HVAC ducting advice

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gpf

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#304310 22-Apr-2023 16:09

I'm installing a 3 bedroom heat transfer kit shortly to our 2005 house in Central Otago.

The 2.7 high ceilings are well insulated with pink batts and all windows double glazed. The bedrooms get very cold in winter (10-12°) while the temperature reaches 33+ up high in the 3.6 stud lounge. At sitting level it's overly hot at 26-28 when our big Yunca log burner is stoked up. Turning on the ceiling fan makes it unbearably hot and doesn't move nearly as much warm air into the dining room and hallway towards the bedrooms as we would like. After lots of online research I'm convinced our situation could be helped with a good HT system.

I've purchased a quiet 200mm fan from Mingfans in Auckland with 200mm R1.0 insulated ducting from heat source room to a carbon filter (to remove wood smoke smells), onto the 200mm fan, another 2-3m before splitting into the 3 rooms. The furthest room is approx 11m from lounge so we might be struggling to make a difference that far, but I'll buy extra insulation blankets (Mammoth 870mm R1. 8?) to wrap everything in if need be.

The house already has a 14kw ducted heat pump system that draws air from the hallway into the system in the roof cavity and then splitting into several ducts to 7 rooms (incl 4 bedrooms). It works reasonably well but we like using the log burner not only for the ambience, but it has a very effective wet-back which heats our hot water in winter.

Finally onto my question: with ducts and outlets for the heat pump system in all bedrooms already, what do you think about the idea of adding a Y junction above each bedroom in a 2-into-1 direction and routing both the heat pump inlet and heat transfer inlet to blow into the single end which connects to the room outlet? I don't mean to run both systems at once, we would choose either heat pump without fire/heat transfer, or just the fire/hear transfer.

My concern is what might occur at the 2:1 Y junction. Will the heat from either source pass through to the bedroom without hinderence from the other incoming pipe not being used, or could it create some sort of back-draft suction that draws the heat away?

The reasons I'm considering the idea are the following: one less unsightly duct vent in each bedroom ceiling; one less "hole" in the ceiling insulation where heat could escape; and thirdly the existing outlets are already in ideal positions in each room.

Thoughts appreciated...

mattenz

190 posts

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#3067056 22-Apr-2023 20:14

Wouldn't you just run the ducted heat pump on fan only? Or if that doesn't do the job, add an extra return in the lounge? Seems a bit silly to add a tinny 200mm fan to the mix.

timmmay

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#3067068 22-Apr-2023 21:17

Yeah just add another return above the fire for the ducted heat pump. Maybe find a way to enable or disable it if required.

Our positive pressure ventilation system has it's outlet beside the ducted heat pump return. That works well.

tweake

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#3067073 22-Apr-2023 21:34

as above i would use the hvac system. simplest could be to put a fixed return by the fire. you could also make it switchable with dampers, but depending on layout there may be no harm to leave it permanently connected. don't forget the filter in the air return. you might not like replacing it if you have high ceilings.

the other option is to get a basic one outlet heat transfer kit and simply pump hot air from fireplace to the air return. a good kit will have a thermostat controller which adjust speeds, or turns it on, when the room temp is high enough. that way you don't cool the room down to much by pumping all the air to the rest of the house. the hvac on fan only.

timmmay

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#3067077 22-Apr-2023 22:00

The original idea of using the current diffusers may not work so well. You might need to put big valve type things inline to make sure the air from the transfer kit went into the room rather than back into the HVAC system.

The simplest and most effective way of using this is probably going to be sucking in the really hot air and pushing it into the bedrooms with dedicated diffusers that push the air almost directly down. If you look in my ducted heatpump guide there's MDO diffusers that would likely work great, pushing the air mostly down but directionally. However, 200mm fan, I'm not sure how much velocity / pressure the air is going to have, i.e. if it's going to be very effective. I'd want to look at the throughput of the pump and consider that along with the total volume of the rooms being heated.

tweake:

the other option is to get a basic one outlet heat transfer kit and simply pump hot air from fireplace to the air return. a good kit will have a thermostat controller which adjust speeds, or turns it on, when the room temp is high enough. that way you don't cool the room down to much by pumping all the air to the rest of the house. the hvac on fan only.

That might be a "good enough" option with no changes to the ducted system. Put an input above the fire, filter, directional outlet blowing towards the ducted heating input. However I would be surprised if it made much difference to temperatures that way. The heat loss along the heat transfer, the heat loss in the ducted system, etc. I wonder if it would be a lot of work for minimal gain.

tweake

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#3067265 23-Apr-2023 10:03

timmmay:

tweake:

the other option is to get a basic one outlet heat transfer kit and simply pump hot air from fireplace to the air return. a good kit will have a thermostat controller which adjust speeds, or turns it on, when the room temp is high enough. that way you don't cool the room down to much by pumping all the air to the rest of the house. the hvac on fan only.

That might be a "good enough" option with no changes to the ducted system. Put an input above the fire, filter, directional outlet blowing towards the ducted heating input. However I would be surprised if it made much difference to temperatures that way. The heat loss along the heat transfer, the heat loss in the ducted system, etc. I wonder if it would be a lot of work for minimal gain.

that applies to heat transfers in general. its own ducting or using the existing ducting, its going to be loosing some heat. you can't get around that.

hence why you also want to temp control it so your only taking excess heat from the fireplace.

andrew75

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#3067332 23-Apr-2023 11:43

When installing the transfer system you also need to factor in trying to get approximately the same length of ducting going to each of the outflows, otherwise resistances and flow will be different at each outflow. On mine I made a bit of a mistake and had one shorter run, resulting in higher flow at that point. You can rectify it by closing the baffle (or adding a separate baffle more proximally in the duct), but this has added a bit of noise at that outlet. Is it an option to just do a single duct into the output of the ducted heatpump? You can get quite a bit of airflow from the MingFans 200mm fan with 200mm ducting especially if the run isn't too long. Make sure you have the inlet close to the fire.

gpf

35 posts

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#3068279 26-Apr-2023 23:24

Thank you for all your feedback and ideas. I hadn't thought of connecting the heat transfer output into the existing HVAC but I don't think it's right for our situation. Yes I could cut a 200mm hole where the return ducting rises from the hallway into the main system, but everything looks huge: 250 or maybe 300mm ducting running in all directions that feed 9 outlets scattered around the house and including 2 largish outlets in the heat source room (logburner in lounge) and two more just outside that in the dining room, so I'd be feeding the same areas I'm trying to take the heat from, with 4 of the 9 ducts.

Secondly, the idea is to relocate the logburner heat without involving the HVAC system. Running the HVAC in fan-only mode as well as the transfer fan seems excessive. While I imagine it would cost considerably less to run the HVAC in fan-only mode than heating mode, it has no smarts built-in, so we have to control it manually with a hallway switch. Hardly ideal with a smart switch on the heat transfer fan if I have to jump out of my seat to turn the HVAC on and off when needed.

I do like the sound of moving cold air from the cold rooms to the heated room, as heat loss from the insulated ducting would less of an issue, but with a concrete floor, the only location for inlet vents would be the bedroom ceilings - which is the warmest part of the room and the outlet would have to be the 3.6 high lounge ceiling, again where the heat resides. Do you think this would it still work in that scenario?

Back to my original question, which I don't think anyone has answered yet:

Does anyone have an opinion on airflow if I plumb a "reversed" Y junction before each existing bedroom outlet so both the HVAC and heat transfer systems are plumbed into the reverse Y on one side, so the air flows out the single end into a short duct before entering each bedroom? See diagram below. I mean to only use *one* heat source at a time: either the heat transfer fan while the lounge room is hot from the fire, *OR* to use the HVAC instead of the fire when we want to. If they are both plumbed into the Y junction (in 2:1 direction), is there likely to be any back-draft/turbulence caused by the unused Y junction arm, which could hinder the airflow from reaching each bedroom?

If you don't think it should affect the flow I'll try that configuration first. It'll mean one less vent in each bedroom ceiling, and I can utilise the existing HVAC outlets which are in the perfect location in each room.

Andrew75: I won't be able to have the same ducting lengths without adding copious amounts of extra ducting to the close rooms (the first room is about 6m after the fan and the furthest will be approx 11m from fan). I'm planning to branch off the 200mm ducting after the fan with a 150mm duct to the first room via a 200 to 200/150 Y, continuing the 200mm ducting to the remaining rooms into a 200 to 150/150 Y to split the flow between the other 2 rooms.

By the way the 3 speed fan has the following throughput (Lo/Med/Hi): 572, 710, 850 (m3/h). I've got a Tuya temperature sensor in the lounge and in the bedrooms and will setup some rules to turn on the fan (connected with Tuya smart switch) when the lounge sensor is higher than x degrees and when a bedroom temperature is below x degrees, then a second rule to switch it off when either the lounge temperature drops or the bedroom temperature reaches x degrees. I'll experiment with the fan speed to find what works the best.

Moving onto an insulation question: I've purchased R1.0 ducting to help protect against the local Central Otago cold and I suspect I'll need to buy extra insulation, something like Mammoth 870mm blankets to wrap the ducting as well. They have an R1.8 blanket and go up to R4 (for a LOT more cost). Do you think the R1.0 + R1.8 blankets would be enough for the cold here? It gets down to -5 some nights.

Y junction configuration

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andrew75

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#3068286 27-Apr-2023 06:48

Hard to know about back flow through the Y connectors. Suspect you'd get some, the only way to stop it would be electric dampers but that would get expensive and complex. Having the y as close to the outlet as possible would help I suppose. Have you got the fan and ducting yet? An experiment with a trial mock-up would be a way to find out.

On my system in Auckland I have a very long run from one end of the house to the other, R1.0 duct wrapped in a 1.8 blanket. I measure effectively zero heat loss from the intake to the far outlet on a cold night. I'd imagine your nights are much colder than ours, but suspect it would probably still be ok.

Manual control of the systems isn't that bad. I just turn ours on when we light the fire and turn it off when we go to bed. In addition to heating the rest of the home a big plus of the system is it stops the fire room getting crazy hot.

Not sure if the 'piping cold air' into the fire room would work. Sounds ok in theory but in practice I'm not so sure.

If I was you i'd be looking for the easiest solution which is probably to plumb it into your existing system. Larger diameter ducts are good - more airflow... Do you need the existing outlet in your fire room - or can you 'repurpose it' for the for the heat transfer system? You could even branch off it directly into your fan before feeding back into it and preventing backflow with a single electric damper. Presume you'd need a lot of extra insulation for all your ducting though.

timmmay

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#3068291 27-Apr-2023 07:29

I'm not sure if using the same ducts is a good idea. You will basically pressurise the HVAC system ducting, there's no telling where the air will go, but probably all over the place. You'd probably need valves or dampers. Separate outlets would make sense, I linked to some I thought would be good above. Additional outlets = additional heat loss of course, they're holes in your ceiling.

tweake

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#3068462 27-Apr-2023 16:40

using Y connectors means air will be pushed back up the HVAC. how much will depend on how much restriction there is between the Y and the room from the duct and grill. likewise the hvac will push back through the heat transfer. motorised Y damper is the best way to switch between the two.

check the hvac doesn't have a ventilation electrical connection.

moving cold air is a bad idea. all that will happen is the fan will lower the air pressure in the bedrooms and suck in cold outdoor air in and push hot air up and out of the main room. where as sucking cold outdoor into the main room won't hurt due to the fireplace and pushing warm air into the bed room pushes out cold air.

Handle9

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#3068468 27-Apr-2023 16:56

tweake:

moving cold air is a bad idea. all that will happen is the fan will lower the air pressure in the bedrooms and suck in cold outdoor air in and push hot air up and out of the main room. where as sucking cold outdoor into the main room won't hurt due to the fireplace and pushing warm air into the bed room pushes out cold air.

Nah. The air will take the path of least resistance which is inside the building envelope. The only way you'll get meaningful air pushed out is if you have a sealed room. With the right sort of diffuser you'll also get some slightly helpful air movement inside the heated room.

You'll get very marginal leakage but you're talking about a low volume fan moving relatively small volumes of air.

It's not going to help heat the bedrooms much but it's not going to do what you are describing otherwise you'd have the same effect running extract fans in bathrooms or range hoods in kitchens.

tweake

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#3068486 27-Apr-2023 17:54

Handle9:

tweake:

moving cold air is a bad idea. all that will happen is the fan will lower the air pressure in the bedrooms and suck in cold outdoor air in and push hot air up and out of the main room. where as sucking cold outdoor into the main room won't hurt due to the fireplace and pushing warm air into the bed room pushes out cold air.

Nah. The air will take the path of least resistance which is inside the building envelope. The only way you'll get meaningful air pushed out is if you have a sealed room. With the right sort of diffuser you'll also get some slightly helpful air movement inside the heated room.

You'll get very marginal leakage but you're talking about a low volume fan moving relatively small volumes of air.

It's not going to help heat the bedrooms much but it's not going to do what you are describing otherwise you'd have the same effect running extract fans in bathrooms or range hoods in kitchens.

sorry i did describe it badly. "all that will happen" is not correct.

marginal is debatable, especially with air leaky homes. how much depends on how good the air return is, but generally they are undersized at the best of times and sometimes not installed at all. unless the house is super air tight your always going to suck some outdoor air in. but its also the room with the least amount of heating. if you suck in air into the lounge no one will notice because of the heat source.

and yes you get the same effect with bathroom fans and range hoods. its even worse as its pumping air outside and sucking in replacement air from outside.

Handle9

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#3068549 27-Apr-2023 21:41

The vast majority of times the return is open doors. Air is going to go through the 2.5 square metre opening rather than through a very high resistance cracks. These are fairly small fans running at relatively low static pressures, not positively pressurised systems.

Heat transfer kits rely on low volumes with a lot of energy from a fire rather than higher volumes with lower energy density.

mattenz

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#3068761 28-Apr-2023 11:19

I'll give another plug for adding another return to your existing system before you go involving another fan. Your HVAC system doesn't care if you're supplying the same areas that you're trying to take heat from, in fact that's what it's supposed to do: circulate air around the house, and create an even temperature. If it's not smart enough, then you could look into a Wifi controller.

tweake

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#3068910 28-Apr-2023 15:02

Handle9: The vast majority of times the return is open doors.

another words no return was installed in the room.