Can you get radiant cooling using forced air systems?
doc_ankush
| Posted in Mechanicals on
I am planning on installing my VRF ac unit air vent above a metal false ceiling so that the ad cools the metal ceiling and the cold ceiling cools the room by radiant cooling . Is this practical ? Has any one any experience about something like this ? Or if you have heard of someone who might be using a similar system ?
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You'd better be very aware of the dewpoint . Besides radiant cooling you may get indoor rain also .
This is just not a good idea .
What problem are you trying to solve by doing it this way? On the face of it, it seems like a bad idea, but if we knew what you were trying to achieve, we might have other suggestions on how to achieve that goal.
And what climate are you thinking of doing this in?
But isn't this how hydronic radiant systems work using metal ceilings with small pipes over it ? Can any one suggest a few tips on how this system can work properly ?
I am in India where we have high humidity for 3 months . I don't like forced air systems , but I don't have access to good hydronic radiant installers Here . Maybe if I can devise the system to throw air into the room during monsoon and over the ceiling over rest of the year ? They don't sell radiant ceiling panels here . Moreover if I let the ac return air to come from above the ceiling only , won't the ac use less electricity since it will have to cool only the area above the ceiling . Maybe if I keep temperatures above dew point ?
Ankush,
The most energy-efficient way to cool your space is to use your air conditioner the way it was designed to be used -- to blow cool air into the space you want to cool.
Your scheme -- to use the cool air to lower the temperature of a metal ceiling -- will require more energy (since it is an indirect method that will require you to over-cool the plenum space above your ceiling) and will cause condensation (also known as indoor rain, as Richard said). Don't do it.
Radiant cooling only works well in fairly dry climates, with low dew points to the outdoor air. Those low outdoor dewpoints during hot weather are nowhere to be found in India, even in the driest desert regions of Gujarat & Rajasthan.
To do radiant cooling effectively you need the dew point of the outdoor are to be below 10C, whereas during hot weather the outdoor dew point averages are above 15C and often above 25C, even in the driest parts of India.
Enter your location into Weatherspark, and pull up a dew-point graph, and scroll out to view the season of interest:
https://weatherspark.com/#!dashboard;a=India/Gujarat/Kheda/Jodhpur
About 10C/50F is really about the highest dew point at which radiant cooling is fairly easy. Above that it requires specialized controls, and above 68F/20C it's next to useless, even with dew-point compensating controls.
Dana . Maybe you have not heard .
Thanks Rich--that's a very interesting and useful study!
As for its implications for Ankush, that system had a "dedicated outdoor air system" that took care of dehumidification. It would be unusual to do that in a smaller building like what I assume you are considering, but it would be possible. But setting that up with proper controls would be challenging, and if you have a lack of contractor skills to do a more conventional system, doing a less conventional system is going to amplify that problem.
But if you dislike forced air, maybe we should consider designs that overcome the aspects you don't like more directly. For example, using a delivery system with a very low air velocity, which could be a mini-split head with the fan on low, or sized such that with the fan on auto, it usually runs on low. Or it could be a ducted system with large ducts and registers and/or more registers that a normal design would call for. The extreme of this is called "displacement ventilation".
Charlie ,
Just as with hydronic systems there are very few that design properly as you have described . DOAS systems should not be disregarded in smaller buildings or homes . Especially the homes most often discussed here at GBA .
Make no mistake , I do prefer water based systems . That aside I a m a firm believer that whatever system is used MUST be designed properly .
Maybe we need to be more aware of contractor skills and not just rush to the ductless mini split that will amplify the lack of same over time . we are in poor supply of qualified men / women now , why make it worse by not requiring they KNOW what and HOW to design proper systems .
I always see here how everyone talks about German and European stuff in general . They embrace renewables and have very few ductless systems as a whole . 99% of German homes and buildings have hydronic systems . I remind you that efficiency is a mandate there .
http://forum.heatinghelp.com/discussion/155897/how-a-german-home-maximizes-energy-efficency#latest
I might point out that Solar thermal is far from Dead in Europe , CHP is used extensively and that PV is also alive and well although not the end all be all how some view it . They take a whole different approach , one that this country should look at .
Thanks a lot for the responses guys . I think I will set up a small experiment in my shed and see if that works . Will update . I am dead set on using a radiant system so let's see how this experiment goes . I had read this on the net somewhere about a Samsung building in the U.S. which is using the system I had described . There were no details and I can't seem to find the site again . Will post when I do . Thanks
Richard/Ankush: The large commercial building radiant cooling system would not make it without the mechanical dehumidification. Without a reasonably air-tight and ventilation-controlled & humidity controlled building it simply wouldn't work- you'd have puddles of condensation everywhere.
Given the nature of this forum my presumption was that this is for a residence, not a large commercial building where that level of sophistication in the mechanical systems are financially rational. But perhaps that assumption is simply wrong (?).
Ok got the link .
http://energydesignresources.com/media/1783/EDR_DesignBriefs_radiantcooling.pdf?tracked=true
Check page 6 regarding restaurant in Munich . This is what I was trying to create . Anyone heard of this ?
And this is for my home . I am trying to do this only in one room , approx 250 sq ft . If it works , then might do it for the whole house
Ankush,
Quoting from the document you linked to: "The [ceiling] panels function as large diffusers." This is a forced-air heating and cooling system, not a radiant system. The ceiling is leaky. The forced air is delivered through the ceiling cracks, so the ceiling acts as a forced-air diffuser.
Ok . So this will slow down the air .right ? What if I install perforated metal ceiling or perforated pvc ceiling ? Will that work ? Or will condensation still be a problem ? If cold air comes down slowly through the ceiling won't that give more uniform cooling in the room ?
Dana , the biggest problem we have suffered over the past several decades is those who refuse to see a building as a building . As if ASHRAE Standards only apply to everything but residences .
Yabbut, residences are DIFFERENT from buildings, y'know? ;-)
The gross levels of oversizing of mechanical systems seen in residential settings are rarely seen in commercial buildings, but that's not to say the average commercial HVAC installation is anywhere near perfect.
I'm not sure whose residence would be calling for radiant cooling in India (Shah Rukh Khan ka ghar shayad? ;-) ) What type of building is this (what is it used for)? Ventilation rates and cooling loads for commercial buildings with high occupant density are much higher than for homes, which has a direct affect on the amount of ventilation and cooling needed.
With any type of radiant cooling, the temperature of the radiant surface needs to stay above the dew point of the air on the room. That means the humidity in the ventilation side the building needs to be reduced, and the room air humidity controlled. It can be done, but it's not a simple problem when outdoor dew points are as high as they are in India. It's a lot easier in California than any location in India (except at high altitudes where cooling isn't really needed.)
I am an architect on the Front Range of Colorado considered a Zone 5 Dry Mixed Climate. I've been reading of the increasing use of radiant cooling in European commercial buildings employing various types of radiant ceiling panels. But they are also using the radiant floor tubing system for cooling. These are energy-wise buildings with lower internal loads and are satisfying a high percentage of the total cooling loads in European climates mainly in the northern reaches of the continent. There is a dewpoint sensor linked to controls keeping the floor only a few degrees above the interior dewpoint to alleviate condensation problems. Dehumidification can control the dewpoint to allow higher capacity in hot humid weather.
I am planning to use this strategy to cool a ductless 'pretty good house' project using the floor as well as a solar exposed mass wall. There are interesting ceiling panels being produced by Messana Radiant Cooling - a pre-plumbed gypsum panel faced unit that appears to install pretty seamlessly.
There is no reason to ignore your hydronic distribution system and always think that air distribution is the only way to go. The experience the Euros.have shows that circulating water temperature needs to be only 3-6 degrees below ambient indoor to effect an acceptable amount of exchange.
Dave,
Hydronic cooling systems can be made to work, but they are more appropriate for large commercial buildings than for single-family homes.
Downsides:
1. Dew point sensors require unusual controls. These sensors and controls are expensive, subject to failure, and hard to get repaired by local HVAC contractors.
2. You'll probably need a whole-house dehumidifier in addition to your radiant cooling system. That adds expense.
3. Hydronic distribution systems, boilers, and chillers cost more than ducts, furnaces, and air conditioners. Unusual systems are hard to get serviced. The main reason that duct-based systems have a bad reputation has nothing to do with effectiveness or occupant comfort. These systems have a bad reputation because almost no one bothers to design duct systems, and the systems are sloppily installed. (In the U.S., residential duct systems are are often leaky and located outside of the building's thermal envelope.) A well designed and properly installed duct system can be effective and can provide high levels of occupant comfort.
Dave,
You live in the right climate for this. And it sounds like you are going into it with a good understanding of the issues. However, I think you may want some capability to control humidity. It looks like there were a few weeks last year when you had oudoor dew points between 60 and 64. That would lead to indoor humidifier in the 60s with no indoor moisture generation, and perhaps in the 70s with. You might opt to tolerate that, but if you are aiming for excellence, I think the ability to dehumidify and or cool conventionally with dehumidification would be called for.
You could include some Chiltrix fan coils for cooling with dehumidification (along with the capability to send them colder water without sending it to the ceiling panels), include a dehumidifier with minimal ducting, or include dehumidification capability in the ventilation system ( as is normally done in commercial systems like this.)
In your climate, you could probably get away with a design that doesn't have any dehumidification, but at the expense of some reduction in comfort and some risk, especially if you have occupants who sensitive to dust mites, etc. Happily though, if you do include dehumidification capability, it won't work terribly hard or cost as much energy as it would in the Southeast.