Electric water heater heat loss measurement, I just did a test.

Forums › Home Workshop DIY › Electric water heater heat loss measurement, I just did a test.

Huchiz

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#185361 19-Nov-2015 14:09

The model is a Rheem 135L low pressure one, manufacture date was 3/2001.

Here is how I did the test. Set the thermostat to 60 degrees and let it heat until stopped, measure the temp on the kitchen tap. It was 51.5 deg. Cut the power and not to use any hot water for 24 hours. Measured the temp on the same tap again, it was 43.5 deg. Environment temp was around 18 deg, though it will be higher in the closet.

So the 24 hours temp loss is about 8 deg, for 135L of water it is about 1.26KWH. (4.2*8*135*1000*1/3600*1/1000) Considering during normal use the temp will be higher in average, so let's say the normal daily heat loss is 1.5KWH max.

So is that figure normal for a 2001 year model? If I were to add a wrap on that tank, can I expect the heat loss will be cut by half or anything? Cheers.

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Vocus

#1431159 19-Nov-2015 14:13

what's the temp of the outside of the cylinder when it's heated up?

Huchiz

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#1431160 19-Nov-2015 14:13

ubergeeknz: what's the temp of the outside of the cylinder when it's heated up?

Sorry I didn't check that, it was a small closet just for the tank.

timmmay

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#1431172 19-Nov-2015 14:29

Seems like a significant drop. Completely different situation, but I have a high pressure tank, not sure size but family sized, I guess 200 - 300L : it's about 5-6 feet high. I turned it off one day and forgot, 30 hours later we realised but the water for showers was still very warm - this is around 5 average length showers later.

I'd insulate it. I put a wrap on my two year old cylinder, mostly because I had one lying around, and because it's in the ceiling space and it gets pretty cold up there in winter. Saved a heap of cupboard space :)

ubergeeknz

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Vocus

#1431189 19-Nov-2015 15:04

I'd estimate it's warm given that level of heat loss ... a wrap should help but there's only really one way to find out for sure ;)

One other thing; are you sure none of the pressure valves are leaking?

Huchiz

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#1431199 19-Nov-2015 15:20

ubergeeknz: I'd estimate it's warm given that level of heat loss ... a wrap should help but there's only really one way to find out for sure ;)

One other thing; are you sure none of the pressure valves are leaking?

The pressure valve is a APEX 7.6m one, it seems there is no way to manually test it. (?)

But I have once measured the vented water by putting a drink bottle on the end of the pipe outside the house. From memory the 1L bottle was not full after a full heating process (from 20 deg to 60 deg). The copper relief pipe is only warm during the heating period, otherwise it is cold.

So I kinda guess the valve is working fine?

wellygary

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#1431200 19-Nov-2015 15:22

According to this ,
https://www.energywise.govt.nz/at-home/water/saving-money-on-hot-water/

they began to start decently insulating HWCs in 2002, so your 2001 one could do with a wrap...

ubergeeknz

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#1431224 19-Nov-2015 15:33

Huchiz:
ubergeeknz: I'd estimate it's warm given that level of heat loss ... a wrap should help but there's only really one way to find out for sure ;)

One other thing; are you sure none of the pressure valves are leaking?

The pressure valve is a APEX 7.6m one, it seems there is no way to manually test it. (?)

But I have once measured the vented water by putting a drink bottle on the end of the pipe outside the house. From memory the 1L bottle was not full after a full heating process (from 20 deg to 60 deg). The copper relief pipe is only warm during the heating period, otherwise it is cold.

So I kinda guess the valve is working fine?

Yeah that sounds fine if no water is dripping out anywhere else. A wrap it is then!

Surprisingly enough our very old (1980 ish) cylinder is cold to the touch, it doesn't even warm the HWC to any noticeable degree. So while standards now dictate good insulation, this must have been exceptional at the time.

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Bung

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#1431228 19-Nov-2015 15:50

wellygary: According to this ,
https://www.energywise.govt.nz/at-home/water/saving-money-on-hot-water/

they began to start decently insulating HWCs in 2002, so your 2001 one could do with a wrap...

That link says pre 2002 tanks are PROBABLY not well insulated. I have a 1997 tank that is so it may depend on size and manufacturer.

Aredwood

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#1431465 20-Nov-2015 00:32

A far better test would be measuring the power consumption over say 24 hours. While not using any hot water in that time. The problem with measuring the water temp at the tap, is that the water always forms into layers which are at different temperatures. (thermal stratification) This means that the temp near the bottom of the cylinder probably dropped by alot more than 8 deg.

If the element is less than 2.4kW (it will probably be a 2kW for that size low pressure cylinder) Temporally disconnect it's power cable. And wire a lead onto the cylinder. So you can run it with one of those plug in power meters.

Huchiz

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#1431466 20-Nov-2015 00:55

Aredwood: A far better test would be measuring the power consumption over say 24 hours. While not using any hot water in that time. The problem with measuring the water temp at the tap, is that the water always forms into layers which are at different temperatures. (thermal stratification) This means that the temp near the bottom of the cylinder probably dropped by alot more than 8 deg.

If the element is less than 2.4kW (it will probably be a 2kW for that size low pressure cylinder) Temporally disconnect it's power cable. And wire a lead onto the cylinder. So you can run it with one of those plug in power meters.

Thanks, that's good advise. It is 2KW rated, and the wiring on the connection block looks easy enough for me to try. Thanks.

Hammerer

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#1431477 20-Nov-2015 03:34

The issue of hot water cylinder insulation has come up before and this 2012 thread is useful: Hotwater Insulation for a modern (already insulated) cylinder

The NZ standard should be considered the minimum acceptable rather than the optimum economic. Even with new hot water tanks it is economic to add more insulation particularly with:

Larger cylinders. Sizing your cylinder is important. If the cylinder is too large (or your usage is low) then your energy losses will be greater which means the tank will have lower efficiency. The larger your cylinder then the larger your energy losses but the unit cost of insulation is cheaper based on surface area or volume. See UK Investigation of the interaction between hot water cylinders, buffer tanks and heat pumps page 35 Table 4.3.8 comparison of cylinder sizes.
Drawing off less hot water. See same paper.
Tall or thin cylinders. The larger ratio of surface area to volume gives a correspondingly greater loss of heat which is exacerbated by convection currents that form more easily the higher the heat loss for a given volume. See the next paper for some nice colour diagrams showing stratification and convection.
Little existing insulation. "Providing outer tank wall insulation reduces mixing considerably." See the Abstract of Natural Convection in Stratified Hot Water Storage Tanks

It is worth being aware that a thin layer of insulation can increase heat loss if the surface area increases more than the thermal resistance. See Critical Radius of Insulation, p.9-10 Steady Heat Conduction PDF

NZ advice: Factsheet on saving with hot water

Overseas advice: Superinsulate your hot water tank

jonathan18

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#1431507 20-Nov-2015 08:11

A serious question - how does one insulate a gas HWC?

Hammerer

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#1431590 20-Nov-2015 10:31

jonathan18: A serious question - how does one insulate a gas HWC?

EnergyWise NZ says "Note that you can't put a cylinder wrap on a gas hot water system" and cylinder wraps say things like "Limitations : Do not use on gas fired hot water cylinders".

When we replaced our hot water cylinder we excluded internal gas hot water cylinders as options because the flue increases heat losses from the house and if the cylinder is outside then they are not well enough insulated for our temperate damp and windy climate. The problem is that, even if you do safely insulate a gas cylinder, most of the heat goes out through the flue/chimney and it is unsafe to hinder or block that airflow. This means that the saving will be a lot less than for an electric cylinder. Insulating the pipes is safe and is more worthwhile because on longer pipe runs for external gas cylinders.

SmarterHomes NZ says "gas cylinders need to be located in a well ventilated area and flued to remove exhaust gases. This leads to long pipe runs. Heat loss from gas hot water cylinders is large but it’s not safe to put a hot water cylinder wrap on a gas cylinder." and "Gas hot water cylinders can't be fully insulated as they have an exposed flue in the middle where the flame heats the water. They lose about three and a half times as much heat as a similar electrical storage cylinder. Hot water cylinders are now sometimes installed outside the house to save space. The cold and rain outside mean that the cylinder insulation needs to be particularly good." My emphasis added to that quote.

I've seen a few external gas water heater packed around with polystyrene blocks or the like.

In the US and UK people do insulate their gas hot water cylinders:
UK insulating tanks, pipes and radiators "Savings based on a typical gas-heated home" give a payback of less than six months
USA Hot Water Heater 'Blanket' "On gas water heaters, keep insulation at least 6 inches from the flue. If pipes are within 8 inches of the flue, your safest choice is to use fiberglass pipe-wrap (at least 1-inch thick) without a facing. You can use either wire or aluminum foil tape to secure it to the pipe."
USA Blanketing Your Hot Water Heater "A note of caution about gas hot water heaters: The installation of insulating blankets or jackets on gas and oil-fired water heater tanks is more difficult than those for electric water heater tanks. It’s best to have a qualified plumbing and heating contractor add the insulation. If you install it yourself, read and follow the directions very carefully. Keep the jacket or blanket away from the drain at the bottom and the flue at the top, make sure the airflow to the burner isn’t obstructed, and don’t insulate the top of the tank—the insulation is combustible and can interfere with the draft diverter."

USA Rob in Super insulate your hot water tank converted his to gas cylinder to electric.

jonathan18

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#1431600 20-Nov-2015 10:44

Thanks, Hammerer, that's really useful information. Worrying as to the degree of heat loss in a gas versus electric cylinder!

Mine's an internal HWC (in the laundry), and that cupboard is certainly nicely warm. Looks like the only thing I can do is to insulate the pipes (and only as far as they exit the cupboard, which is not far). I'm assuming I can just use a standard foam pipe insulation, or are there better ones than this?

Our previous house, when we bought it, had the gas HWC outside; it broke down while we owned the house and, while we kept the replacement in the same place, we replaced it with a modern electric cylinder designed for outside placement. TBH, I'd rather not have reticulated gas given that since we put in ducting air con the only uses for gas are hot water and cooking (stove top), where the latter could easily be replaced by a gas bottle (which is exactly what we did at the last place).

Hammerer

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#1431639 20-Nov-2015 11:34

In the US, insulation has to be a minimum of six inches from the flue. In NZ, I don't know if there is a relevant regulation and I don't think I've seen anything specifying a safety clearance. Certainly you'd want to avoid hindering any airflow around the draft hood/diverter and pilot light.

As an aside, while you're looking at it you might want to look at other issues with gas cylinders http://www.hotwatercylinders.nz/blog/internal-natural-gas-hot-water-cylinders/

It's only recently, 2009, that domestic gas hot water heating has had to consider the efficient use of hot water. Previously, the emphasis was entirely on the just the cylinder itself through the minimum energy performance standards (MEPS). I consider this another acknowledgement of the lower efficiency of gas heating systems. I'm slightly disappointed that there hasn't been a product specifically developed to insulate inefficient gas water heating systems. Maybe companies consider it to be too risky or it would put people off gas water heaters.

There are probably benefits from insulating only the sides of a cylinder even if you don't/can't insulate the top, bottom, flue, draft hood, pilot light, etc. Insulating the sides would be expected to reduce the convection flows down the outside wall of the cylinder thereby reducing mixing and heat losses. However, there are two factors that probably count against this. First, the flue up the centre of the tank effectively makes a narrower cylinder and the thinner the cylinder then the greater the energy loss due to convection. Second, heating the center of the cylinder creates a natural flow up to the top of the tank which is more likely to come down at the outside wall because the cylinder is effectively thinner. In other words, the natural convection flow from heating the water at the center flue may increase convection losses at the outside of the cylinder because that is where the natural convection flow ends up circulating down the outside wall.

On the other hand, if you hinder that flow at the outside by insulating then it might slightly reduce the flow at the centre or it might force the natural convection down further from the outside wall giving you a double benefit. Figures 8 and 9 in this article http://www.sciencedirect.com/science/article/pii/S0360544214011189 might help to visual the natural convection flows.

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