Ventilation and Cooling for An Airtight Home With Radiant Heat
I am trying to determine the best ventilation strategy for an airtight home in Climate Zone 5A that will be using radiant floor heat as its primary heating system. The house we have designed is a 3040sf ICF home – 1040sf basement, 2000sf main level. The home’s foundation and main level walls will be built entirely out of Quad-Lock ICF forms and will utilize Quad-Deck flooring for a main level concrete floor over the basement. We will have a truss built roof with R-50 blown in cellulose insulation in the attic.
In terms of the H.V.A.C. acronym, my current assumptions are as follow:
Heat – Provided by radiant heat in concrete slabs on both lower and main levels.
Ventilation – Probably going to need an ERV to add fresh air to the house while conditioning incoming fresh air and capturing moisture in outgoing air as necessary.
Air Conditioning – We will need to provide cooling for the warmer months so we will have an air handling unit with evaporative and condensing coil connected to ductwork providing cool conditioned air to various rooms throughout the house.
Where I’m confused is whether the ductwork for an ERV and the Air Handling Unit are one of the same? I’m looking for a simple diagram to understand their relationship to one another but having a hard time finding anything. Everything seems to show an ERV and its ductwork belonging to itself and not coupled with a cooling system.
Please excuse me if what I’ve described is not accurate as I still have a lot to learn when it comes to the HVAC design of high performance homes.
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Radiant floors in well insulated and air sealed homes don't provide all that much in terms of comfort and unless you're using a heat pump, it will not be green. When used in slabs, they're hard to control due to the time it takes to warm/cool them. They're also extremely expensive for a redundant and limited system.
Since you're installing ductwork, an air-to-air heat pump can easily handle both heating and cooling.
https://www.greenbuildingadvisor.com/article/all-about-radiant-floors
The problem with that article is it spends more time on what to call a radiant system than actually addressing the most important issue when using them - ventilation & air filtration. This is the extent the article even mentions them:
"The main disadvantage of hydronic heating systems is that they don’t provide a convenient way to integrate air conditioning. (Most air-conditioned homes in the U.S. use ductwork to distribute cool air; since these homes need ducts for cooling, they usually use the same ducts for heating as well.)"
So first, let's remember HVAC is about comfort. If we've done radiant regardless of its cons (it's already in an insulated slab) how do we ventilate & filter the air in a new, air-sealed home that either has no AC or uses radiant cooling? Even with ducted AC, what happens during winter? We've installed radiant in three homes we've owned. The first two were historic homes in northern Utah that were somewhat leaky and had no, or low, AC loads. One had a roof-mounted, ducted swamp cooler.
We're building this current home in Arizona (zone 4b), where this year we had 20+ days over 100, with a high of 106. Can we run an ATWHP/air-handler on fan-only during the winter so it would continue to filter the air? If so, what tells it to turn on if not thermostats? If we decide to do radiant cooling (yes, I know the cons), and therefore need a completely separate ventilation/filtration system, do we use an upsized ERV-type system? Our house calculates out to a roughly 170 cfm ERV need, which as currently drafted would pull from bathrooms and dump into AC ducting. If we separated the systems, I'm guessing we'd need more vents than just those removing vapor from showers.
I know all the arguments against radiant and am not looking for more. I promise I'll report back on exactly how pointless it is in a better-than-code/almost-passive house once I've lived in it; Maybe we'll save another hopeful, radiant-loving person the crushed dreams we experienced - modern systems don't even give you warm floors. For now, I could really use insights on what has actually worked for stand-alone ventilation with radiant heat.
https://www.greenbuildingadvisor.com/question/cold-climate-air-to-water-heat-pump-auxiliary-heat-ac#comment-271457
I actually read your post along with the others in that thread. Our daughter lives in Boulder so I'm familiar with your climate. Do you still find that you don't need AC? I'm not as concerned about not having warm floors - that was kind of a tongue-in-cheek comment. Our real challenge is moving enough air to ventilate and filter without ducted AC, since I'm looking seriously at radiant cooling (or ventilating in winter even with ducted AC). It recently dawned on me why I loved our basements so much in Utah. Being at least partially underground, they're cool even during the hottest days of summer. I thought cool floors might feel weird until I remembered that's what's so nice about basements. My kids used to fall asleep on the floors since they felt so cool.
I came across the term "air displacement ventilation" this morning as a possible solution. And down another rabbit hole I go ... :(
> Do you still find that you don't need AC?
Do you mean fan coils or some other type of forced air system? Yup. The house is able to maintain temp without any direct air cooling. This summer, we've had quite a few upper 90s/100F+ days and every room in the house has remained in the low 70s even though we coast (don't actively cool) from 12PM - 11PM.
Haiku fans are installed in the living room, media room, and all bedrooms as well. The air movement improves the performance of the radiant and masks the slow, 3-4F rise of temps in the afternoon when cooling is off. Even if we decided to keep the house at a constant temp, I believe ceiling fans improve comfort and would still install them.
> Our real challenge is moving enough air to ventilate and filter without ducted AC, since I'm looking seriously at radiant cooling (or ventilating in winter even with ducted AC).
Can you better describe your goal here? Our Zehnder ERV system has worked extremely well. The only extra things I did was to add pass through grills above doors that are often left shut (bedrooms, bathrooms, the laundry room) and to use occupancy sensors in the bathrooms to automatically trigger the boost setting. I purchased a portable CO2 monitor (Aranet 4) and levels stay low so long as I keep up on ERV filter changes.
Unless you live in a climate where the outdoor dew point is reliably and significantly below a comfortable indoor dew point you're going to need some sort of dehumidification.
Each person in a house releases about five pints of water a day into the air, from bathing, breathing, sweating, cooking and cleaning. That water vapor needs to be removed or the house will become uncomfortably and dangerously humid. If the outdoor dew point is lower than the desired indoor dew point the vapor can be removed by ventilation alone. But if he outdoor dew point is higher than the indoor dew point ventilation and any air brought in by infiltration just make the house more humid.
A comfortable dew point is around 57F -- that's equivalent to 77F with 50% RH. Unfortunately I can't tell you quantitatively what "reliably" means in this context, nor what "significantly below" means, other than to say that you're going to have to move a lot of air to remove any moisture if the dew points are close.
That said, in most of the US, summer dew points are not reliably significantly below 57F.
Scsiguy Post #9 - your AC needs are basically what we had in Utah. Here, we had a high in July of 108 and more than 20 days of 100+ temps. We definitely need to add cooling/dehumidification of some sort.
As for moving air, the required cfm for a normal EVR system for our house (meaning it's used to exhaust from bathrooms and wet areas) is less than 170 cfm. It just can't imagine that would be enough air movement by itself, especially for dehumidification. If it dumped into a ducted AC system (supposing we went that route rather than radiant cooling), then the "boost" of the AC air might be enough. Perhaps winter on fan-only would be fine, too. That's what I'm trying to sort out. I feel like we should run the ducts regardless of whether they supply heat or AC, just for the ventilation/filtering/dehumidification factor alone.
BTW, we're in Sedona AZ where we have periodic monsoons, but are otherwise fairly dry, though not as dry as you'd think.
DCcontrarian - post #10:
>Unless you live in a climate where the outdoor dew point is reliably and significantly below a comfortable indoor dew point you're going to need some sort of dehumidification.
I just checked and the dewpoints for July (the hottest month) were max: 68; average: 50.64; min: 16. Our neighbor was just talking about needing to add dehumidification at his place (normal construction, no air sealing, nothing fancy).
Am I right to think a normal ERV system is inadequate? Based on the calcs we have, we need less than 170 cfm for our place, assuming exhausting wet areas only. That isn't much air movement.
Replying here, because I guess we've gone too deep to link to the actual messages.
The ERV is a great help in maintaining indoor humidity levels. At least that has been my experience. When the monsoon flow hits here (more rarely than in the past), the house significantly lags the change in outdoor humidity levels. But the ERV is designed for efficient balanced ventilation, not dehumidification. I wouldn't try to combine these roles.
Instead, I'd add a central hydronic fan coil somewhere at the top of the house (where air is most hot/moist) that runs at a low fan speed and is treated by the control system as a dehumidifier. This is exactly what we pre-provisioned for in my house, but found we didn't need it. The Tekmar system we used will happily run the fan coil at 45F while delivering 50-55F water to the floors depending on dew point.
Vapor diffuses pretty rapidly. This is why you can successfully install a central AprilAir steam humidifier that discharges to any suitable room and have it work effectively—ours vents into our first floor living room but RH is within a a percent or two of there throughout the second floor. So I'm not convinced you need distributed dehumidification if your latent load is small.
> Scsiguy Post #9 - your AC needs are basically what we had in Utah. Here, we had a high in July of 108 and more than 20 days of 100+ temps. We definitely need to add cooling/dehumidification of some sort.
Radiant cooling is cooling. So long as you have enough radiant surface area (the amount required varies by flooring and floor covering choices) and HP capacity, a system can be designed to reject the sensible load of your building even at 108F. The thing it cannot do is deal with latent load. This requires understanding the 95/99% dew point at your location and how well the building envelope and supporting systems insulate you from high outdoor RH levels. From there, you can easily calculate how much latent capacity you need to design in and match that with published information about equipment (e.g. hydronic fan coil latent capacity at different supply temps or dedicated dehumidifier).
We are planning a geothermal system for our radiant loop. Do you have any suggestions as to what cooling and ventilation strategies I could use in tandem with this heating system for the cooling months?
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Dedicated ducting is aways the best but then you end up with a lot of ducting costs/space. The next best is the exhaust ducted system here where the supply is shared with the air handler.
https://www.finehomebuilding.com/project-guides/energy-retrofit/ducting-hrvs-and-ervs
In those cases you will need to run the fan on the air handler even in the heating season, as long as it is an ECM blower, the cost of this is pretty minimal. This way the whole house air cleaning can be handled by the air handler filter as well.
P.S. Many people that have done modern builds with heat pumps and in-floor heat never end up running the in-floor heat. Too slow to respond and with a low load place, the floor is never that toasty warm feeling you got in old leaky houses. Basically the ducts you are putting in to cool the place can easily handle heating the house as well, might as well skip on the redundant floor heat system. Resistance mat floor heat in bathrooms, entrance mudroom and kitchen is still a good idea though.
Akos:
> P.S. Many people that have done modern builds with heat pumps and in-floor heat never end up running the in-floor heat. Too slow to respond and with a low load place, the floor is never that toasty warm feeling you got in old leaky houses. Basically the ducts you are putting in to cool the place can easily handle heating the house as well, might as well skip on the redundant floor heat system. Resistance mat floor heat in bathrooms, entrance mudroom and kitchen is still a good idea though.
Yeah, the tubes are already in the polished concrete floor. I suppose we could skip the expense of manifolds and circulation pumps if we didn't hook it up though. I'm also seriously considering radiant cooling (loved growing up with cool basement floors). Either way, I agree we need ventilation, filtration (forest fires), and likely dehumidification which all require ductwork anyway. We do have access to install ductwork right now and will be installing it, but I'm not sure how much the duct/vent sizing will change depending on whether it's for ventilation/filtration/dehumidification only or all that + AC. We're using am AtoW heat pump anyway. If we finish the radiant manifolds & pumps, we'll have with two distinct systems which could operate as all water-piped heat & AC, or all air-delivered heat & AC. (seriously, why...) The non-negotiable part with our tight home is a ducted "inside air treatment" system of some kind, maybe that should be through an oversized ERV with its own fan delivery system at higher volume like an air displacement ventilation system, or maybe through the air handler used to deliver AC as well.
I really appreciate your, and everyone'sm weighing in.
Typical bedroom needs about 3" duct for fresh air. If you want it quiet, you want that bumped up to 4". In a well insulated house, the same bedroom you would have to bump that up to about 5" duct to heat and cool it. If you are already running ducting the cost delta of the slightly larger ducting is pretty much noise.
An ERV will filter outside air but won't do much for indoor air. I run into this in my own home (heated floor + mini split +ducted ERV with hepa) as there is a lot of dust in the house. As soon as my mini splits die, definitely replacing it all with a ducted unit with a proper filter.
Air to water makes sense for existing house with hydronics. If you are doing a new build and you need AC, stick to air to air. Modern modulating heat pumps are cheap, very efficient and offer a level of comfort that is not even comparable to older single stage heating systems. In a well sealed and insulated place the temperatures are already so even that you simply don't need any fancy HVAC.
>"If you are already running ducting the cost delta of the slightly larger ducting is pretty much noise."
Meaning, in your experience a larger duct is more noise? I had thought it was the opposite. Is there any "penalty" for oversizing ducts if space isn't an issue. I realize it seems ridiculous that I don't know about something so seemingly simple as ductwork, but we haven't had true ducted AC in our historic homes in almost 30 years, and even then it was a ducted swamp cooler (yikes). I don't remember what it might have been like before that.
When you replace your system after the minis die, are you planning on tying into your existing ERV ducts and running it all through a central ATAHP with air handler? Do your EVR ducts run into and out of all major rooms or are they exhaust only?
Even though it's new construction, our house falls into the category of "existing" since it's already in the slab, but yes, I can see the wisdom of Air to Air and simplicity on the next go-round ... trust me on that.
The cost delta is "noise" -- ie, you won't even notice it.
If you must have your air to water, here is what I would do.
Set up the air to water to heat your main floor slab. No cooling, no hot water, just the slab. If you really must have cooling, set it up to cool the slab to a reasonably warm number that avoid any of the condensation issues.
For the 2nd floor get a ducted heat pump. Set this heat pump up to also supply the rooms bellow where you want proper cooling. Connect your ERV to the intake of the unit. This will do your fresh air distribution plus whole house air filtering. It should also do the bulk of your dehumidification. I would skip on the dehumidifier and only add a stand alone unit if you have high humidity issues down the road.
There as some issues with your HVAC design. Seeing 200CFM for a room with not much exterior and 100CFM for a bath. Those numbers about 2x what I would expect in a high performance build. I didn't look through the details of your man J but something feels off.
P.S. A stand alone heat pump water heater is cheaper and usually more efficient than an indirect. It also means you don't need a 2nd tank to bring the water up to proper tap temperature when it gets cold.
P.S.S the 2nd floor heat pump could be a fan coil unit hooked up to your air to water. The cost between the two plus the extra controls is about the same, I would take the separate unit to provide some redundancy. Also an air to air tends to be more efficient since it needs one less heat exchange and can run at higher delta T.
You may want to contact Building Equinox about their CERV2 heat pump to see if it fits your needs. Would recommend a room by room heat loss calculation and zone accordingly. Most rooms in my house only turn on the heat when it drops below -10F.
We actually do have a room by room manual J that was done, which is guiding our sizing.
Wow, -10F? That's a well-built home indeed!
We have radiant heat (Warmboard) - the bathrooms and tile foyer are set to 72F, other room set to 69F. So the bathrooms supply heat to the rest of the rooms. In a well insulated and air sealed house you don’t need radiant everywhere.
Good luck on the new build!
[responding to post #12 because we've nested as far as we can]
I'm not aware of any methodology for engineering humidity control. Heat loss and heat gain are well understood and straightforward to model using Manual J. I'm not aware of anything equivalent for humidity. I do recall someone posting here a site where you could look up 99th percentile dew point data for a location, but I can't find that post.
So I have no way of beginning to answer the question whether, for example, a 170 CFM ERV provides sufficient dehumidification in your climate.
[replying to #16]
"This requires understanding the 95/99% dew point at your location and how well the building envelope and supporting systems insulate you from high outdoor RH levels. From there, you can easily calculate how much latent capacity you need to design in and match that with published information about equipment (e.g. hydronic fan coil latent capacity at different supply temps or dedicated dehumidifier)."
Easily, huh?
The problem is that latent capacity is entirely dependent upon conditions. This is actually the heart of the reason why we don't have a well-developed methodology for engineering humidity, because traditional air conditioning is largely self-modulating, the more humid it is inside, the more latent cooling you get. Let's say it's 77F inside your house and your air handler is at 50F. If the air is at 70% RH you get an SHR of 48%, if it's at 50% RH you get 72% and if it's at 30% RH you get 100%. So long as you have enough of a sensible load to get the thermostat to turn on you'll pretty much get the dehumidification you need.
Now where I live, Washington, DC, we have humid summers, and with conventional AC the humidity can still be a bit higher than is desirable. I log temperature and humidity in my house, and from June 15 to September 15 of this year I averaged 63% RH with an indoor temperature of 77F. I'd prefer it to be in the 50's, basically I don't have enough sensible load to get the AC to run enough to handle the latent load.
The last thing I'd want to do is add cooling that is 100% sensible.
At least around here, the most humid days are when it's not super hot. We had a day this summer when the dew point hit 77F: outdoor temperature of 78F, raining, outdoor humidity 99%.
If your thermostat is set at 77F, on a day like that your AC is hardly going to run at all. It's rainy out so there's hardly any solar gain, and the outdoor temp is right around the thermostat setpoint. There's almost zero sensible load, so the AC doesn't run. But the humidity is thick.
Hence the popularity of dehumidifiers.
But the thing about dehumidifiers is they are quite efficient space heaters, they return to the space all of the latent heat that is extracted by removing humidity, in addition to the heat from the energy required to run them. So not only do they extract humidity, they also provide a sensible load that encourages the house AC to run more.
Now, from an energy perspective running a dehumidifier just so that you can run a 100% SHR cooling source like radiant cooling makes no sense. It's like leaving your windows open in the winter to get your radiant heat to run more so you can have toasty toes.
[Replying to posts 16 & 18]
DCC - due to our monsoons, there are days here like what you describe in DC. I created a spreadsheet (attached) showing the daily high dew point, time of high dew point, temp at time of high dew point, and the high & low temps for that day. Maybe this helps? I was surprised that from June - August this year, we had 26 days where the outside dew point was over 60, including 3 days @ 66 degrees and 4 @ 68. The dew point also dropped to as low as 5 degrees during that time. Today before a brief shower, the RH was 13%, which is pretty normal - if there's no monsoon. We also have long shoulder seasons here, and even when it's hot it tends to cool down quite a bit overnight. (Yes, that adds to the arguments against in-slab radiant.)
Scsiguy - I'm still trying to wrap my head around the 95%/99% idea. I'm not sure I do understand it.
So, from these posts, it seems if we can design a system with the capacity to handle dehumidification, since that's our biggest design challenge, it should also be able to handle our fresh air & filtration needs. Great.
I'm still not sure if that system would be better as a higher velocity, stand-alone ERV/dehumidifier, with supply & return ducts running to most rooms (rather than merely exhausting wet areas), OR if it would be better as a hybrid system using an air handler & dehumidifier paired to the ATWHP, which also runs on fan-only in winter.
Am I close? Is radiant cooling even possible with those numbers (especially in light of the ridiculousness of opening windows for hotter floors)?
Imagine if you can a chart that has heating/cooling on the Y-axis and humidification/dehumidification on the X-axis. Cooling is up, heating is down, humidification is to the left and dehumidification to the right.
You could divide that chart into regions. So in the northwest corner you have cooling and dehumidification, which is what a conventional AC provides. Moving clockwise you start needing even more dehumidification, so it would be AC plus a dehumidifier, then just a dehumidifier, and further clockwise is heat plus a dehumidifier. Then heat with ventilation, then heat alone, then heat plus a humidifier. Keeping going clockwise, next would be humidification plus cooling (a rare combo), then cooling alone, cooling plus ventilation, and then you've made the complete circuit back to cooling and dehumidification.
It's possible to design a system for any point on that graph. It's theoretically possible to design a system for every point on that graph, but in practice you wouldn't do that because it gets expensive. What you do instead is try to figure out what parts of the graph are actually going to happen in your house and design a system for that.
Traditionally, HVAC design has only focused on the north-south axis, heating and cooling. And for that you look at the 99th percentile temperatures, the temperatures that it's warmer than or cooler than 99% of the time, you figure the other 1% of the time the house has enough heat capacity that it's not going to matter if the system is a bit undersized for a few hours.
I see two problems when you start trying to bring humidity into the equation. The first is I don't know where you would get the data to figure out how much time you spend in each part of the chart. It may be out there, I just don't know where to find it. What's tricky is that it's determined by both outdoor temperature and humidity, but also occupant behavior. The highest humidity removal need of the year may be in December when you have 100 people inside for your Christmas party.
The other problem -- and this is a crucial point -- is that when it comes to dehumidification, the 99th percentile approach isn't going to work. When humidity gets excessive you risk condensation. There's really no acceptable level of condensation. A year is 8760 hours, 1% of a year is 88 hours, 88 hours a year of condensation is a big problem.
There's also a practical problem that if you need a system capable of operating in that northwest quadrant, why have another system?
> The other problem -- and this is a crucial point -- is that when it comes to dehumidification, the 99th percentile approach isn't going to work. When humidity gets excessive you risk condensation. There's really no acceptable level of condensation. A year is 8760 hours, 1% of a year is 88 hours, 88 hours a year of condensation is a big problem.
Designing for 99% latent load is just fine assuming, again, appropriate controls. There will be no condensation anywhere except in any fan coils or dedicated dehumidifier that is still active and driving toward the indoor humidity target. The controls will just be unable to set the floor temp low enough to achieve the target indoor air temp because doing so would cause condensation on the radiating surface. Covering the final 1% is possible, but doing so usually means the system is less efficient when operating in more common conditions. This is just a design choice, not a safety one.
Can you post your Manual J? That would make this all much less speculative. The conclusion that "dehumidification is our biggest design challenge" doesn't make sense to me. You aren't building in Louisiana.
> Am I close? Is radiant cooling even possible with those numbers (especially in light of the ridiculousness of opening windows for hotter floors)?
Are you talking about opening windows at night? In our house, we do this when it makes sense (outdoor temps lower than the house, outdoor humidity is low) and don't bother adjusting the t-stats. Its not like we are mechanically cooling a bunch of air that will just get sent out a window. The open windows just cause the air and floor temp targets to be satisfied sooner than if we hadn't let in the night air.
You're showing a fundamental misunderstanding of what Manual J does.
It's concerned with heating and cooling, and capacity for that. It doesn't care about interior humidity levels, it assumes that the cooling system will take care of that.
The humidity used in Manual J is average humidity at the 99% temperature. So on the hottest day of the summer, how humid is it outside? This is used to insure that on that 99th percentile day there is enough total cooling capacity, Manual J doesn't ensure that you have enough dehumidification at other times.
In many places the hottest days are not the most humid. So where I am, the design temp is 92F, the design humidity is 42%. That's a dewpoint of 66F, which is high but probably closer to the 50th percentile for the summer here.
The Manual J that was performed when building my house included several design points to understand how resilient the system design would be to deviation from the bin data values and/or if we failed to meet our air infiltration target. The software package used by our energy modeler allowed them to easily change any of the input parameters and simulate the result. Are you worried about that annual party you host in summer? Up the occupant load. That muggy day with moderate outdoor temps? Plug in that condition as the design point. Someone leaving a window open? Up the infiltration rate. etc.
Yes, the results from these other scenarios are not a true "Manual J", but they can inform your design choices, including the situation where you want to handle higher latent load than the standard Manual J design point would select for you.
>The conclusion that "dehumidification is our biggest design challenge" doesn't make sense to me. You aren't building in Louisiana.
What I meant is that it seems from this discussion that dehumidification would be the parameter, which if met, the other ventilation needs (general fresh air and filtration) would also be met. It is pretty dry here, but it does feel like Florida on and off during the monsoons.
>Am I close? Is radiant cooling even possible with those numbers (especially in light of the ridiculousness of opening windows for hotter floors)?
I was quoting DCContrarian's comment about overcoming low floor temps with radiant - the "ridiculousness" that while chasing that warm-feeling floor, you abandon all the work you've done to create a tight house and open all the windows to force the floor heat to turn on. One of the big reasons we were able to go without cooling in our historic homes with radiant, is that we'd open all the windows and night and close them up before it got too hot in the morning. The historic double hung windows allowed us to create a bit of convective flow by opening the tops a bit on the sides of the house which were shaded. They also had 10'4" ceilings.
Expanding on your suggestions seem like the solution for our house - an ATWHP air handler paired with dehumidification & filtration, which is working in concert with a ducted ERV to bring in filtered outside air and exhaust wet rooms only, since the AC delivery moves/filters/humidifies the air in the rest of the house. In the winter, everything works the same, but without cooling the air. Does that sound right?
The system design is being done by a professional, but I'm not sure how much he's dealt with radiant before, which is why I'm asking while the framing is still accessible. We haven't installed the upper floor tubing yet and I asked if maybe we should just do air heat up there (1500 s.f.). He said it was an all or nothing situation - all radiant heat or all forced air, since every time the upper floor called for air heat it would overheat the main's radiant. I don't know if that's true when we have 4 manifolds on the main, each with actuators that can monitor and control each individual loop if wanted. We've now paid two people to design this: one who knows everything about radiant, but doesn't design for AC (he works in cold climates), and one who knows all about ventilation and AC, but I don't think has worked with radiant much. There just isn't much info available on AC & ventilation with radiant, hence my questions.
For future readers, now we know that: 1) an ERV in the typical stand-alone configuration of exhausting from wet areas & dumping to living spaces isn't enough since it doesn't move enough air or account for filtration or dehumidification of inside air; 2) a hybrid system with a ducted air handler, where an ERV exhausts from wet areas and dumps into supply ducts is the best solution because it has the volume to also dehumidify and filter inside air while the ERV accounts for outside air; 3) radiant cooling isn't very effective where I am (zone 4b) because dew points are too high to allow the floors to cool without condensation.
I've attached my manual J, but this program shrinks things and it may not be readable. We shall see.
"He said it was an all or nothing situation - all radiant heat or all forced air, since every time the upper floor called for air heat it would overheat the main's radiant."
That is a very curious statement.
[Reply to #33 and #34]
[Post #33] >"That is a very curious statement."
Which is why I felt I had to dive in and try to understand this better - I don't want to have our design based on a fundamental misunderstanding of either system and I only feel somewhat confident with radiant, not ventilation, though even that is still rudimentary. To be sure, I did not want to learn this, but here we are.
[Post #34]
>"There as some issues with your HVAC design. Seeing 200CFM for a room with not much exterior and 100CFM for a bath."
I will ask about those. I also noticed a 5000 btu cooling load disparity in one room between the radiant manual J and the new one I posted. All others were pretty close. Will be checking that as well.
Re: water heater - I had already bought a separate Rheen hybrid heat pump water heater. This is complicated enough.
I think I now know what to ask for on the design:
- ATWHP sized to provide water to the radiant heat systems & air handler
- Radiant manifolds & actuators as now designed to down regulate heat output as required
- Air handler & ducting sized to provide AC, ventilation, filtration, & dehumidification (which can also provide heat for quicker response time).
- Controls/sensors to run the air handler on fan-only even in winter
- ERV to supply outdoor air that exhausts from bathrooms (+ maybe laundry) and dumps into supply air
If I did decide to mess with radiant cooling - as a curiosity more than anything - I will have basically built two completely separate means of heating and cooling the house with the only overlap being an air handler, that acts as a fan-only, for ventilation needs. (I imagine there are cheaper ways of adding a fan.)
It's amazing how simple it seems when you understand it even a little bit better. Next time, if there is one, a simple ATAHP without radiant, will suffice.
I can't thank you all enough DCContrarian, Akos, scsiguy, and others for your wisdom and insight! Thank you! I'll report back to add to our practical knowledge base.
> This is actually the heart of the reason why we don't have a well-developed methodology for engineering humidity, because traditional air conditioning is largely self-modulating, the more humid it is inside, the more latent cooling you get.
I find this a very surprising statement. The Manual J for my home estimated the latent and sensible loads for the house independently, and from there it seems quite straight forward to design a system that can achieve design targets for both temperature and humidity.
> Now, from an energy perspective running a dehumidifier just so that you can run a 100% SHR cooling source like radiant cooling makes no sense. It's like leaving your windows open in the winter to get your radiant heat to run more so you can have toasty toes.
The whole set of arguments here feel strained to me.
"Fan coils (AC) have variable SHR depending on indoor humidity levels."
Sure, but what matters from a system standpoint is the design indoor dew point, design outdoor dew point, estimated infiltration rate (including from ventilation), and occupant load. A Manual J (at least the one I had done) converts this into sensible and latent loads. From there, you can select equipment that either has the right SHR ratio and capacity to serve both temp and humidity management roles, or split them. If you want radiant cooling (which is a *subjective* decision), it can make sense to split these roles.
If the indoor humidity is higher than the design target, you're in some kind of "recovery from setback" situation. Whether using a fan coil or dedicated dehumidifier, dehumidification capacity rises in this scenario. That's a bonus (i.e. improves recovery speed), but not a major concern from a design standpoint.
Controls exist today that can be deployed in a residential setting that will independently run dehumidification from sensible cooling. So the floor isn't being cooled in your 77F day scenario - that really would be nonsensical and a reason to throw out the controls. Only the dehumidifying fan coil or dedicated dehumidifier is running which is exactly what you want because it most directly targets the dimension of excursion from the desired indoor conditions.
But the real argument here has an underlying subtext which is doing this well is expensive. I've never argued that. The OP asked if it is possible. It is, and the systems to implement it have been around for a long time (10+ years).
" From there, you can select equipment that either has the right SHR ratio and capacity to serve both temp and humidity management roles, or split them. "
The SHR numbers published with HVAC equipment has zero engineering value. Zero. It's only use is in qualitatively comparing two pieces of equipment.
The SHR ratio for the chosen equipment needs to be calculated from the AHRI rating data adjusted for the design conditions. Many manufacturers and distributors provide extended data tables (measured performance at non AHRI conditions) and calculators to do this for both DX and hydronic coils. At least some people seem to be quantifiably estimating the expected equipment performance when making their selections.
"Controls exist today that can be deployed in a residential setting that will independently run dehumidification from sensible cooling."
I'm sorry this is a fundamental mis-understanding.
You can't have latent heat removal without sensible heat removal. The way to remove moisture is to cool the air, you can't do that without, well, cooling the air. Latent always includes sensible. The converse of course isn't true, you can have 100% sensible cooling.
In some situations -- depending on climate, building construction, occupancy pattern, etc. -- you can extract as much latent heat as you need to, and still not extract as much sensible heat as you'd like, so you can then get to your comfort point by using a 100% sensible cooling source like radiant. But there's no guarantee that this surplus sensible cooling load is going to exist. Where I live -- and in vast swaths of the US -- it doesn't exist. Where I live, for much of the summer, in many houses, you can cool to meet 100% of the sensible load and still not meet the entire latent load. Which is why dehumidifiers are popular, as they are almost everywhere on the east coast.
Are there places with surplus sensible loads? Sure. You clearly live in one of them. But it's dangerous advice to assume that everywhere is like where you are.
But if you live in a place without surplus sensible loads and wish to implement a 100% sensible cooling system in conjunction with latent cooling, the only way to do that is artificially increasing the sensible load by introducing heat into the building. That's what a dehumidifier does.
To put a finer point on this, if you live in a place without surplus sensible cooling loads, in order to meet your latent needs you need a system capable of meeting 100% of your sensible cooling loads, and then some.
So why add a whole other cooling system when 100% of your cooling needs is already being met?
> You can't have latent heat removal without sensible heat removal.
And yet you know that this is exactly what a dehumidifier does—well, it actually adds some sensible heat.
> Are there places with surplus sensible loads? Sure. You clearly live in one of them. But it's dangerous advice to assume that everywhere is like where you are.
Is this my advice? Is this what I'm assuming? You keep talking about the East coast, but last I checked, the op is in Sedona, AZ. I was trying to help answer their specific questions, related to their specific conditions and goals, not convince you or anyone else that hydronic radiant systems are the best solution for all applications.
You've already made your point that you wouldn't bother with a radiant system and why. You should be able to do that without distorting what I've said with straw man arguments.
Provided by radiant heat in concrete slabs on both lower and main levels. https://1depositca.com/