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Community and Q&A

Night-time setback w/ radiant floor

David Meiland | Posted in Mechanicals on

I was talking to a heating contractor about a radiant floor system, and we don’t agree regarding night-time setback. I’d appreciate some light shed on this.

The system has a typical slab on grade with R-10 under it and PEX tubing cast in, and another area of framed floor with PEX underneath with aluminum distribution plates. The boiler is a 30kw electric with multiple elements and outdoor reset.

The heating contractor advised the homeowner that setting the night-time temp too low would cause the system to work very hard in the morning, to get the slab back up to temperature, and therefore waste energy.

My opinion is that lowering the setpoint at night is a good thing, as the system will put fewer BTUs into the house, saving energy. When the system ramps up in the morning, it will reheat the slab without any loss of efficiency. Fewer total BTUs will be put into the house. This is not a gas boiler that might have efficiency issues related to part loads or short cycling or whatever else.

I think the heating contractor is confusing a period of high demand in the morning with low efficiency. What say you?

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Replies

  1. Cramer Silkworth | | #1

    David,

    What's the rest of the home's thermal envelope like? Very well insulated, R-50+, or what? If it is, and the slab is sizable, then you may not even experience a night setback, whether you set one or not - the slab's mass combined with a well insulated envelope may carry it through the night with only a small drop in temp.

    The system won't "work hard" to bring the slab back up to temp (it may be at full load, but the heat's not being lost any faster than it normally would), but it will take some time, such that you'd need to set the system to come on a couple hours before you want the morning temperature reached. Some fancier thermostats do this for you by learning how long it takes the system to reach setpoint and come on earlier to compensate. So that combined with when the night setback takes effect and you may only have a few hours overnight to cool down.

    Fortunately, though, this is all just operations tweaking - as long as the boiler is sized for design conditions (and I imagine it is and then some), and you can just play with the controls to minimize energy use while maintaining comfort without risking much.

    Also, 30 kW electric?!?! Cheap electricity at the site, I take it?

  2. Keith Gustafson | | #2

    Setbacks on high mass radiant are usually a loser.

    If the setback is too deep and the concrete loses significant temp, it will take hours to reheat, and the residents may be uncomfortable. Radiant just does not recover like baseboard or hot air

  3. David Meiland | | #3

    A loser in what sense? Difficult for the occupant to manage satisfactorily, or an energy loser because it's somehow inefficient?

  4. Kevin Dickson, MSME | | #4

    David,

    Your contractor is dead wrong. Electric boilers are always 98%-100% efficient, no matter what the operating conditions.

    Not all heating contractors understand energy efficiency.

    Use my simple test: ask them "Can you explain to me the difference between a Kilowatt and a kilowatt-hour?"

    Keith's answer is also right but no heat is "wasted" with deep setbacks, in fact, heat is saved. Uncomfortable occupants are what contractors dread and they will tell white lies to avoid the possibility.

  5. Keith Gustafson | | #5

    I don't think it is per se an energy loser, just that radiant slab is not sized to enable quick recovery. Even if it was, the thermal mass effect would keep putting out heat hours after you wanted it to stop.

    Look. the perfect high mass radiant/ outdoor reset system has a slab just enough warmer than room temp to make up for losses. The perfect reset curve means that as the outdoor temp rises and falls the slab temp rises and falls and it is all very comfy. If there is a slight mismatch, you have a nice warm floor radiating to you and you don't notice. Now if you shut off the heat source, the room temp will lag by several hours but the floor temp continues to drop. In the morning your thermostat turns back on but the physics of thermal mass say you need to put 'x' btus into the slab before it will rise in temp, and thus allow the room to rise. If you put enough btus into it quickly enough to ramp room temp up then the mass will cause it to overshoot, and how does having the room at 78 degrees 3 hours after you go to work save energy?

    If I was convinced that I could save fuel setting back a high mass radiant system, I would probably have a complex timer that kept the slab temp up no matter what the thermostat called for.

  6. Doug McEvers | | #6

    It may have been discussed but you could run tubing at different depths in the slab for various recovery times. Shallow depth for quick morning heat, deeper tubing for thermal storage. Is off peak electricity an option?

  7. Dan Kolbert | | #7

    David - in the 2 well-insulated homes we've built with radiant slabs, we've found that it takes a long time and strains the system if the heat had a set back. One client went away on vacation, turned the heat down to 50. Took almost a week to get back to temp, and she had very little hot water while she waited.

  8. GBA Editor
    Martin Holladay | | #8

    There's a theme developing here... High-mass systems have disadvantages...

  9. James Morgan | | #9

    @Doug - going shallow with the tubing may give you some improvement in response time but we're still talking several hours rather than a few minutes. High mass systems are slow response systems: that's just the way it is, the inherent consequence of the thermal flywheel effect, and that is supposed to be their value after all. If you want fast response radiant heating that would justify the setback you need a low mass system. Radiant tube under a thermally lightweight suspended floor would offer some improvement but the best and simplest arrangement would probably be the steel or aluminum wall-hung panel radiators that are standard in Europe. These are often fitted with individual thermostats for efficient zoning and they can take a room from chilly to toasty in ten minutes.

    Of course as others have pointed out the whole issue of setbacks and the consequent issue of recovery times is mostly of relevance to homes with substandard insulation. Get the envelope right and you will be comfortable with any kind of heating system, just set the thermostat and forget it.

  10. Garth Sproule 7B | | #10

    Dan
    A week to recover in a well insulated home?? There has to be something wrong with the system. I would first check to see if all the floor loops a circulating. They can sometimes get air-locked.

  11. Doug McEvers | | #11

    James,

    Thank you for the detailed response. I am with you, build well and simplify the systems. Our idea of comfort is often misguided due to experience with cold and drafty buildings. Superinsulation has changed this and HVAC for such homes can be central and simple, providing uniform comfort throughout.

  12. Aj Builder, Upstate NY Zone 6a | | #12

    In my area a normally tubed radiant cellar floor slab cannot be set back at night. A vacation home here is set at 50 degrees unoccupied. To raise the temperature to 66 for them once, well I never saw it get that high during a day that I checked. Boiler is Triangle tube condensing unit that outputs twice what they need for lowest temps.

    4" slabs should never be set back nightly and if a vacation home, figure out the degrees the temp can be changed in a day and then install a remote controllable tstat.

    One great thing about this home. Temp is the same from cellar floor to peak of great room. And at 50 degrees the place feels warmer than a 1950s air leaky ranch at 60. Radiant is comfy.

  13. Kevin Dickson, MSME | | #13

    Keith,

    You're right that overshooting a room temperature will waste heat. My house has a 2" thick pea gravel slab and it never overshoots in the morning after a setback. The newer Honeywell thermostats can "learn" the temperature response of your house and compensate to prevent overshoot in any house.

    Setting back a well insulated house (PGH) with any thickness slab won't save nearly as much energy as setback will save in a old, conventional house. That's because the heat you save is directly proportional to the temperature of the house. A PGH with a thick slab will only cool off a couple of degrees overnight, so the setback doesn't save much heat.

    But I highly recommend nightly setback for ANY house, and the reason goes back again to comfort:

    Consider the master bathroom. A 68F slab feels cold and uncomfortable with wet, bare feet. In a PGH the slab heating system is rarely on, so the chances of the slab being warm when you need it at 6am is very low. Setback is a great way to ensure a warm slab at 6am

  14. Aj Builder, Upstate NY Zone 6a | | #14

    Kevin, your 2" slab is on what? Location? Wish we all stated our location in our name title.

    What is the finish of your slab? Home superinsulated?

  15. Monty Worthington | | #16

    I have a similar concern - newly built house that is only mildly efficient - inefficient compared to most homes discussed on this site! We did what we were comfortable with based on our limited budget and design requirements for our very temperate climate (Zone 3 Marine - SF Bay Area).

    We have been in the home only a month now - our first week we hit our design low temps for several mornings (30° F). This was when the system was just fired up so we hadn't worked out the setbacks, how to program the remote correctly, and optimum temps for each of our zones. We have very basic digital thermostats (Lux PSP511LC). I now have a 3° setback for the overnight hours, and the middle of the day for each zone. As an example I have to set the thermostat to turn on at about 5am to have the master bedroom hit 70° by around wake up at 7am - middle of the night and during the day it is set to 67°F, this room, and other rooms with large amounts of windows, get warmer after the sun hits them (no window coverings yet). We do get lots of winter days with 30° mornings and 65°+ afternoons - we are kind of in shoulder season for our entire "winter". We might have the heat on in the morning but can have the windows open in the afternoon when it is nice out for fresh air and a nice breeze. The first 10 days in the home we used 45 therms. Some of this was wasted as two thermostats were wired to the wrong room/zone and called for heat for wrong zone continuously for those first days!

    I guess my question is does it make any sense to look at the a more advanced thermostats as an upgrade to hopefully prevent overshoot - or would it even us save the cost of the thermostat in energy savings? What is the consensus for Radiant slabs? Eliminate the setback and just run our current thermostats at 68-69° for all zones all the time? Why have zones in that case??? Or is there a better solution.

    Further information: We are using a condensing NG tank heater for both DHW and the radiant heat. We have never had a cold shower or tub fill even with all the heat zones calling for heat at the same time - when we need hot water we get it.

    I like to sleep in cooler temps (down to 65°) which feels good in the winter - so don't really want to set it and forget it at one temp if I can. Plus we have a section of the house that we don't use much that we can turn down when not used.

    Interestingly people have noticed the warm floors even during the day - Suberbowl Sunday we had a party and people in sock feet noticed the warm floors. The heat hadn't been on since at least 9:30am, but with the some sun hitting them they stayed nice and warm. I have measured the surface temp at 73-74° when the air temp on the thermostat is showing 70-71 - either during a zone call or just after.

  16. David Meiland | | #17

    Does your system have outdoor reset?

  17. Aj Builder, Upstate NY Zone 6a | | #18

    Monty, for drastically changing room temperatures, one installs forced air not radiant. You could adapt yourself to your radiant system by leaving the bedroom at sleeping temperature and leaving the living quarters to another set point. But overall I think you are on the right track by limiting your setback temp change.

    Radiant is not forced air. That you now know.

  18. Monty Worthington | | #19

    David Meiland:

    Funny you should ask that - We were expecting outdoor reset, and talked about that with our heating contractor - however, based on his invoice and the fact that he went over budget (way over budget) I think that it was eliminated somewhere without me being involved. I don't think it was put in even though I discussed it with him and he said outdoor reset was his preference. I am not sure I trust him to put it in, which I am pretty sure he didn't, at this point - too many problems along the way. Long story anyway...

  19. Monty Worthington | | #20

    AJ BUILDER: I have had a radiant heated home (electric) before. I actually hate forced air. What I was hoping to glean from people here with more experience than I have is what is the typical setback (if any), or the best practice for temperature changes, and what alternatives we might have. I think I am getting close to what will work - would love any help or tips than others can offer

    Based on my last post - it seems there are things we should have had installed, like outdoor reset, were not put in - I had to fight to even get a condensing tank heater for our home.

    We are a shoulder season type area with moderate winters - so using even a small mod-con boiler with a storage tank wouldn't work as well for our needs compared to a condensing NG tank water heater, at least based on my research. I had to fight the local installers and contractors to convince them to use my choices. Unfortunately there are very few new homes built where we are and almost no contractors to choose from who have any knowledge in radiant heat in our area.

    I feel like I am on the right track on heat amounts - would love to get some tips to help. I will look at outdoor reset - again I though that would be part of our system going in - need to figure out why it isn't.

    Thanks for the comments!

  20. David Meiland | | #21

    Monty, I used to live in the East Bay and installed a staple-up system under the wood floor of my one-story bungalow... a low-mass system. It had a simple gas boiler with a couple of programmable stats, no outdoor reset or anything else. It took a short while but I learned the programming routine for that system. When the temperature swings were in the 30 degree range, it was necessary to start gaining or losing heat 3-4 hours in advance. This was a quick-responding system... 7/8" fir plank subfloor, 5/16" hardwood over, well-insulated underneath and I forget what the water temperature was. I don't see how you could easily get much of a nighttime setback with a high-mass system with those outdoor temperature parameters. Up here in WA, we get very stable temps in the mid 30s and low 40s for weeks at a time, maybe a 10-degree swing if that. It's a lot easier to ramp up and down.

  21. GBA Editor
    Martin Holladay | | #22

    Monty,
    I'm sorry to hear that you are having a hard time fine-tuning your high-mass radiant slab system. I don't really have any advice for you.

    However, I have some advice for other GBA readers: if you live in a mild climate like Monty, and your design temperature for your heating system is 30°F, you don't need much of a heating system. What you need is a good airtight envelope with high levels of insulation and excellent windows. If you have those features, you won't need much heat at 30°F -- certainly not a high-mass radiant floor with a condensing gas water heater. Your internal loads -- the refrigerator, lights, and TV -- will provide much of your home's heat. A few ductless minisplit units are all you will need for cool nights.

  22. David Meiland | | #23

    Martin, for a second there I thought you were going to recommend... radiant ceiling!

  23. Monty Worthington | | #24

    Thanks Martin and others for the comments.

    We looked into ductless mini-splits and almost switched to them. There a few things that led us to where we are now that I will share - it might help others in a similar situation choose something better.

    1. We have all of our daily living space up stairs (reverse floor plan). In that space we are pretty open and could have easily used a single mini-split. We have planned for it in case we want one in the future by placing 220V power and a pad outside where the outdoor compressor/condenser would be placed.

    2. We switched to baseboard radiators upstairs for quicker response - and lower costs - from our original design of warmboard. It is working pretty well and comes on mostly in the mornings, rarely at night. I will be curious to see how it responds as we move through the seasons. We are using them on the same water system as the downstairs.

    3. Our 3 bedrooms, baths, and a utility room are downstairs - and we have an L shaped downstairs floor plan with bedrooms on the 2 ends and the corner - I think, and was told, that this would have required 3 ductless mini-splits minimum. We weren't sure we wanted that - and we didn't want a ducted system. Further, I couldn't find anyone locally who was putting in the ductless mini-splits.

    4. We could have gone with hydronic baseboard throughout the home; looking back now that might have been the most logical solution.

    5. Our building site really leaned towards a concrete slab & foundation due to a height restriction. We had to cut into a hillside a few feet at the back of the house to get our 2 stories. A slab as part of the design made that easier - If we wanted a different floor we would have had to go deeper and costs would have gone up.

    6. We wanted finished concrete floors for our downstairs - the slab also made that easier.

    7. We went with a condensing gas water heater as we felt it offered us the best solution for our design - especially compared to what others were suggesting locally. So far it seems to be doing pretty good. Original specs from our home designer were for a Mod-Con boiler which is even more overkill I think. We also wanted to stay away from a shared tankless NG system that the local contractor and supply house were both pushing, and wanted something more efficient than a standard tank water heater. I think if we had used a standard NG 30-50 gal tank heater we might have have had some cold showers in the mornings when heat was called for based at the same time as 3-4 people getting ready for work/school. Some of our other suggestions were 2 NG tankless heaters - one for the DHW, one for the heat or a buffer tank downstream of 1 Tankless. The same people making those suggestions were pushing things like Insul-tarp among other things.

    Looking back at it now I agree with Martin the high mass floor system for our climate probably wasn't the best choice - even if we do have a decent building envelope due to our local Green Building codes - certainly not near PH or anything close to what most on this site have or recommend. We have pretty good windows, good insulation, and decent air sealing - we could have done better.

    I would have loved to have someone help me go over all of the options that had better knowledge of the latest technology, experience installing them, and the service skills to do any future repairs and maintenance. Our town is built out and has been for a long time. Sites like this help, but you can only gain so much online, and if you install something that no one can service down the road you might be stuck.

    A perfect example of this occurred when we sold our previous house with electric radiant heat panels in the ceiling - the buyer hired an inspector to look at the system who was a radiant heat "expert". He told the buyer that she wouldn't see low energy bills like we had provided. We had 18 years of utility bills showing the highest monthly bill at $110. We had 4 people in the home, including a baby, she would be a single person in the home. I expect her bills will be even lower.

  24. Aj Builder, Upstate NY Zone 6a | | #25

    Monty, you have been trying to shove a square peg in a round hole.

  25. Laura T | | #26

    Just wondering if you might be able to provide some sage advice as we live in the same area (SF Bay Peninsula) and are considering radiant heat for our remodel but want to make sure we are using the right system for our climate.

    We have a typical 1950's ranch rectangular ranch house that has gas forced air heating. It has been partially updated (windows, insulation shot into walls) but we still need to add (more) insulation to the attic and underneath the floors. We have a small kitchen that we will convert into an office. The new part of the house will be built behind the garage making the house an "L" shape" and will be about 1000sqft and include a kitchen/family room, small guest room and bath (note: the original kitchen in the old part of the house is very small and will be converted into an office/den). Of that 1000sqft about 500sqft already exists built on foundation (and used as a playroom). We will be adding 500sqft more.

    We are thinking that we would install hydronic in only the new part of the house and leave the original part of the house (bedroom) on the gas forced air system. At first, we thought we would retrofit the entire house, but decided that might not be a good idea for may of the reasons Monty states (it's only the bedrooms, we like the colder nights, no one is in that part of the house during the day so we turn down the heat very low). However, we think we'd still like it in the main hub of the house (kitchen/family room) where we will spend most of our time after school, meals, etc. and it seems as though we'd keep that part of the house on a more consistent temperature throughout the day even though we are gone for school. Does that make any sense - to have the split system?

    Also, I think we are limited to a staple-up/aluminum fin method because half of the room already exists and is in place, but then recently I read about Thermalboard and sandwich methods and am wondering if that might be a better solution.

    For the system, we are leaning toward a closed system with an independent heat source.

    Does this make any sense?

    Thank you.

  26. GBA Editor
    Martin Holladay | | #27

    Laura,
    Your idea will work.

    However -- and I apologize if I sound like a broken record -- but a much simpler and cheaper heating system is all you need if you build a decent thermal envelope. After all, in San Francisco you hardly need any heating at all, because you never get any cold weather.

    If you include a fairly airtight envelope, good windows, and thick insulation, you could easily heat your addition with a ducless minisplit air-source heat pump with a single indoor head.

  27. Laura T | | #28

    Hi Martin –
    Sometimes it just takes more than once for the info to sink in. I had not even considered mini-splits simply because I didn’t know it was an option (or quite honestly knew what they were). I just started reading about them and I think that is what I had in my last apartment for aircon. I will certainly look into them for this remodel (esp for air con!).

    The appeal however, of the radiant heat is the feel of it. Having lived in the Midwest in a radiator heated house, I just loved the feeling – no noise, no dusty air blowing around. Plus, we are a no-shoe house so the feeling of warmth at our feet is very attractive (and no shoes means no extra dirt and dust coming into the house). That said, we want to be prudent with our choices so I will look into mini-splits more.

    As for cold, well it’s relative. Surely it’s not the harsh Midwest/East coast winters but it can get down to 32 even though it might get to 55 in the day. Plus the winter means rain and – 55 degrees and rainy is miserable. The North side of our house (front) has several redwood trees that shade the house so it feels very cold. The addition/remodel faces South however with not many trees, and we will have several windows and we are building as efficiently as we can given we need to tie to existing house We just did some rough and preliminary calculations and we are estimating we will require 20-25,000 BTUs for the remodeled space. So maybe a mini-split is the way to go. Or if we do decide on radiant, we would go with a domestic hot water heater with a heat exchanger to keep potable water separate rather than having an independent closed system as I originally thought.

    Thank you for all the time and effort you put into this site. I greatly appreciate it!!

  28. GBA Editor
    Martin Holladay | | #29

    Laura,
    If you new addition is well insulated and tight, your floor will rarely feel warm, because the hydronic tubing won't be circulating hot water very often. So don't think you'll have a "warm toes" feeling very often.

  29. Aj Builder, Upstate NY Zone 6a | | #30

    Laura, if you need less heat it is best to add radiant to less sqft. Then the area that is heated is warm to the tootsies.

  30. Laura T | | #31

    Good point Martin. Thanks again. I will have to really consider this proposition and learn more about the mini-splits especially because I think it will wind up being a much more affordable solution. My initial estimates to install radiant are about $10-12k : $6-7k for materials including the heat source and perhaps another $4-5k for installation. We have an accessible crawl space, but it is a crawl space so it will be labor intensive. I think it will be difficult to install above the subfloor as we are tying into an existing room and can't really change the floor height. Definitely need to do more research. Any suggestions on where to get good info on mini-splits?

    Sorry, AJ, I'm not sure what you mean. Are you saying to selectively install the PEX in limited areas?
    Thank you again for your time.

  31. Michael Chandler | | #32

    To Monty
    What is the temperature of the water going into your floor? It sounds to me like its too hot. Since you have zones I'd recommend that you set the bedroom zones to the temp you want for sleeping and the living room zones to the temp you want for living and leave it there.

  32. Aj Builder, Upstate NY Zone 6a | | #33

    Laura, yes.

  33. Monty Worthington | | #34

    Michael:

    In answer to your question - the temp going into the floor I believe is around 120° F

    As a further follow up: When I first posted we had just moved into the home. Now that we have been living in our home for a few more weeks things have stabilized a bit. One item that seemed to help the most was getting cellular shades on our bedroom windows. We have a lot of glass in our home, and while they are good quality double pane windows, think we were getting a lot of heat loss and convection with so much exposed glass, causing me to up the temperature to feel comfortable.

    With the addition of the cellular shades the rooms have held temperatures much better and the rooms feel more comfortable at a lower temperature. In addition, on warm sunny days we have less overheating.

    On your comment about zones: At one point in the design phase, so long ago, we looked at doing 2 or 3 zones max, but our HVAC installer wanted to break it up into more zones. I am not sure how much we are benefiting from that now, but it is what we have. Our bedrooms are all on the downstairs slab - our living space upstairs uses baseboard radiators. I think we might have been able to have less zones than we have. A simpler system would have been better I think now.

    I have a few plans in the works to tune the system to meet our needs and get the water temps and zones fine tuned even more. In a month or so we will turn the heat off until probably November anyway. There are still some tweaks to the water temperature to do, getting the zones temps set, adding some extra insulation in the utility room, insulating a few pipes around the HX and pumps that hasn't been completed, and adding perimeter insulation to a few areas of the slab that also aren't completed yet. With those changes I think our system will run even better and be more efficient.

  34. Michael Chandler | | #35

    Monty
    120 is way too hot to run into a slab. At 50 degrees exterior temp you would probably be looking at 85 to 90 into the slab and at 20 degrees outdoor temp you would be looking for something in the 105 to 110 range. The calculations are more involved including flow rate at the pumps, emissive area in the floor, average daily and even hourly weather data (esp if a high mass system where you have extended lag times) and solar thermal gain. you would still want to run 120 or hotter into the upstairs radiators so you need a system that can handle two different temperature zones. The Taco iSeries 3-way mixing valves are good for tempering hot water down to a slab temp and throttling it down further on mornings as the outdoor air warms up to compensate in anticipation of reduced heat loss as the day warms up.

  35. Michael Ginsburg | | #36

    Having designed and built a successful hydronic radiant floor heating AND COOLING system in Tucson, AZ (recently received the NAHB Research Committee GOLD EVHA award for hot climate zone), I am intrigued by the varying responses to your questions.

    Is your contractor thinking that the morning time period of human activity is going to require additional heat from the floor because your above ground thermal envelope is insufficient to maintain internal air temps? What's the internal temp loss overnight without any system operation? In the bay area, I can't imagine the slab (since you insulated it) would lose that much thermal energy UNLESS you have significant heat loss in your envelope.

    You say you have an R10 insulated slab on grade but is your above ground thermal envelope sufficient to hold and maintain the ambient air temps within 3-4 degrees overnight without any system operation?

    In Tucson, our temps range from about 30 - 75 in winter but the ambient internal temp loss overnight is only about 3 degrees (without any system operation) because the above ground thermal envelope is like an 'ice chest' holding and maintaining the temps generated from the warm 'ice block' of the thermal mass (insulated) concrete floor.

    If you are using an electric water heater (30kw) I don't think it matters when the time of use is. A good radiant system (including a very good thermal envelope) has to be managed in a way that prevents over heating.You just have to learn the nuances.

  36. Robert Bean | | #37

    This graph may shed some light on the relationship between floor temperatures, flux and operative temperature. As noted by Michael Chandler...the calculations are involved with the fluid temperature in the pipes being a function of the flux, flooring characteristics (i.e. fin efficiency) and tube spacing. All the gory details can be found in the various chapters in the ASHRAE Handbooks.

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