OK already, reviewing Psychrometrics 101. Working on a tricky radiant ceiling design and you guys are distracting me. Thinking Yakima, WA and evaporative cooling now.

Yes, I will design a radiant slab, but would prefer radiant ceiling for cooling since I have perpetually cold feet:-).

Sorry Rob, you were right, as usual.

I looked at the dew point graph. for a month (july) dew points would regularly cross over the threshold required for radiant floor cooling to be efficacious in most cases (about 55-56 degrees typ). His details may vary but he is definitely going to need some dehumidification capability on this system unless he's ok with just shutting down the cooling for several hours a day.

If you are only looking at surface temp of the slab he's ok, but I think that's a questionable design practice and I would always want to make sure the distribution piping outside the mech room was not condensing for radiant cooling.

One of the aspects of general HVAC design, if considered at all, are the internal gains generated by plants, plants, humans, cooking and showers. If you don't want the "wet mirror" affect, you will have to pay attention to ventilation, an ERV and yes maybe a means of de-humidification for a very tight house.

I would always want to make sure the distribution piping outside the mech room was not condensing for radiant cooling.

Seems like a good goal (you want mild supply temps for efficiency anyway), but insulating the pipes isn't also an option?

I'd like to see a complete residential radiant cooling design with ERV delivered dehumidification (DOAS) and some instrumentation (like a WEL) to see how well it performs.

Yes, but you can't easily insulate manifolds. I'm sure dana will mumble about spray foam again but that makes the manifold unserviceable and is not an option I would advocate.

Plus, chilled water pipe insulation is seriously business. You have to do it very well. I prefer to obviate the need for installation perfection if at all possible.

we are not residential but we are heavily instrumented. it works great here. when we run the shower we see a temporary bump of about +1% RH when doing boosted exhaust with our ERV. I do have a client on nantucket for whom humidity control has been a problem. So far we are diagnosing it as an infiltration problem, but haven't gotten blower door done yet to see for sure.

So far we are diagnosing it as an infiltration problem, but haven't gotten blower door done yet to see for sure.

You might also look into pressures. A nice tight house that looks great with a blower door will leak badly if half the rooms are at +10 pascals and the other half are at -10 pascals. The -10 rooms will have high humidity.

that would drive air from room to room (not that we care about that) but if the house is rated for leakage at 50 pascals how would +/-10 drive unforseen in/exfiltration?

If a room is at -10 pascals with respect to the outdoors, it will be sucking in humid outside air. A house that maintains the same pressure (0 pascals) as the outdoors in every room will perform much better than one that has non-zero numbers. Blowers door tests do not measure infiltration caused by these pressure differences. Ie, blower door tests aren't very accurate for what you actually care about (ACH under normal operating conditions). You could use a tracer gas test for that, but I doubt it would work with a ERV or other vents operating (maybe this could be accounted for).

Picky, picky, picky:-). Trace gas won't be a problem if the bathrooms aren't properly ventilated. Thus the challenges of renovation. We are working on two older homes with new additions and energy codes. HVAC always a challenge, sometimes the old house must be ignored, for ACH but still needs to be heated and cooled. Easier with hydronics and high velocity, ERVs are not in the cards since it will be hard to pull a negative on a 1920-1950 circa houses with a few bath fans for the addition.

yes, but you measured at 50 so I guess I am just saying that's infiltration you could be aware of... but I see your point. these ERVs won't really generate much more than 10 Pa and we do air pathway specifications through the house to ensure we stay very low on pressure differences in the home... zero chance of that issue here.

it is an open cell foam house, but the indoor humidity tracks outdoor very quickly. unlikely that it's vapor migration through the foam... infiltration is a very likely culprit. hopefully we can nail that down in the next few months.

Generally, I suspect that radiant+DOAS cooling isn't a good fit for poorly sealed, insulated or balanced buildings in humid climates (it needs careful attention to all three).

absolutely not. but this home is not supposed to be poorly sealed or insulated... the whole thing was spray foamed. so... we'll see.

I'm in the process of figuring out whether the 2nd tank is cost effective

Please keep us posted on any details. I would think that more successful examples, designs and operating data for residential radiant cooling would be good for the radiant industry. If current controllers don't handle indoor and outdoor temp/humidity, two tanks, radiant loops and a chilled water coil, that's a solvable problem.