HRU - Not what I consider the best value especially for a homeowner. I'm a 57 year old retired , at 46, scientist who worked for many years in the field consulting in analytical materials labs. I worked with lots of instrumentation and it all had various systems to move heat or do various things with materials and electricity. I also was exposed thoroughly to developing methods to test how plants who make things run their production lines. Anyway I say this kind of product doesn't look like a good value to me for a couple of simple reasons. 1. The initial cost . If it's $100 or less well I can't complain. It's only a box with a tube basically. So what you pay is money that's out of service for you and not working elsewhere. 2. The freon has to be plumbed into the unit. This is more cost and introduces something I would never do in a laboratory unless it had more gains than this ... A leak would ruin the A/C unit and cause downtime plus the cost of the A/C unit. This isn't a good plan. If the unit came with the A/C mfg design I would consider it a part of the method the plant uses to build the entire unit. But aftermarket just does not ever make much sense to me unless .. there is no other way. I think there's another way. 3. The efficiency ratings I read are 10%. That is exactly the gain I got when I water cooled my coils on a home refrigerator unit. I got a similar reading when I cooled a regular home A/C unit with water. The amount of water to cool my parent's 4 or 5 ton , 1935 (it's still running) A/C unit is about 6 gallons / hour. The amount of water usage is very low. So I say this: Use water to cool your coils. You'll lose the heat energy to the water if the water's thrown away but assuming it's at home the water can be used so it's not wasted. And if you want some heat that's in the water then plumb a regular $50 heater core or small radiator on the cooling coils of your A/C unit. If the size doesn't fit .. yes you'll have to reconsider the design. Maybe you can only run 1/4" copper tubing firmly attached to the coils .. But you'll be running water through the secondary coolling system .. not freon. And a failure of that system won't affect the safety or even the operation of the A/C .. as long as you haven't blocked the air flow. If not for the probable high initial cost . I'm guessing $400 and the plumbing into the freon line I would be positive towards a unit like this. Which I am via the design ideas above. 4. Another method to cool coils without putting water directly on them is to use a 'swamp cooler' design around the coils where air comes in to pass over. I realize this won't work as well in Houston or Florida. However air that is 90F and wet will still carrry away more heat than air that is dry and at 90F. Any gain of humidity with pads and a bit of water over them will give you some gain of efficiency. And there is the fact that your compressor will run longer.. ours has been running in 100F Summers since 1935. And I personally know it's been running since 1975 without any maintenance or downtime and the costs of running it are low. This will occur with both plans of my idea or the HRU. One things I don't quite get in the marketing is the absolute heat temp of the output of the HRU. If the compressor is to run cool it 's going to need cool water. The outflow from the unit we have , from 1935, is at about 15F above the incoming temp. While this would help to preheat water I would not expect hot water nor would I want hot water. I 'd want my compressor to run as cool as possible. You decide. I have not run extensive testing. I'm using mostly experience but not any calculations on the HRU except the specs I've seen. I have an idea. Seriously. Why not put a 100 gallon tank of water in your car and use your engine to heat it to about 180F as you drive around? Then when you get home you'll have a day's worth of very very hot water 'for free'. Seriously. This is the exact design used for carpet cleaning vans which use engine heat to heat the steam either preheating or fully heating. I don't know how they can get to 212F without the engine overheating but they do , for about $3000, offer a heater that runs off the engine and heats the clearning water. why not do the same and then 'plug in your water tank' when you get home for an whole night's worth of hot water ? There are ideas out there. They're not dumb. Just weigh the cost and risks. Andre

It would be a shame to tear out the steam radiators from your 1924 house. Since the boiler is only 5 years old my suggestion is to have a steam expert evaluate the system to determine what repairs are needed. It could be a simple repair like replacing radiator valves. Installing a furnace an the attic will be expensive. I assume the registers are currently in the ceilings, and the height is more than 8 feet. This works well for cooling, but not optimum for heating. Consider the comfort a steam system can provide in making your decision.

Make sure you have covered angled vents (kind of like this https://www.amazon.com/Ventilaider-Deflector-Low-Profile-Non-Adjustable-Sidewalls/dp/B09CZL59WZ/ref=asc_df_B09CZL59WZ/?tag=hyprod-20&linkCode=df0&hvadid=532423223930&hvpos=&hvnetw=g&hvrand=16860139863354245117&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9016908&hvtargid=pla-1485715464394&psc=1) where it shoots the air across the floor instead of straight up in the air. It mixes with the cold air as it casually rises. The air will rise any ways, but you also want to have atleast and 1/8th to a 1/4 of an inch thick glass windows, and then tungsten oxide or pure silicon dioxide for storm windows, they're very strong, they can be thin and still very strong but they will cloud or freeze. You can also roll mylar plastic and glue it on the walls and plaster over it and then paint and it wont let any heat out, or in. If you don't want thick windows mylar works too well lol. Sometimes they don't suck fresh air in through the furnace and they try and recirculate it through the furnace to minimize heat loss or rise in the gas or electric bill. But less oxygen in the house lowers the furnace's flame's burning efficiency. And then no pollutants will get pushed out through the pressure relief vents. If there's no pressure relief vents it can be hard to open the main doors. In some other really large houses and I would just leave the furnace running because of having suction fans on for smoking or because of methane, carbon dioxide build up, other pollutants. And it would cool the house down and the furnace would stay on I almost just put a little heater on a vent sucking air in and a suction fan sucking air and smoke out. Smoking sucks if you cant light and take a few puffs where ever you're standing. But air purifiers do get rid of smoke. You may need air flow restrictors on your vents so that it allows pressure to build up in the duct at the vent and shoot the air down the longer ducts to the vents that are further from the plenum (place above or below the furnace where all the ducts intersect). Measure all the ducts from the plenum to the vent, and add the length together. Divide each duct's length into the total of all the duct's length's from the plenum. Say the total is 100 feet, one duct is 45 feet, one is 25 feet, one is 20 feet, and one is 10 feet. The shortest duct needs the most resistance to cause a pressure build up 10 ÷100 is .10 so it's 10%, subtract 10% from 100% you get 90%. You want 90% restriction on the vent. Say the vent is 40 square inches or you might want to put the restrictor on the duct. The duct is 20 square inches you want to only let 90 percent of that air through so you want to block it. You would only want 2 square inches or circular inches in total size of the hole or many holes if you want to drill a bunch of small ones in a metal, wood, or plastic plate. And you do this calculation for each duct and then you put the thermometer for the thermostat in the coldest part of the house. The furnace will run longer but it will be evenly heated, if you have hot spots or cold spots you may want to add more vents, You could do the same type of calculation if you want to add multiple vents to one duct. HVAC systems can get very sophisticated especially if we start trying to monitor humidity, pressure, drafts, oxygen levels, pollutants, and stuff like that. Furnaces almost became like computers, but there's a lot of methods or a different kind of engineering that isn't mechanical, or electrical. It's also possible to heat the duct without burning anything by having two magnets close together or moving back and forth. One of my old furnaces wasn't electrical or gas. I used two air hydraulic lifts (like the car lifts in garages) to spin something, one goes up and pushes the other one down and then it goes back up and pushes the other one down and it just keeps going on and on forever, unless it's switched off. Nothing was electronic besides tvs, computers, and telephones. Even light bulbs were just heated crystals and we didn't mind wicks being burned regularly, it was just the oil that sucked, it started as whale oil, but eventually they started using petroleum and everyone almost died of cancer. I use to make records out of pure carbon, I was standing by a pile of carbon from burning wood and some one yelled something to me and then I would hear the same thing again a little quieter, and I said why do you yell it at me and then say it again quietly really close by. He said I didn't and I said whatever. Eventually I fell down and I heard myself fall and complain. Record players werent electric at first anyways and they winded themselves back up and you just had to switch the rolls. They kind of forced this whole electricity everything thing on us, but electricity was more simple, but everything became motors, timers, belts, sensors and then eventually microchips, instead of gears and pulleys, and all these other methods.

You might want to consider a new thermostat with programmable temps. No need to remember to re-set the temp. The system will adjust the temps gradually so you can wake up to the correct temperature without "shocking" the system. Love my Honeywell heat/ac. also, my Honeywell has a wi-fi app that lets me adjust the temp remotely. I didn't think I would use this but I actually do.