Ceiling heat to “assist” hot water cylinder

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#1960923 20-Feb-2018 12:45

robfish:

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).

Sure, but why not just turn it off at night? My experience with smells is with my own very old house.

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#1960985 20-Feb-2018 14:11

As previously mentioned, running a cold water loop into the roof space will be more efficient than raising the ambient temperature.Standing losses are around 1.5 to 2.5 kW/day in modern hot water cylinders (NZS 4305:1996).

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#1960998 20-Feb-2018 14:49

timmmay:Sure, but why not just turn it off at night?

The main purpose of the SmartVent (or DVS or HRC etc.) system is to eliminate condensation.

The free heat (especially with a concrete tiled roof) is an added bonus.

Rob

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#1961014 20-Feb-2018 15:08

robfish:

timmmay:Sure, but why not just turn it off at night?

The main purpose of the SmartVent (or DVS or HRC etc.) system is to eliminate condensation.

The free heat (especially with a concrete tiled roof) is an added bonus.

I have HRV, but its largely counter intuitive. When its hot in the day (usually summer), the roof is hot, so you don't want the heat in the house. When its cold in the day (mostly winter), the roof is cooler than the house, so you don't want the cold. I find its useful (for heating / cooling) for about ~2 hours of the day (dusk mostly) At the extremes of the day not so great. Otherwise, only good for ventilation.

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#1961030 20-Feb-2018 15:14

With double glazing and a ventilation system on 3-5 hours a day (not much during the evening / night) we get little to no condensation.

Aredwood

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#1961065 20-Feb-2018 15:35

You will probably make bigger savings just by installing a smart timer circuit to the power to the cylinder.

Add extra insulation as others have said.

Also check the cable size of the power cable to the cylinder. I see lots of cylinders that have a 3KW element running on a 1.5mm2 power cable. Even with the cable in open air, it gets quite warm.

Assuming that the element is 3KW, 20M long cable, partially surronded by insulation,1.5mm2, You will be wasting 100W of power whenever the element is on. Change to 2.5mm2, loss goes down to 56W. Change to 4mm2, loss is 34W.

If you don't need quick reheating, easiest way to reduce cable losses might be to swap to a lower wattage element. or a twin element. And add a second switch, so you can use both or just 1 output. EG, have 1.5KW for overnight reheating, and a switch that gives you 3KW if you then need faster reheating.

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#1961084 20-Feb-2018 16:14

mAYH3M:
As previously mentioned, running a cold water loop into the roof space will be more efficient than raising the ambient temperature.Standing losses are around 1.5 to 2.5 kW/day in modern hot water cylinders (NZS 4305:1996).

.

There is actually a newer standard AS/NZS 4692.2 2005.

The max heat loss for a 180L cylinder is around 1.7KW/Hr per day. That is at a 55deg temp difference between stored water temp and ambient air temp.

Note I don't have access to that complete standard, But I found a summary graph. And had to try and pull some values from it. As max allowed heat loss varies with cylinder size.

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#1961248 20-Feb-2018 22:50

robfish:

The main purpose of the SmartVent (or DVS or HRC etc.) system is to eliminate condensation.

The free heat (especially with a concrete tiled roof) is an added bonus.

What happens in winter? I thought these systems pumped in hot air in summer (when you want the house cooled), and cold air in winter (when you want the house heated).

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#1961268 21-Feb-2018 06:25

The top of the line systems have heaters and heat transfer units but even the basic systems have some "smarts".

At very low temperatures (in the ceiling) the fan slows down but remains effective against condensation. At mid range temperatures the fan speeds up and at high temperatures it turns off. There are sensors in the ceiling and in the living areas.

We have a large family and had tried dehumidifiers which where only partially effective. Forced ventillation systems have been totally effective (even in Christchurch where condensation is a common problem.
I doubt that you would find someone unhappy with their similar system.

Rob

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#1961293 21-Feb-2018 07:36

The most basic units are just a fan with no smarts. Mine has a heater inline, but it's 1kw and really just takes the chill off, plus as it's rarely used you get the smell of burning dust. Previous house owner put it in. If I was doing it I'd get a heat exchange unit, possibly integrated with a heat pump, but they'd probably cost $10-20K.

As I said above, I just have my cheap dumb unit which pulls air in from outside on a timer. In summer it goes something like 6am - 10am and 7pm to 10pm, in winter it's more like 11am to 4pm. I have it come on for half an hour at night. I don't need it to do anything about condensation though, double glazing takes care of that, particularly as the house is dry from having this thing on during the day.

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#1961378 21-Feb-2018 10:02

Probably the best place to discuss Dvs HRV and other diseases is in their respective threads. A very effective way of reducing condensation is to open your windows.

With regard to this thread however, as suggested the greatest advantage would seem to be the cold feed to the cylinder could be routed through the hot roofspace. Whether the cost of the piping will offset the power savings? I would suggest it would be marginal.

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#1961505 21-Feb-2018 13:18

The power consumption difference of using warm roofspace air to warm the cupboard immediately around the HWC to reduce the temperature differential ΔT across the HWC insulation and thus slow the heat flow from the HWC will be immeasurably low. In fact, it may even be counter-productive as you'll be constantly replacing the air with new which has a cooling effect due to mass flow and the air immediately around the cylinder cladding (the boundary layer) being disturbed.

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#1961513 21-Feb-2018 13:27

Aredwood: You will probably make bigger savings just by installing a smart timer circuit to the power to the cylinder.

By far the best option for an existing installation.

Aredwood: Add extra insulation as others have said.

That can actually be counter-productive on 'modern' cylinders though. There is an optimal level of insulation due to the increasing surface area with additional insulation and most manufacturers would be working within that range.

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#1961718 21-Feb-2018 17:02

cadman:

Aredwood: Add extra insulation as others have said.

That can actually be counter-productive on 'modern' cylinders though. There is an optimal level of insulation due to the increasing surface area with additional insulation and most manufacturers would be working within that range.

not true. the effect you are talking about is for small pipes (<1cm) like get used in refrigeration. Any additional insulation would help. A good tool is a FLIR camera and then you can see where you need to an the insulation.

elpenguino

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#1961769 21-Feb-2018 18:57

elpenguino:

Heat transferal is exponentially related to the temperature difference between the 2 areas.

I see the edit button only hangs about for a certain time.

In the interests of accuracy this should say logarithmically.

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