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Heating Up the Attic

02/27/2019 8:59 PM

Hi guys, I am currently exploring dissertation ideas and have been looking into the heat recovery potential from domestic gas boilers and ovens. More specifically, I am seeing if it is viable to collect heat from these appliances during or after their use, and then pump this heat into the attic of your standard UK home. The purpose of this would be to warm up the attic space, therefore increasing the temperature in the attic, and thus decreasing the temperature difference between the attic and the rooms below it. Hence, decreasing the overall heat transfer between these areas, which would lead to a more insulated home. I made a quick sketch of the concept to illustrate this mechanism (apologies for the quality as it was done rather hastily on the train ride home) https://imgur.com/a/HE4Fmpt.

So, essentially what the above sketch depicts is a mechanism which utilizes the heat from the oven and the boiler flue gasses and runs them through a heat exchanger to warm up the ambient cold air coming into the attic.
I was hoping to get some feedback from you guys on; how viable this concept is, the variables I need to account for when trying to construct this system and also any suggestions on areas of research I could pursue to enhance my knowledge of heat recovery systems.
All advice and suggestions are welcomed and greatly appreciated.

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#1

Re: Heating up the Attic

02/27/2019 9:05 PM

If you want the heat in the living space, put the heat in the living space. Insulate the attic

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#2

Re: Heating up the Attic

02/27/2019 9:21 PM

Insulating the attic is the best way to reduce heat transfer from the house. If you pump excess heat into the attic it will quickly be lost, as heated air rises. If the basement is unheated, pumping recovered heat that is not needed (at that moment) in the living area into the basement would decrease the demand for heating in the home, as the recaptured heat will warm the living area as the warm air rises from below.

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#3

Re: Heating up the Attic

02/27/2019 9:37 PM

The heat in your attic is already lost heat. You should reduce not increase the heat that reaches your attic.

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#4

Re: Heating up the Attic

02/27/2019 10:06 PM

This makes no sense....most of the new gas furnaces have very little waste heat, if any...The best dollar value is insulation to improve efficiency by preventing heat loss, which , you guessed it, is lost through the attic...Next would be to upgrade the windows to double or triple glazed and insulated doors, also to prevent heat loss...Most important is an airtight living space, air leaks around doors, windows, through electrical outlets and switches facilitate losses, once again to the attic....There is plenty of room for improvement in insulating houses and sealing living spaces, but pumping waste heat into the attic is not one of them....

..."

  • 90% AFUE Furnaces - The 90% plus is the last furnace group, and can include furnaces with 90%, 92%, 95%, and 98% efficiencies. They’re also called condensing furnaces because they expel water. They shouldn’t be confused with geothermal units or water source heat pumps, which are entirely different units. The water comes from the condensation of the gas fumes in the secondary heat exchanger. Unlike other furnaces, condensing furnaces have two heat exchangers. The first heat exchanger absorbs heat from the gas exhaust, and the exhaust goes through a second heat exchanger where it is cooled again. The hot gas exhaust cools down so much that condensate is released. The water is generally diverted to a pump or drain outside the home. These furnaces are the most efficient, and some only waste a few percentage points of heat. A correctly installed 95% furnace will turn 95% of the gas put into the furnace into heat and will waste only 5%, maximizing your heating energy savings."...

https://www.kennonhvac.com/blog/2017/january/which-furnaces-are-the-most-efficient-/

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#5

Re: Heating up the Attic

02/27/2019 10:15 PM

Insulation is the key to retaining heat inside the house, but a house cannot be perfectly insulated and still allow fresh air. It is desirable to bring in fresh air from outside and remove stale air from in the house without losing too much heat.

A counterflow heat exchanger will allow the warm air from in the house to transfer its heat to the cooler air entering from outside, minimizing heat loss. With a counterflow heat exchanger, the cold outside air moves in one direction and the exiting inside warm air flows in the opposite direction. Between the two flows, there is the maximum amount of surface area and highest heat conductivity. With the counterflow design, the cool air coming in can be at a higher temperature than the warm air exiting to the outside, as shown in the diagrams below.

"Hence, in a cocurrent design, the temperature of the cold stream outlet,

Tc,outTc,out is always lesser than that of the hot stream outlet, Th,outTh,out. Therefore, the heat transfer is restricted by the cold stream's outlet temperature, Tc,outTc,out.
On the other hand, in a countercurrent design, the restriction is relaxed and Tc,outTc,out can exceed Th,outTh,out. Hence in this design, the heat transfer is restricted by the cold stream's inlet temperature, Tc,inTc,in.
Therefore, to achieve greater heat recovery, a countercurrent design is preferred to that of a cocurrent design."

https://www.quora.com/Why-is-a-counter-flow-heat-exchanger-better-than-a-parallel-flow-heat-exchanger

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#6

Re: Heating up the Attic

02/28/2019 4:11 AM

The way to go is to insulate and draught-exclude in the first instance in order to stop the leaks. Cheap and easy.

Heat recovery and heat pumping equipment versus space heating equipment need to be considered on each case on its merits. A solution justifiable in Bognor Regis will not be the same as the one justifiable in Lerwick.

Also, there is no such thing as a <...standard UK home...>; they are all different. Each one has an energy performance rating, the details of which may be found in the sales prospectus for the property. This document suggests the improvements that will prove to be economically justifiable in the future of the property and the potential for improvement.

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#7
In reply to #6

Re: Heating up the Attic

02/28/2019 1:00 PM

Will Brexit effect the flow of hot air?

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#8
In reply to #7

Re: Heating up the Attic

02/28/2019 2:44 PM

The flow of hot air is forever subject to the laws of thermodynamics. It's the amount of flow that's affected by politics. Hot air will inevitably rise until it becomes trapped by greenhouse gases in the upper troposphere. Described by some as a sort of "digitus impudicus".

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#9
In reply to #7

Re: Heating up the Attic

02/28/2019 3:41 PM

The amount of turbulence may change but that's all.

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#10

Re: Heating Up the Attic

02/28/2019 11:45 PM

The cold air entering the attic is replacing warm air exiting the same space. Typical attic designs provide/encourage ventilation to prevent excess moisture (condensing/mold formation) and high temperature (roofing destruction).

Your theory is sound, but the key will be to extend the timing of the waste heat addition to the attic spaces. The actual energy you save will be very high during the time you broil a steak in the oven, but that heat will be gone quickly, and the resident time in the attic will be very short as convection takes over. If your heat exchanger has storage capability, such as being made from soapstone, then your heat might stay with the building, if the rafters can take the weight.

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#11

Re: Heating Up the Attic

03/01/2019 2:21 PM

Couple thoughts: why the heat exchangers? Just vent the hot air into the attic.

The warmer attic might help reduce snow build up on the roof, a big problem here in the northern states.

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#13
In reply to #11

Re: Heating Up the Attic

03/01/2019 8:57 PM

"...Just vent the hot air into the attic...."

There are some good reasons to avoid venting hot air from various household processes directly in the attic. Excessive moisture comes first to mind. For stove grill and oven venting, grease buildup over time could be problematic. For any process buring hydrocarbons, venting could risk asphyxating occupants below.

.

"...The warmer attic might help reduce snow build up on the roof, a big problem here in the northern states. ..."

The warmer air provided intermittently might also readily build ice dams in many situations causing increased roof loads and potentially catostrophic roof failure.

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#12

Re: Heating Up the Attic

03/01/2019 6:52 PM

May be worth looking at Scandia Hus design. Condensation may be an issue.

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#14

Re: Heating Up the Attic

03/02/2019 11:05 PM

As many have already stated, putting heat into the attic, low level ‘waste’ heat is a bad idea for many reasons. I’ll make a short list here so that I can be sure to include my view:

1) attic space is really the ‘outside’ and the air here is in equilibrium with the outside air. 2) dumping moist combustion air into the attic may invite or incite unwanted biological activity (mold, mildew, bacteria) in the space or insulation.

3) capturing the Heat energy in the living space would have to be done through heat exchange to protect the air quality in the living space from unwanted gases in combustion or other processes in concentrations not consistent with living space air.

4) take other systems such as high recycle turbines and think about how they recover low level heat. Cost effective heat transfer becomes unattainable when the temperature difference isn’t there to start, and straight mixing of the fluid(s) with the living space air is not acceptable

Good luck but I think a better approach is preheating feeds to other systems such as heat pumps. Maybe the best bang for the buck for ‘waste’ domestic heat

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#27
In reply to #14

Re: Heating Up the Attic

03/05/2019 11:31 AM

"Preheating feeds to other systems such as heat pumps", care to elaborate?

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#15

Re: Heating Up the Attic

03/03/2019 6:18 AM

Why are you thinking of venting the hot air? put it through a heat pump and use the heat to heat the hot water cylinder, If a condenser boiler is used in conjunction with a mains pressure water cylinder, it is the best combination and because the condenser boilers are so efficient the flue pipe is plastic & with the boiler running you can hold the exhaust and it's barely warm such is the efficiency at extracting the maximum energy from the burner.

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#17
In reply to #15

Re: Heating Up the Attic

03/04/2019 8:01 AM

I'm a bit out of date on heat-pumps, Bazzer, but is corrosion still an issue ? 30 odd years ago I recall a contact who got out of the business for that reason. The garden situated pump sucked up heat from the air fine, but all that process inducted moisture>>>corosion. Heat pumps are a way foreward. Any comments you have are most welcome.

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#22
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Re: Heating Up the Attic

03/05/2019 5:45 AM

The heat exchanger I'm talking about work the reverse way that a car radiator works, it takes the heat from the waste from the other appliances forces it through the heat exchanger to recycle the heat, this is much simplified but it is how it works. On the corrosion front I'd hope that the manufacture's will have addressed that issue, Heat pumps that take heat from the atmosphere are a different thing, they pull air in extract the heat & use that heat to heat water, whereas the heat exchanger takes heat from the building, (or vessel ) and extracting the useable heat to do the same thing without drawing outside air in.

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#16

Re: Heating Up the Attic

03/04/2019 7:31 AM

Hi

Recovering the waste heat from the flues of boilers and stoves is a good idea but heating up space you don't live in is not so good. It won't improve your energy utilisation, carbon footprint or wellbeing. Heating the basement with the waste heat would be a good idea but 90+% of UK homes don't have one and we tend not to use them as public spaces when we do, so that idea seems to have limited appeal. Also, ovens tend to simply dump waste heat after use into the living space anyway so it might be difficult to justify the cost of pumping it elsewhere.

You could use the recovered heat from flues to heat water. There are proprietary heat exchangers available which go into the flue and extract heat from the flue gas. If you have a system boiler with a separate hot water storage tank it might be quite straightforward to put an additional heating coil in it and circulate a heat transfer medium between the two. This would work well with a wood-burner and a non- condensing boiler but you will get less benefit with condensing boiler as most of the saving has already been planned for by the boiler maker. You would also need to pipe the condensate from the bottom of the flue to a drain or purpose build soak-away remembering that the condensate is acidic and corrosive.

Have fun!

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#18
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Re: Heating Up the Attic

03/04/2019 10:14 AM

Lowering the flue gas temperature seems like a good idea but it has additional risks. If the flue temperature drops below 100°C at any point along the rise then condensate can form on the flue liner. This condensate will often include acids that will eat away and corrode the flue liner. Incompletely burned hydrocarbons can also form a gooey creosote layer in the flue. This goo is the potential fuel of a future chimney fire.

Here is a very rare risk but with some very serious possibilities. If the outside air temperature is cold enough a temperature inversion inside the flue stack has been known to happen. This will prevent poisonous gasses (CO) from safely leaving the firebox by the flue and flooding the occupant space. CO detectors do save lives but...

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#19

Re: Heating Up the Attic

03/04/2019 11:33 AM

Reduce Fenestration, Insulate and if you are a multi story home turn the heat on, on the first floor and the cooling on, on the second floor.

Single story insulate the insulation and get that R value maxed.

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#21
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Re: Heating Up the Attic

03/04/2019 2:25 PM

"... .if you are a multi story home turn the heat on, on the first floor and the cooling on, on the second floor..."

.

er....what? Care to expound?

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#20

Re: Heating Up the Attic

03/04/2019 12:50 PM

I have an idea that uses the excess heat in the attic. I live down South in Alabama so there is plenty most of the year. If you could duct the heat from the attic through your clothes dryer, the heating element would probably never turn on. in addition, most dryers that are in conditioned space have to take the air that is cooled by the AC (70°F) and heat it up to the 135° F that most dryers use. This air is then rejected outside. the makeup air has to come from outside which then puts more load on the HVAC. The attic air is just heated outside air in the summer and instead of using inside air should provide some relief to the HVAC and also to the requirement for the heating element in the clothes dryer. I haven't done the cost analysis, but should save a bit. Possibly even be too hot on occasion.

I estimate about $0.30 to $0.50 per load. We probably run the dryer 7-10 times a week. That is a family of six.

Thoughts are welcome.

Norm

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#23
In reply to #20

Re: Heating Up the Attic

03/05/2019 10:38 AM

Very interesting idea, I will definitely look into its viability and cost efficiency.

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#24
In reply to #20

Re: Heating Up the Attic

03/05/2019 11:06 AM

Here's an idea, instead of ducting the heat down to the dryer, put a clothes drying line in the attic and cut-out the dryer altogether, save electric too.

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#25
In reply to #24

Re: Heating Up the Attic

03/05/2019 11:14 AM

Yes, you could do but there's less effort involved in just chucking the clothes in your dryer, also depending on the design of your attic it might not be practical to go up to the the attic each time you want to dry your clothes.

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#29
In reply to #25

Re: Heating Up the Attic

03/05/2019 12:25 PM

Do what I did, put the attic under the house and the basement under the roof, Ta Da! also don't do what I did once dig a basement when living on a barge, it didn't end well.

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#26
In reply to #20

Re: Heating Up the Attic

03/05/2019 11:17 AM

Also, if I may ask, how did you go about generating your cost estimate per load?

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#28
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Re: Heating Up the Attic

03/05/2019 11:34 AM
(5,600 watts x 0.667 hours) / 1,000 = 3.73 kWh
3.73 kWh x $0.09 = $0.34 per load
$0.34 per load x 24 loads a month = $8.16 per month
$8.16 x 12 = $97.92 per year

https://homeguides.sfgate.com/average-cost-hour-run-dryer-68320.html

https://www.thespruce.com/how-much-does-it-cost-to-run-an-electric-dryer-1387954

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