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278 posts

Ultimate Geek
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Topic # 230342 20-Feb-2018 00:23
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I’ve been tracking the temperature in my house and roof space for quite sometime now. My house is mostly single level with hot water cylinder in a moderate sized cupboard in center of the house. I notice the temperature in my roof space on some on these hotter summer days peaking between 40-58 degrees Celsius.

I was thinking whether that heat would be better used if I had a small DC fan that drew that hot air into the hot water cupboard (to supplement the cylinder by reducing heat loss). I get that a typical HWC internal water temp is around 60 degrees and they are pretty well insulated, so there is zero chance of it *maintaining* water temp, but being the ambient temp of the cupboard itself is likely a good 15-20 degrees lower than that of the roof, would drawing the hotter air help minimize the on/off state of the cylinder helping reduce power? The distance from ceiling to top of cylinder in cupboard is about a meter, and obviously only a sheet of gib board apart. A low wattage DC fan (like a PC power supply fan) would mean low power consumption. I still need to wrap the cylinder and I get this is more efficient than the above solution, but would combining both be worthwhile doing? Any downsides? Waste of time?

An ESP WiFi chip would do the temp comparison and only run the fan should the roof cavity temp be high enough.

I’m trying to think of every little power saving idea, obviously it started with LED lights, heated towel rail timers etc, but I figure all the other little things also add up, and it’s also an excuse for a new project...however I don’t want to waste my time with this if the difference will be negligible.

Thoughts?

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  Reply # 1960705 20-Feb-2018 08:09
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When I renovated my bathroom I put the hot water cylinder into the ceiling. I didn't notice any significant difference in hot water bills.

 

Side effect of what you're suggesting is you're pumping hot air into your house, which will make your house warmer, probably smellier, as insects and rodents might be able to get into the roof.

 

All in all I wouldn't do it. Just insulate your cylinder with a wrap, though if the cylinder is modern not sure that's even worthwhile.





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  Reply # 1960732 20-Feb-2018 08:34
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We have a forced ventilation system which prevents condensation as well as pumps warm air into the house.

 

I often arrive home around 5:30 on cold (but clear) Christchurch days and the whole house will be warm so it definitely is a power saving.





Rob

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  Reply # 1960739 20-Feb-2018 08:55
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robfish:

 

We have a forced ventilation system which prevents condensation as well as pumps warm air into the house.

 

I often arrive home around 5:30 on cold (but clear) Christchurch days and the whole house will be warm so it definitely is a power saving.

 

 

If it stays on at night it can pump cold air in.

 

My ventilation system used to bring ceiling cavity air in, but the air quality was poor, particularly in my 100 year old house with who knows what up in the ceiling cavity - well I know some things, like mouse droppings and three kinds of insulation. I bring in fresh air direct from outside now, on a timer that I change for winter or summer. We leave the heating on low during the day in winter, so the house stays warm through. Makes a big difference





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Ultimate Geek
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  Reply # 1960743 20-Feb-2018 09:11
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timmmay:

 

Side effect of what you're suggesting is you're pumping hot air into your house, which will make your house warmer, probably smellier, as insects and rodents might be able to get into the roof.

 

 

It's pumping directly into a closed cupboard, not into the house.  I could add a one way vent into the floor to help push any cool air down and out if it did become an issue.  And if I left the doors open then yes it would enter the house, but that defeats the purpose of having a nice warm and dry cupboard

 

The side effect is it also seconds as a "drying room" (or cupboard) - the added heat could help dry any towels, linen etc out too.  

 

My largest concern is quality of the air, so possibly a small filter may help to minimize any dust.

 

What I want to know, is 1. efficiency, whether its actually worthwhile and 2. whether the cost of running a small wattage fan and wifi chip to control it, would outweigh the cost difference saved on the HWC thermostat turning on/off

 

Here is the past week of roof/house/outside temperature data:

 


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  Reply # 1960747 20-Feb-2018 09:24
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My completely unscientific opinion is that the difference it would make would be so small as to be almost immeasurable as far as running costs are concerned.  





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  Reply # 1960761 20-Feb-2018 09:43
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chimera:

 

It's pumping directly into a closed cupboard, not into the house.  I could add a one way vent into the floor to help push any cool air down and out if it did become an issue.  And if I left the doors open then yes it would enter the house, but that defeats the purpose of having a nice warm and dry cupboard

 

The side effect is it also seconds as a "drying room" (or cupboard) - the added heat could help dry any towels, linen etc out too.  

 

My largest concern is quality of the air, so possibly a small filter may help to minimize any dust.

 

What I want to know, is 1. efficiency, whether its actually worthwhile and 2. whether the cost of running a small wattage fan and wifi chip to control it, would outweigh the cost difference saved on the HWC thermostat turning on/off

 

 

If you pump air into a cupboard it has to go somewhere. Without a vent it will go through the cracks in the door into the house.

 

I think this will have negligible effect on heating costs. It might make a nice drying cupboard, if you have a vent, but I'm still not sure it's worth the bother.





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  Reply # 1960783 20-Feb-2018 10:23
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Very scientific!

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  Reply # 1960804 20-Feb-2018 10:53
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If you really want to be scientific you'll have a heck of a lot of calculations to work through. How much heat do you really think will make it from the cupboard, through two layers of insulation, and into the water?

 

Try putting an insulated container of water into the ceiling for a few hours, take the temperature before and after.





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Ultimate Geek
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  Reply # 1960819 20-Feb-2018 11:06
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timmmay:

 

If you really want to be scientific you'll have a heck of a lot of calculations to work through. How much heat do you really think will make it from the cupboard, through two layers of insulation, and into the water?

 

Try putting an insulated container of water into the ceiling for a few hours, take the temperature before and after.

 

 

??? I don't expect the heat to get "into the water" whatsoever.  As initially stated, I expect the heat to help create enough ambient temperature in the cupboard and around the HWC to help reduce heat loss from it.  Its certainly not about heating the water, its about minimising temperature loss due to external ambient temperature being too low.  That said, I also mentioned the HWC is well insulated and would be heat wrapped too, so "scientifically", how much loss would a normal HWC lose via the outside of the cylinder (I think convection?)  If its minimal, then yes this is a waste of time - hence my question, as I don't know enough about thermal dynamics and hot water cylinders to make a call.

 

Put a HWC in the snow

 

Put a HWC in the sun

 

I can guarantee regardless of the HWC's insulation, one will lose heat faster than the other.  Again, the question is whether that heat loss is negligible - or another way, that the cooling effect on the outside of the cylinder is enough to draw heat away from the internal water temperature.  

 

I can take measurements, via clamps to measure HWC power, temperature sensors in the cupboard and roof, even set this up and perform tests - but to save me the time, someone who has some sort of knowledge about thermal dynamics maybe able to explain "scientifically" why this is or isn't a valid idea.

 

 


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  Reply # 1960825 20-Feb-2018 11:17
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I suspect heat loss is minimal. When I put my hand on the wrap around my modern hot water cylinder it's rarely much warmer than ambient. I've read that there's virtually no benefit to even using a wrap, but I had one so I used it. I'll leave the details to someone who has better knowledge or can explain it better.





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  Reply # 1960828 20-Feb-2018 11:21
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timmmay:If it stays on at night it can pump cold air in.

 

We have installed 6 "SmartVent" systems (3 of our own houses and 3 sons' houses)

 

In all cases, even though cool air was pumped into the house overnight (and was compensated with extra heating), the warm air in the afternoon meant that the power  consumption decreased.

 

Regarding smells from the ceiling - the refreshed air in the ceiling will decrease those smells anyway (it hasn't been a problem at any of the 6 houses we tried).





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  Reply # 1960831 20-Feb-2018 11:22
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If you have a modern-ish cylinder and you are going to wrap it, I would not bother about increasing the ambient heat in the HW Cupboard,

 

You will get more "Bang for your buck" looking at lagging any HW pipes that are exposed,


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  Reply # 1960832 20-Feb-2018 11:23
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Heat transferal is exponentially related to the temperature difference between the 2 areas. 

 

If you increase the temperature of the cupboard to that of the water in the cylinder, then in theory there should be no loss to 'ambient'.

 

Every degree of the cupboard's temperature increase decreases the tendency of the heat in the cylinder to 'escape'.

 

As much as I love a good gadget-based project, you would get the most bang for buck by installing a cylinder wrap. 

 

 




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Ultimate Geek
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  Reply # 1960833 20-Feb-2018 11:29
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Cheers Gary, yes indeed this would be a very good idea, in winter it takes a good 5-10 seconds before hot water comes out the taps.

I guess the idea would only be worthy of a drying cupboard as opposed to having any effect on the hwc itself / minimizing running costs. Still, maybe worthwhile doing - as most people do - we put towels and linen in the cupboard to keep them warm and dry produced from the heat lost from the cylinder. I suspect the cupboard is maybe a few degrees warmer than the house.

May need to take some core sample data before and after to determine whether it’s useful or not!

Thanks for all your replies.

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  Reply # 1960837 20-Feb-2018 11:36
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Perhaps add a loop to the cold water intake, so that water runs through a coil in the ceiling space before entering the cylinder. So you're pre-heating the water, and let the insulation help with maintaining heat


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