Replies

1. 
   DCContrarian|Sep 03, 2020 10:42am|#1

   Do you want a ducted AC or do you want to use the radiant heat tubing for cooling as well? If you're thinking ducted there are no special concerns, just don't hit a pipe when you cut the holes for the ducts.

   If you want to use the tubing, here's what you need to know. The reason they call it "air conditioning" and not cooling is that AC does two things, it cools the air but it also removed humidity. Without removing humidity the air is cool and damp and uncomfortable. If the dew point of the air is above the temperature of your cooling equipment you will get condensation, which will lead to mold growth as well as making for a slippery floor.

   So you'll need a way to provide humidity control as well as cooling. As far as I know there is no way to remove humidity from air other than running it over a cool surface, so you will have to have some sort of air handling equipment. In a traditional AC both steps are done at the same time, the part of the cooling that goes to humidity removal is called "latent heat" and the part that goes to cooling the air is called "sensible heat."

   So the first step would be to run a Manual J, which will provide calculations for your latent heat and sensible heat. Then you need to design a system where your piping provides an appropriate level of sensible heat, and a separate system provides dehumidification as needed. Then you need a control system to tie it all together, basically you can't let the coil temperature get below the dew point of the air. In your zone if your coils provide sufficient heat in winter they should have no problem with the sensible heat in summer.

   There are people on the internet who claim to have this working, but I've never seen anyone describe in detail how it all works. Here's an example:https://www.pmengineer.com/articles/93157-looking-up-to-radiant-cooling

2. 
   DCContrarian|Sep 04, 2020 12:36am|#2

   I've been thinking about this a lot because I asked a similar question a few months ago. The general consensus here was that it was a bad idea although no one made a case that was persuasive to me.

   Having thought about it more, I think the reason radiant cooling isn't common has to do with sensible heat ratio. Recall from my post above that air conditioning consists of both cooling air and removing humidity, both of which remove heat from the air. The heat that is removed by cooling the air is called sensible heat and the heat that is removed by removing humidity is called latent heat.

   With a radiant cooling system you are seeking to separate the sensible heat removal from the latent heat removal. The reason for this is that the radiant system is only capable of sensible heat removal because it has no way of dealing with the water. The problem is that the only way to remove moisture from air is to cool it, so you can't have latent heat removal without also having some sensible heat removal. Sure, you could negate the sensible heat removal by putting the waste heat back in -- that's the way a dehumidifier works -- but it makes no sense to do that if you need sensible cooling too. If you already paid to remove that heat you may as well dump it outside.

   So you can't have latent cooling without some sensible cooling. The question becomes, if you want to have part of the system that provides only sensible cooling, how big is that going to be relative? This is where the sensible cooling ratio comes in, which is the ratio of sensible cooling to total cooling. You want to know two things: what is the sensible cooling ratio needed for the whole system, and what is the sensible cooling ratio of the part that provides dehumidification? You already know the ratio of the part that provides only sensible cooling, it's 100%. Together those two parts have to provide all of the needed cooling.

   The second question is easier to answer. If you assume that you're output air is at 100% humidity, the colder it is the lower the sensible cooling ratio. To give some idea of the magnitude, if you take air at 85F and 50% RH and cool it to 60F the ratio is 72%. At 50F it's 58%, at 40F it's 56% and at 30F it's 56%. You can't really go below 32F or your coil will freeze up, so 56% is the lowest sensible cooling ratio you can achieve.

   Now, what is the sensible cooling ratio of the system? It depends on your climate and the building. Looking online I found estimates in the range of 80%-95%. I'm going to assume that humid climates like the east coast are at the lower end of the range.

   So if your system sensible cooling ratio is 80%, and your dehumid has a sensible cooling ratio of 56%, what does that leave? It's not a problem of simple addition. But say you need 10,000 BTU/hr of total cooling. At 80% you need 8,000 of sensible and 2,000 of latent. If your dehumidifier has 56% SCR to get those 2000 BTU/hr you need 4500 BTU/hr from the dehumid device. You also need 5500 BTU/hr from the radiant loops. So roughly speaking, half and half.

   So I think the problem with hydronic cooling is that you essentially still need a conventional HVAC system to handle humidity. Granted it's half the size, but having two separate systems at half the size is going to be more expensive and complex than one full-size system.

   Now, the higher your sensible heat ratio the more sense a hydronic system makes. In a dry climate I could see having a mini-split in "dry" mode to provide all the latent cooling, while hydronic coils provide sensible cooling in each room.