisplit as supplemental cooling to the radiant would also take the edge off if it starts to get sticky inside, and may well be worth having, even if you use the radiant floor for cooling as well as heating.  Many have a "dehumidify" mode,  to run for optimal latent-cooling efficiency with minimal sensible cooling, if for some reason it isn't condensing enough in standard cooling mode.

Thanks for the info. Could the humidity caused by radiant cooling cause any damage to the subfloor/joists or under the floor?

Radiant cooling is very expensive to install. The dewpoint is critical. Infiltration rates and exhaust rates must be controlled precisiely. Floor covering materials must be wisely chosen.Instead of radiant heating and cooling , and using minisplits, I would seriously look into VRF systems.Mitsubishi and Sanyo (now Panasonic) make such systems.By carefully zoning and advanced inverter technology these systems could easily give you high efficiencies and the same comfort with a lot less headaches and less future problems.Anytime you deal with water there are maintenance issues.

Radiant cooling doesn't cause humidity- it just doesn't remove humidity the way ducted & ductless force air cooling does, which means you would have to control the humidity separately. But in the relatively dry air of Silicon Valley that's neither difficult nor expensive.

If you limit the low-temp of the floor to 55F with a floor thermostat and keep the interior under ~50% RH you won't have condensation at the floor unless you had a lot of carpeting (which would be bad for the efficiency of radiant heating anyway.) Many people normally keep the interior air under 50% to eliminate dust-mite allergies issues anyway. At 78F & 50% RH the dew point is 58F (3F above the floor limit), but a 55F floor is already about as cool as you 'd want the floor to ever be for bare feet, and if it's not keeping up the the sensible load you be turning up your other air conditioning (mini-splits etc.) anyway, which will further reduce the humidity in the air, lowering the dew point. It takes a number of hours of operating 3F below the dew point to end up with visible water on the floor, or significant adsorption of moisture into the wood, but if you want to be ultra-safe raising the low-limit of the floor a few degrees or reducing the upper bound on the relative humidity of the conditioned space air to 40% is an option.

Fortunately in Silicon Valley most of the summer humidity can be adjusted by the ventilation rate, since the average dew point of summertime air is in the mid or low 50s. But during the most-humid days you'll need other mechanical systems for drying the ventilation air. This is in sharp contrast to the eastern half of the US where summertime dew points are SO unfavorable for radiant cooling that it needs a lot of special care & attention. The average summertime dew point in Boston is in the mid-60s, and the further south one goes along the eastern seaboard the higher those dew point averages are, making radiant cooling an expensive proposition of dubious value. In Boston the sensible (temperature-only) cooling loads aren't all that high and the latent loads (humidity) are the lions share of the total load.

In Santa Clara the opposite is true, and in a mid-sized home a portable 70 pint room dehumidifier set up to drain automatically can keep up with the latent load for the whole house as long as you aren't over-ventilating during the most-humid days. Moisture problems affecting wood are all about the averages. The summertime outdoor dew points in your area are ~55F. If you use that as the low limit for the floor temp, the average temp of the floor will be much higher, and even if the room air dew point exceeds that at times, any moisture adsorption accumulated during the day under a high air conditioning load will be released during the periods of lower sensible load when the floor temps are higher (literally most of the time, even on peak cooling days.)

Whether you can meet the entire sensible load with a chilled floor at 55F+ temps depends on many factors, but primarily it's about unwanted solar gain which can be designed out to a large degree. But if it can't, A: the supplemental air conditioning will dry out the air for you, and B:The cool floor will still provide a large comfort benefit, if not quite as much as a chilled ceiling might.

The cooling effectiveness of chilled floors is less than that of chilled ceilings or walls, since the cool air will pool at the cool floor rather than continuously convect, and the cooled air film insulates the floor from the rest of the room. (It doesn't take much of a ceiling fan to break that up though.) Unless you design & simulate carefully I wouldn't count on it for much more than 2/3 of the peak loads, even if it does a pretty good job on the average load, but the actual amounts will be very house & system design specific. I only throw it out there as a useful adjunct to using an air source to hydronic heat pump (like the Daikin Altherma) for at least a good portion of your sensible cooling. Since you would already have the radiant floor tubing in place, the cost adder for the cooled floor option is fairly small.

To make the radiant cooling the total solution would be more expensive and more design input than a separate systems approach, but in your climate the moisture accumulation risks are low as long as you don't try to hyper-chill the floor just because you CAN. Even a 60F floor would provide a goodly of the sensible cooling, and would have very few condensing hours over a summer for you, but would have nearly 100% condensing hours were you anywhere on the eastern seaboard of the US, and would leave the home muggy/clammy even if it kept up from a temperature point of view. If you lived in a more humid climate I'd be very much advising against using it.

Thanks Dana1 and others.

My choices are:

1. Just in-floor radiant heating & cooling

2. Just mini-splits for heating and cooling

3. In-floor radiant heating and mini-splits for cooling/heating (expensive)

4. Install in-floor radiant heating & cooling during construction. Observe for a year and then
    install mini-splits if needed.

Can you please help me decide? Thanks.

I lived in the silicon valley for 15 years. It's quite comfortable. I believe you'd love the radiant floors in your climate so I'd choose your #4 but I'm not certain you'd have to chill the floors. On the second floor, I'd put in a mini split because it can get a bit hot with the island effect of the other buildings from what I remember. I think the experts on this site (not me) seem to like this Daikin unit which sounds like could also heat your hot water. Did you decide if your going to do slab on grade?

Posted By Roundeye on 21 Jan 2012 12:26 AM
  ..... Did you decide if your going to do slab on grade?

No slab. We will have crawl space. Thanks.