Radiant hydronic system plus forced air augmentation

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Old 01-07-07, 09:13 PM
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Radiant hydronic system plus forced air augmentation

We are finally building our retirement home in Logan, Utah, and have run head first into a reluctant HVAC guy. He was selected by the general contractor, but hasn't convinced me that he knows what he is doing. He wants to size everything based solely on square footage of the house! (He even thinks that we need 4 tons of AC for 1800 sq. ft. of main floor and a few more tons for the 900 sq. ft. of loft/office and bonus room in the half story. This would be for a house at 4500 ft. elevation in northern Utah. Our house in Phoenix, AZ only has 4 tons for 2100 sq. ft.)
I have to put in a forced air system in case we want to install AC later on. The compressor part will not be installed until, and unless, we think it is actually needed. I want him to use the forced air system to augment the radiant hydronic system, which gets installed under wood and tile floors between floor joists. I figure the hydronic system can be the main source of heat, but since changing the temperature of the house with that alone takes many hours, I want an air handler that has a hot water coil.
I am thinking that the radiant system is the "base" load and the forced air system is the "peaking" load. The air handler part can handle any temporary perturbations in the force, so to speak. He is trying to convince me that an ordinary gas furnace is the way to go, even tho there will be radiant heat under our feet. He is young and I suspect he is being asked to do something that is a totally new experience for him.
Is my thinking bogus here?
 
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Old 01-07-07, 10:36 PM
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It certainly sounds like you could use a better contractor. You could also go with radiant and oversized panel rads in the other rooms. On every rad have a TRV (small non-electric temp control). Run all the circuits from a manifold and also try to keep it all one temperature. Do the A/C hi high velocity ductwork and tie it in to any forced ventilation.
 
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Old 01-08-07, 07:37 AM
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I second Who's advice. Find someone who knows what they're doing. I'm all for young people in the trades, but in this situation, you don't want your house to be the place where he cuts his teeth. If he's not trained up, then he's not trained up. Simple enough. Either he knows how to do heat load and a/c load calculations, or not.

I would be even more worried about a rookie radiant installation. It's not rocket science, but it does take a fair bit of skill and experience to get right, and fixing mistakes can be very painful.

Radiant plus panel radiators with thermostatic radiator valves would be a great way to achieve the "base load" plus "boost" you're after. Keeps the heating as all one system, and allows the A/C to be its own system. I like that approach.
 
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Old 01-08-07, 03:02 PM
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Agreed, except

You need to chase off either the GC or the HVAC sub (or both) with a big stick.

I disagree with Who on the high velocity system. Since you are still in the planning/building stage, I certainly would use a conventional duct system. High velocity systems are CRITICAL in their design & installation. They also reduce the SEER rating of your cooling system significantly.
 
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Old 01-08-07, 03:15 PM
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The same guy did my daughter's house. Their gas furnace short cycles a lot, making the whole house humidifier they installed a waste of time. I would select a slower fan speed to see if that helps, but it isn't my house and my daughter doesn'st seem to think I know much unless it is convenient, or finanacilly advantageous, for her to do so. I favor a conventional ducting system, but an air handler that runs at a slower speed for a longer period of time for better heat distribution and less stratification. Not sure how that works with gas furnaces, as I have never had one before. Always lived in warm climates except one time and that house had baseboard electric.
But it is decision time, so I have to do something.
 
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Old 01-08-07, 03:41 PM
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Fan Speed

Many modern gas furnaces are available with ECM motors & controls which can be set for a constant very low speed. This helps greatly with comfort & thus reduces energy consumption. When the furnace is firing they will ramp up to maintain the set temperature rise across the heat exchanger.
 
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Old 01-08-07, 04:11 PM
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Bill: There is nothing bogus about your thinking. IF properly engineered and installed, your proposed system would definitely be a high-end system. There would need to be some careful consideration of control but it is certainly a do-able concept.

I agree with Grady that since this is new construction that you do NOT want a high-velocity A/C system. Just as Grady mentioned, high-velocity systems need greater care in engineering and installation than low-velocity systems. High-velocity (contrary to what many will tell you) tends to be noisy.

Since you have made the decision, at least tentatively, to have A/C ductwork installed the addition of a hot-water coil in the air handler plenum is not a serious increase of cost and could give you far more comfort and controllability of your indoor environment.

I would NOT recommend that you go with a "simple" forced-air heating system as your contractor is recommending. If necessary, hire your own HVAC engineer and installing company.


For what it's worth I did these kind of design scenarios as part and parcel of my job for over thirty years, mostly commercial and industrial but also a few high-end residences.
 
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Old 01-08-07, 05:10 PM
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Easy guys...

When I hear the word loft, I tend to think that there isn't that much space for normal ducting... that and the fact it was 1:30am had me thinking hi velocity. Personally I'm a major mini split ductless fan and they have an even worse SEER rating yet somehow they just don't cost as much to run... - go figure! ;-)

Bottom line, everyone here agrees that if the contractor is doing HVAC calcs by the square foot, they really don't know what they're doing as well as they should. Get someone who knows what they are doing. You'll be paying for how much that guess was off for a very long time.
 
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Old 01-08-07, 05:32 PM
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If it was strictly up to me, I would not even have AC. I have lived in similar climates before without it. Didn't even have a swamp cooler! But modern people accustomed to soft living may have different ideas come the day we have to sell the place. I am already 60, and the nursing home or assisted living is only about 25 years away, if I am lucky.

The loft/office area is over the kitchen dining area, and there is a large bonus room off to one side of it, total area upstairs is about 900 sq. ft. That includes a 7x12 room in the middle that can be used as an upstairs equipment room, where I can put an Apollo 2.5 ton/42K BTU coil air handler that I already have. Being in the middle should make the ducting rather simple. That should take care of any upstairs needs. That will all be pre-plumbed, wired, etc. but actually installing the air handler will get done later. If I have the main system feed the office portion of the loft, it may be enough without adding the upstairs system. The idea of a mini-split system, like motels?, is appealing as well. That could easily fit under the window at the end of the bonus room.

The pressing need is deciding on an air handler for the main floor and eventually the basement, when it gets finished. I will make the contractor do a load study for sizing. Knowing that a study was done and equipment sizing is reasonable is worth a few hundred bucks, easily.

Can you guys advise me on brands and models? I like the idea of flexibility in fan speeds, etc.
 
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Old 01-09-07, 02:24 PM
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A/c

I'm a bit confused. The "motel" systems are usually not mini-splits but packaged terminal air conditioners or heat pumps installed thru the wall (think overgrown window unit). Mini-splits are true split systems with the indoor & outdoor units connected via refrigerant carrying tubing & wiring. They just have no ducting & are now available with some very high SEER ratings. I installed a Sanyo this past summer with a SEER rating of 16. Some are even higher.
 
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Old 01-09-07, 02:56 PM
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So a motel type has a great big hole in the wall, while a mini-split will just have small holes for refrigerant tubing, and the outside unit can be close to, or far away, as needed. Right?
 
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Old 01-09-07, 03:19 PM
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UtahBill

That is correct. Motel type (Package terminal unit)= Big hole. Mini-split (ductless) = holes for tubing, wiring, & drain only.
 
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