Where to put AC ducts on hydronically heated homes

ontariomomAugust 4, 2012

Hi GW -- I have cross-posted this on the building a house forum -- sure hope that is okay.

For those of you who have hydronically heated homes, where did you place your AC duct work? We have been back and forth many times with our HVAC contractor trying to find both an efficient and asthetically pleasing duct placement. I feel very frustrated with the process.

Here are the details on our home:

- Original home was a raised ranch

- 2 story addition was placed on the back half of home (part that looked like a standard bungalow before due to grade)

- main living space is very open concept (except a bedroom, laundry room and bath. Upper level contains two baths and four bedrooms.

- All of our ceilings including basement, main floor and upper floor are 8 feet high

- All heat will be supplied by underfloor hydronic heat

- The total square footage not including basement is 2900

We have discussed using either one large air handler or two smaller ones. My priority is to avoid drop down boxes in the main living area (basement would be okay) as our ceilings are only 8 feet. We are likely going to have one air handler in the new attic over the new second floor and frame that area and drywall in a section so it is conditioned space. We are not sure where to place a possible second air handler, although theoretically we could also do another conditioned attic space in the old attic (the original attic that did not have an addition built over it).

Please tell me what you worked out, and where you had to give up ceiling height for drop down boxing, or closet and/or wall space for duct runs.

I look forward to hearing how others have solved this problem. TIA!!


P.S. Maybe we should look again at Mini-splits

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If you are able to, please post a blueprint drawing showing both levels, and in particular where closets are located.

I live in a older home built with radiators, (that's my hydronic heat). AC was added about 30 years ago, and we just replaced the original unit 2 years ago. Our AC functions with an air handler in the attic. Upstairs, the duct work was run in closets, and registers are high on the walls. No room returns, expect where we did a recent addition. All doors are undercut. Main return is in the hallway ceiling where I must guess an attic fan was originally located. There is a real return duct run on the addition that was done in the last 10 years. This connects back into the plenum before the filter area on the air handler.

We have AC upstairs (3 BR/2BR/added closet/office/laundry room. AC on main level LR/DR/Kitchen. There is no AC in the basement. The only soffits we have are in the kitchen and kitchen 1/2 bath. The remaining LR and DR spaces are directly below the closets in the bedrooms upstairs, and have ceiling registers. The downstairs return is that hallway ceiling.

You might be able to do something like this, and then have a smaller unit serve the basement and any main level areas not easily reachable from closets upstairs.

    Bookmark   August 4, 2012 at 6:25PM
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What water temperature do you need for your in-floor radiant heating?


    Bookmark   August 4, 2012 at 9:29PM
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Julie and SR,

Thanks for your replies. Julie, the system you have set up would be very workable and meet our needs to minimize soffits as well. I will attempt to post floor plans tomorrow to get opinions on what might work for us. SR, I am afraid I don't know the water temperature we need for the in floor. I know we are using an Elite low Pressure boiler (model 220). On the specs on the boiler it says the maximum temperature is 210 F, but I doubt we will reach anywhere near that maximum. I will ask my HVAC contractor when I next see him.


    Bookmark   August 4, 2012 at 11:03PM
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The reason I asked for the temperature required for your in-floor radiant heating is that it may be possible, though expensive up front, to convert your heating source to geothermal. You could then also cool your house with the same system while never having to install forced air ducts at all. Instead strategically installed supplemental fan coil units that would both heat and cool, fed by small diameter insulated pipes would be all that is required (basically).

Your energy savings would be substantial.


    Bookmark   August 5, 2012 at 1:37AM
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I would also add that when we replaced our AC system we chose a heat pump instead of just AC. We have been very happy with this.

A HP allows us to have "instant on" heating in milder temp of spring and fall, when it can be so warm during the day that you don't need any heat. Then in the evening and overnight when it is a little chilly, voila the heat pump warms it up quickly and very efficiently. Because the coefficient of performance is very good at mild temps in the 40-50-60. We have our system wired up to automatically change over to the boiler at 40 outside temperature by having a temp sensor tied into our thermostat. This way we avoid electric heat strips.

Before, our system had one Tstat for the AC, and second for the boiler. So if you were cold in early October, you found yourself having to switch on the boiler heat at 6 pm and suffer in a chilly house for it to warm up. In our old house that takes a long time.

So our home is much more comfortable with the HP setup.

    Bookmark   August 5, 2012 at 10:15AM
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Hi Julie and SR,

SR, we did talk briefly about geothermal, and concluded we did not have an ideal spot on our yard for the hole, plus of course the cost for geothermal is high. We have taken a fair chunk of our yard for the addition and we are located on a suburban pie shaped lot. I didn't know before your comments that geothermal would work for cooling. I will revisit the geothermal idea with our HVAC guy who does geothermal too.


Thanks for the extra info on heat pumps -- sounds great for the shoulder fall and spring seasons. My parents had a heat pump installed in a home they used to live in. It never really cooled their upper level very well, but I suspect that had more to do with the ducting work or maybe their unwillingness to crank the cool to save $$$. We will enquire about a heat pump for the reasons you cited.

Here are some floor plans of our home.

Main floor (ignore the shading that represented the HVAC guy's first plan for fur downs throughout main level which we will not be doing).

Here is the upper bedroom level. The bedroom level is over the following rooms in the main level: kitchen, main level baths, laundry, great room and dining room. The bedroom level therefore does not fully cover the main level.

Here is our basement level. Unfortunately on the top of the screen there is another room that is not showing that will be a study (the dimensions of the study are roughly 10 feet by 13 feet). Also the tandem garage on the top of the screen is not fully showing.

Let me know if you see any solutions for ducting that will eliminate or greatly reduce the need for fur downs. Thanks for your assistance.


    Bookmark   August 5, 2012 at 2:48PM
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The fact that your home was built without cooling indicates that you won't need much. Why did you stop considering mini-splits? The trouble there is that you might be buying way too much capacity than you need to put them in all rooms that you want them. If you can locate small air handlers easily, you can supply two or three rooms with ducted mini-splits.

    Bookmark   August 5, 2012 at 8:35PM
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Hi ionized and thanks for your reply,

Our original house was not built without cooling, our home is in the final stages of framing for an addition out back and moving of walls/major renos in the front portion. Prior to our large addition/remodel we had forced air heating and AC. The HVAC contractor has almost finished up the in floor hydronic heat which will replace the forced air. He started to work on the AC a few months ago, and to our shock installed large 9-10 inch main trunks in our great room and dining room (I posted about the issue at the time and I think you kindly responded). The contractor then explained for the first time we would have lots of these ducts all over the main level. The contractor stopped the progress as we wanted a new AC plan that would minimize the drop downs (contractor went on to some other jobs, and we focused on other aspects of the build as well). We definitely need AC here, our weather would be like Michigan or Buffalo -- lots of brutally hot days for 3-4 months per year.

As per the mini-splits we are definitely still considering using them (or ducted mini-splits too). We have heard mixed reviews about mini-splits. I think the main draw back we have been told is the mini-splits need replacing say every 10 years or even sooner, so they don't last as long as central air (hence seen as a less permanent kind of AC). Also, if we go ductless, it will be harder to tie in the HRV unit. Not sure if these warnings are justified or not. The architect and HVAC contractor were down on the mini-splits so made as weary. Cost may also be high with mini-splits, but so is making a conditioned area in the attic I am discovering (not sure yet which is a higher priced solution). A high velocity unit might also be worth investigating.


    Bookmark   August 5, 2012 at 9:29PM
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Hi folks,

Just to let you know we are meeting next week with a HVAC design who was a former installer turned designer. On the phone he wanted us to think about using high velocity AC. Any comments on this idea?


    Bookmark   August 11, 2012 at 7:57PM
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Hi again,

So we met witht he HVAC designer yesterday. I think he had a good solution to avoid or near avoid drop down boxes that would be challenging in a home with 8 foot ceilings. He is recommending a high velocity system, and we will locate the air handler and HRV in conditioned attic space. Our ducts are only needed for AC and HRV as we are to be heated with in-floor radiant heat. He did say we could use the High Velocity System to supplement heat if needed. I had a few questions for your folks if you don't mind:

1)Any opinions on using High Velocity AC? What are the biggest drawbacks? On the whole does this seem to be a reasonable solution given low ceilings (8 feet), and heat being taken care of by infloor?

2)For ducts that are running through unconditioned attic space, do they need to be foamed? If not what is the next best option for these ducts.

3)What problems in general are associated with spray foaming? We may choose to spray foam the garage ceiling as there is living space above, and possibly the ducts as mentioned above. Are there any health risks associated with living in a home that has foam (i.e. headaches, problems with asthma, etc). Please tell me about any down side of using foam in addition to the high cost.

Thanks in advance for your opinions.


    Bookmark   August 15, 2012 at 5:45PM
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Any opinions on the solution I posted above?



    Bookmark   August 17, 2012 at 1:53PM
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