Air-source heat pump / Minisplits with humidification
T_Barker
| Posted in General Questions on
Anyone know of any Air Source Heat Pumps with Mini Splits (ductless) that have integrated Humidification that can be controlled to set levels?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
T. Barker,
Q. "Anyone know of any air-source heat pumps with minisplits (ductless) that have integrated humidification that can be controlled to set levels?"
A. No. And that's a good thing. Humidifiers are dangerous. A well-built house should never need a humidifier. If the air in your home is too dry during the winter, that's almost always a clue that your home's thermal envelope has a high rate of air leakage. The solution is to seal the air leaks -- don't install a humidifier. Operating a humidifier can damage your house.
I'm very skeptical of your answer. Even most quality houses built in the early 2000's have inline humidifiers. When it's 0F to -20F for 3 months of the year and inside humidity is 30% or less, my skin tells me I definitely need humidification.
So you're saying a modern super insulated home should keep relative humidity at 40% throughout those winter conditions without any added humidification?
Most reasonably tight houses do not NEED a humidifier, but may need DE-humidifiers to keep indoor RH bounded to under 40%. Maintaining 40% RH through the winter would be on the high side of what's normally healthy for the building materials of a wood sheathed house, since that imparts a strong vapor pressure difference between the indoors and the sheathing. Holding the line at 35% is measurably better for the house, and still pretty comfortable for the humans.
If it's chronically under 30% it's an indication of excessive air infiltration or a high active ventilation rate. Many hot air furnace systems in new house combine ventilation & heating, where outdoor air is being brought in whenever the air handler is running. That sort of system drives excessive ventilation during the coldest weather (when the outdoor air is driest), and not enough ventilation (assuming a tight house) during the shoulder seasons, where the heating & cooling loads are low, with a low duty cycle on the air handler. An automatic humidifer on that type of system is a band-aid fix, compensating for the wintertime dryness created by the excess ventilation rate.
It's better to divorce the heating function from the ventilation function and control each separately. Controlling the temperature with a mini-split and controlling the ventilation rate with an HRV works pretty well. It's possible to set the indoor RH on an HRV system in winter with DE-humidistat controls, that automatically dials back the ventilation rate when the indoor air's humidity drops to the humidity setpoint. (But set it at 30-35%, not 40%).
Very few places are continuously below 0F for three months out of the year (are you north of the arctic circle?), which would be a special case. Or are those the nightly lows for 3 months (which isn't a special case.)
What is your definition of "quality house"? Even today, probably 99% of houses are built to just meet, but not exceed, code. In the early 2000s, that was probably more like 99.9%. Code in the early 2000s means leakage rates an order of magnitude higher than what is considered a tight house today.
I personally wouldn't regard anything over 1ACH50 a tight house, but if Dana says that houses with 3ACH50 or below will not have low humidity issues (over ventilation notwithstanding), I believe him. So I would say those houses you are referring to are either leaky (well over 3ACH50), or over ventilated. Pretty easy to do if you're heating with a forced air furnace drawing combustion air from the outside, or some similar scheme.
Trevor,
I'm not sure what you mean by "code built." GBA has always recommended that builders comply with building codes, so we are all in favor of code-compliant homes.
Air leakage rates for new homes vary widely from one area of the country to another, but in general, researchers note that homes are getting tighter every year. That's a good thing.
Even back in 2000, some builders were paying attention to airtightness. Maybe not where you live, however -- it's hard to generalize.
Martin,
By "code built" I meant "doing the bare minimum that meets code". I thought this was a common usage of the term. Of course every home needs to at least meet code.
My point was that Mr. Barker's skepticism is probably due to the fact the homes he's seen problems in are quite likely to be far below today's standards of super efficiency, despite their perceived high build quality.
The US codes also now have reasonable air tightness standards (currently 3ACH/50 max for colder climate zones ) whereas prior to IRC 2009 the code was silent on air leakage. Below 3ACH/50 most homes would cruise through sub-zero cold snaps with indoor humidity north of 30%RH @ 70F without active humidification. But with excessive ventilation you can dessicate any house when it's that cold out.
While 3ACH/50 isn't a particularly tough standard to hit, many or even most "quality" custom homes built in the period from 2000-2010 would fail that test. Most tract homes at the higher end of that market would too.
Dana,
A 2002 study of 24 new Wisconsin homes showed a median air leakage of 3.9 ac/h @ 50 pa. That's not great, but it shows that even back in 2002, which is 16 years ago, Wisconsin builders weren't too far off of the 3 ach50 target.
If the median was 3.9ACH/50 it makes the case:
Most homes in the studyh failed 3ACH/50 and ...
...half of all homes in the study failed it by an excess leakage of 30% or more.
Of those homes that tested between 3ACH/50 and 3.9ACH/50 I suspect most could be retrofit sealed to under 3ACH/50.
While the ~4 ACH/50 median isn't a huge energy hit over a code-max 3ACH/50, it would be measurable in the wintertime indoor humidity.
Thanks for the thoughtful responses.
I mean no disrespect when I say I'm skeptical of your answer. It's just that without some measured proof I have a hard time believing that even in a brand new super insulated, 0.5ACH/50 house, you can maintain high 30's to 40% RH in the winter in my climate (Northern Ontario, Zone 7A, most of the winter daytime 5F to 0F, and night time 0F to -10F or colder) without supplemental humidification.
The RH outside in those conditions is very dry (i.e. 30%), but that doesn't mean I should suffer at those RH levels inside. RH comfort zone for me is high 30's to 40% RH in the winter. Anything lower is unacceptable. If that causes vapor pressure differentials that can cause problems with construction materials, then I would look at that as a separate issue that needs to be addressed with it's own solution.
As well, I understand that in most houses different air flow requirements for HVAC systems vs. HRV/ERV dictate separating those systems, although I would challenge that assumption with new super insulated, super tight houses. If the HVAC loads for these new houses are so low and happen at a much slower rate, then assuming appropriate thermostatic controls, the HVAC air flow requirements may in fact be much closer, or even equal to the HRV/ERV air flow requirements for fresh air.
Of course who really knows? Because we don't know the CORRECT levels of air flow for HRV/ERV systems because we don't measure the quality of the air in each room or floor and tie that to the HRV/ERV system controls. I have yet to see hardly anyone talk about properly controlled HRV/ERV systems where CO2/VOC is constantly measured and the correct amount of fresh air supplied - to each room (much like current HVAC systems) - on demand as needed.
In the meantime we build these super tight, super insulated houses trying to conserve every last bit of energy and comfort, but then we blast a random amount of fresh air into that space without a) knowing exactly how much fresh air is really required at any particular time, and b) no great emphasis on conditioning that fresh air for temperature (I understand there is some conditioning, but in my opinion it's a weak spot). It's truly absurd when I think about the state of the systems that are currently available.
Even if a house was 100% air tight, you would most likely increase the HRV/ERV air flow so that you have the same amount of total fresh air circulation. In other words, total air circulation (leaks + HRV/ERV induced air) should be similar, it's just a matter of whether it comes from leaks or the HRV/ERV.
In my experience, when you maintain a suitable (as far as we can tell) amount of fresh air circulation via the HRV/ERV system + leaks, there is a definite need for supplemental humidification for human comfort in the winter and spring months in northern climates. The obvious answer is to tie it into either the HVAC system, or the HRV/ERV system, or if you're ductless, then into each minisplit head or each HRV/ERV module.
This interesting discussion leads to a question that I have about HRVs vs ERVs. Trevor appears to use the terms interchangeably. In his climate, would you expect a significant difference in interior humidity based on use of an HRV vs ERV? An additional question is about defrost strategy. ERVs typically require less defrost than HRVs; would that be likely to make a difference in interior air humidity? In addition, would you expect a difference in interior air humidity between exhaust-only defrost vs recirculation?
Good questions. I'd like to see some real life data or experiences regarding ERV's vs. HRV's in cold climates as well.
FYI, see my comment on this thread https://www.greenbuildingadvisor.com/question/general-energy-efficiency-housing-question regarding my experience with my HRV and inline humidifier attached to my Carrier Infinity furnace.
Trevor, having lived in dry climates in the western U.S. most of my life, our perspectives on acceptable humidity are probably quite different. During the summer, I get uncomfortable when outside humidity is higher than ~25% and routinely experience humidity <10%. However, my preference is my own and you need to be comfortable in your own environment.
There are some data out there that address these issues, but it appears that study conditions are so variable that it is hard to make sweeping generalizations. For instance see:
aea.nt.ca/files/download/03feb646baca002
http://www.cchrc.org/sites/default/files/ERV_ColdClimates.pdf
And of course, GBA has addressed.
https://www.greenbuildingadvisor.com/question/hrv-or-erv-in-cold-climate
A lot of the older papers likely overestimate the energy cost of running current ERVs/HRVs with ECM motors (e.g. Zehnder, Venmar X24HRV (Canada) /Broan HRV200TE (USA))
Edit to add: However, I am still interested in the take of others on GBA on whether use of ERV vs HRV along with defrost techniques make any appreciable difference in a cold environment.
" In his climate, would you expect a significant difference in interior humidity based on use of an HRV vs ERV?"
All other things being equal, an ERV will result in higher indoor humidity than an HRV, in winter in a cold climate. With an HRV, there is no moisture transfer between incoming and outgoing streams. So when the incoming air is below the dew point of the interior air, some moisture from the outgoing air will condense and go out the condensation drain (and whatever doesn't condense goes out with the air). So you have moister air being replaced with dryer air. With an ERV, at least some of that moisture (anywhere from around 30-70%, depending on conditions and the particular model of ERV) will transfer to the incoming stream and come back into the house.
"ERVs typically require less defrost than HRVs; would that be likely to make a difference in interior air humidity?"
The cause and effect are backwards. The humidity transfer is what lowers the defrost temperature requirement.
"In addition, would you expect a difference in interior air humidity between exhaust-only defrost vs recirculation?"
Not sure about this, but exhaust only defrost seems like a bad idea to me, as it will depressurize the house if it's well sealed.
Moisture sources are so variable that it's useless to generalize about what any %RH means about some building's air exchange rate.
Similar for generalizing about what moisture level will harm a building.
It's quite possible that a well built house with proper ERV use will be excessively dry and that humidification won't harm the building.
Don't drop ventilation below recommended levels because the humidity is too low.