Load Calculation

[monty]

I've been going with great interest through the various threads regarding maximum load demand calculations, with the very knowledgeable input form both Elkangorito and Crossy!

One thing I have seen many times over is the fact that when using instantaneous water heaters only 33.3% of rated powers is used.

I could understand this for the wall mounted shower heaters, as they are seldom at 100% power setting, mainly due to the relative high temperature of the water supply.

However, nowadays very often multi point instantaneous water heaters are used, and those devices, when there is water demand, will simply run at 100% of their rated power. If the resulting heated water is too hot, this water will be mixed with cold water at the outlet mixing valve. Be it a shower, sink tap or bath tap.

And many times several of those are installed to reduce the length of the hot water pipes.

Which makes it a high possibility several of those will be running at 100% of their power simultaneoulsy...

Your input please?

[david96]

Instantaneous water heaters are in MD calculations (AS3000) in domestic installations as 33.3% of connected load, so if the heater is rated at 15A only 5A is incorporated in the MD calculation. Intantaneous water heaters should not be confused with quick recovery storage water heaters ( 3.6kW/ 45L).

Diversity is applied here it is regarded that in most domestic environments these HW units are only used intermittently, perhaps only 15 mins at a time.

If you have 2 such units in a home there is no reason that at some time both may be running. One may have to use load management here. Instantaneous heaters should be limited to 10/15A each or storage HWS installed, electric or solar.

The problem with instantaneous water heaters is that the load on the distribution system is still the load of the heater not 33.3%. Two 20A units is 40A in a single phase installation and 20A per phase in a multiphase installation. Can cause high short term demands.

They seem to be very popular in Thailand perhaps because they are cheaper than a storage heater system. Most storage heaters are 13W per litre, a 270L unit will supply a family of 5 easily and is rated at 3.6kW single phase. Temp setting 70C.

But you do require good water pressure.

Edited December 3, 2009 by david96

[monty]

All very good and well, but IMO the theoretical MD calculation does not always jibe with real life in Thailand.

It now is in fashion to supply each bathroom, along with the kitchen, with one such instantaneous water heater.

You say it is unlikely that they are to be used simultaneously, I say it is extremely likely.

4 bed/bathrooms, family with 3 kids of school going age. I say you have a very good chance of all of them popping out of bed in the morning on schooldays and hitting the shower virtually simultaneously.

Stretching the imagination a bit can see you have a maid doing the dishes in the kitchen, also with hot water.

Most of those heaters sold in Thailand (Siemens, Stiebel Eltron) have a much higher power rating compared to the smaller shower heaters, namely between 6 and 10 KW. The smaller 6 KW will draw a cool 27 amps.

So if even only 3 of them are on simultaneously you are drawing over 80 amps on them alone. Add to that some A/C units (7 to 10 amps each), maybe a microwave / electrical cooking hob (another cool 10 to 30 amps) / coffee machine, you know, those things done by the maid, which more then likely will be working in a 3 child, up-class big family house...

I say that with a 100 amp single phase supply, you will stand a fair chance of tripping the breaker more then once, or at least be running in overload for a short while where the main breaker might not trip due to the slowness of the thermal tripping mechanism.

Oh, and why not use a central water heater (solar or electric)? Well, it seems we are living in a time of instant gratification, where people are unwilling to wait up to 2 minutes for hot water to reach the tap! And customer is king, so the builder/designer has to comply!

[Gary A]

I had a 3500 watt hot water shower. During the winter here upcountry, the water isn't hot nor even very warm. I wired it with a 20A breaker in the main box and another 20A breaker outside the bathroom door. I bought a 5500 watt heater and was worried about breakers tripping. Neither have never tripped. Turned as high as it can go, it should trip the breakers. I think the wattage estimate is very optimistic. At 5500 watts the water should be VERY hot. It isn't.

My solution is to buy a tank type water heater. I think a 75 liter tank will be plenty. There is no need for a rapid recovery unit so 3500 watts should be plenty of power.

[monty]

I had a 3500 watt hot water shower. During the winter here upcountry, the water isn't hot nor even very warm. I wired it with a 20A breaker in the main box and another 20A breaker outside the bathroom door. I bought a 5500 watt heater and was worried about breakers tripping. Neither have never tripped. Turned as high as it can go, it should trip the breakers. I think the wattage estimate is very optimistic. At 5500 watts the water should be VERY hot. It isn't.

My solution is to buy a tank type water heater. I think a 75 liter tank will be plenty. There is no need for a rapid recovery unit so 3500 watts should be plenty of power.

A 20 amp breaker can supply 4400 Watts, but will be able to run at 25% overload for a fairly long time before the thermal system will trip it!

Indeed, a 3500 Watt heater can be rather insufficient for heating up slightly chilly water! Hence the multi point heaters only start at 6KW and up to 12 KW. The latter usually only available on 3 phase power.

As mentioned above, a central water heater is the most sensible solution, but many people do not like the wait for hot water, along with the perceived loss of water while you are wasting the cold water (which actually is negligible cost wise), a wait which can be lengthy in a big 400 sqm bungalow style house. One is not always able to put the storage heater bang in the middle, so the furthest away located tap can take a while to deliver hot water.

The hot water leaving the tank will also lose heat to the lengthy water pipe, as good as it might be insulated!

With the rather stange experience I once had in such a house, that you set a nice temperature with th mixing valve at the shower, but the water stream kept on getting hotter as the water pipe was getting heated up!

[bkkbill]

Your right on about electric tankless water heaters requiring a relatively high electric power draw because water must be heated quickly to the desired temperature. But there are disadvantages to centrally located heaters such as:

standby losses. Also as you noted in your post energy wasted when hot water cools down in long pipes and while it's sitting in the storage tank.

By providing warm water immediately where it's used, tankless water heaters waste less water. People don't need to let the water run as they wait for warm water to reach a remote faucet. A tankless water heater can provide unlimited hot water as long as it is operating within its capacity.

Equipment life may be longer than tank-type heaters because they are less subject to corrosion. Expected life of tankless water heaters is 20 years, compared to 10 to 15 years for tank-type water heaters.

Just another point of view.

[Gary A]

A friend of mine has a 75 liter tank type that is now more than 15 years old. It finally burned out one of the elements but is still working on the other one. He is having a difficult time finding a replacement element for the Italian made heater.

When I find one, it will be installed in the bathroom between the washing machine and the shower. Both pipe runs will be very short. I need to find one that the thermostat can be set at a cool enough temperature for a shower without bothering with mixing valves.

[monty]

I need to find one that the thermostat can be set at a cool enough temperature for a shower without bothering with mixing valves.

Will be hard to find, it seems most start at 45 to 50 degrees C (113 and 122 Fahrenheit) whereas maximum shower temperature should never be over 40 degrees (104 Fahrenheit). 40 degrees would already be uncomfortably hot!

[katabeachbum]

I need to find one that the thermostat can be set at a cool enough temperature for a shower without bothering with mixing valves.

Will be hard to find, it seems most start at 45 to 50 degrees C (113 and 122 Fahrenheit) whereas maximum shower temperature should never be over 40 degrees (104 Fahrenheit). 40 degrees would already be uncomfortably hot!

a hot water tank should not be less than 65C to avoid legionella. mixing valves needed

I have 200 liter Fagor tank supplying 7 bathrooms, laundry and kitchen. It can take 20 seconds for hotwater to reach a distant tap. I was considering feeding it from solarhotwater. I measured the powerconsumption for a week, and its close to nothing. so I skipped the solarheatar idea. cold watertemp here in Phuket is never less than 25C and the tank is set at 85C.

[elkangorito]

Regarding Maximum Demand, david96 pretty much said it all.

The "diversity" method used in the Australian Standards to calculate MD, is conservative & seems to have worked quite well for a wide variety of family "types" in Australia for the last 20 or so years. The word "types" is important here as Australia is a multicultural country whereby many families have different habits. For example, Asian families tend to be larger in number than Anglo Saxon families. The same can easily apply to some European families (Greek, Italian etc). The "diversity" method of MD calculation does not seem to present a problem in this regard.

Australia (southern half) also has extreme temperature ranges...from as low as minus 15 degrees Celsius (Perisher area) to as high as 55 degrees Celsius (White Cliffs, Lightning Ridge - NSW). Disregarding the temperature extremes, most Australians who live in houses (not units or flats) choose storage type hot water systems. The reasons are as follows;

1] Most families are at least 3 people, who generally bath/shower at the same time of day/night. A 225 Litre storage HWS (Hot Water System) is adequate in this case. Larger families require a larger HWS - usually 350 Litres+. If sized correctly, these units will provide continuous hot water all year round, disregarding water & outside air temperatures.

2] Off-Peak electricity & gas rates are available for hot water storage units. I believe that Thailand does not have an off-peak rate system for domestic residences although I could be wrong.

3] The water quality is generally quite high in most parts of Australia & as such, good quality copper HWS storage systems last a long time (15 to 20 years), although the initial purchase price is high. "Glass lined" units have a shorter lifespan but are cheaper to buy.

4] Solar energy has been used to heat water for over 20 years in Australia. Many (or most) homes use solar energy & most solar HWS systems are of the storage type. It will be difficult to find the "glass tube" type solar HWS in Australia as they don't have a good lifespan & the tubes can develop problems. Glass tubes can also be shattered by hail. The Australian government also offer rebates for approved solar systems.

Instantaneous hot water heaters are cost effective in a small household (1 to 4 people). Any larger than this & I would be considering a storage type system. There are now ways to greatly minimise water wastage with a storage system. It involves valves & a slightly different pipe configuration. Methods such as these are now being employed in Australia since it is the driest inhabited continent on earth. ALL hot water pipes in Australia are insulated to minimise heat loss (plumbing Standard).

With regard to HWS's in Thailand, I would fork out the money for a decent solar HWS. Sola Hart & Rheem are two very good (& expensive) brands that will last a very long time if the water quality is good (low in metals - calcium, iron etc). If the water quality is poor (as generally in Thailand), I would choose a "glass lined" storage HWS (not solar).

With regard to Legionella, it is not a requirement or necessary in any way, to have the water temperature above 55 degrees Celsius. This bacteria lives everywhere (in the soil, in ponds, puddles etc) & is not a problem until water vapour is inhaled. It thrives at temperatures of 20 degrees Celsius but is destroyed at temperatures at or above 70 degrees Celcius.

Whilst one may ask, "My storage type HWS doesn't heat the water to above 70 degrees C. Can I have a problem with Legionella?"

The answer is NO. You will not have a problem. The reason being is that small storage systems (like the ones used for your home - less than 400 Litres) generally have a high turnover of water. This will not allow dangerously high coliform levels to exist.

The next reason is that Legionella bacteria need lots of oxygen, which is very limited in a storage system.

The long & the short of this is that Legionella bacteria is not a problem in domestic hot water storage systems. Of course, if you want to wash in a "cooling tower", you are asking for trouble.

[katabeachbum]

Regarding Maximum Demand, david96 pretty much said it all.

The "diversity" method used in the Australian Standards to calculate MD, is conservative & seems to have worked quite well for a wide variety of family "types" in Australia for the last 20 or so years. The word "types" is important here as Australia is a multicultural country whereby many families have different habits. For example, Asian families tend to be larger in number than Anglo Saxon families. The same can easily apply to some European families (Greek, Italian etc). The "diversity" method of MD calculation does not seem to present a problem in this regard.

Australia (southern half) also has extreme temperature ranges...from as low as minus 15 degrees Celsius (Perisher area) to as high as 55 degrees Celsius (White Cliffs, Lightning Ridge - NSW). Disregarding the temperature extremes, most Australians who live in houses (not units or flats) choose storage type hot water systems. The reasons are as follows;

1] Most families are at least 3 people, who generally bath/shower at the same time of day/night. A 225 Litre storage HWS (Hot Water System) is adequate in this case. Larger families require a larger HWS - usually 350 Litres+. If sized correctly, these units will provide continuous hot water all year round, disregarding water & outside air temperatures.

2] Off-Peak electricity & gas rates are available for hot water storage units. I believe that Thailand does not have an off-peak rate system for domestic residences although I could be wrong.

3] The water quality is generally quite high in most parts of Australia & as such, good quality copper HWS storage systems last a long time (15 to 20 years), although the initial purchase price is high. "Glass lined" units have a shorter lifespan but are cheaper to buy.

4] Solar energy has been used to heat water for over 20 years in Australia. Many (or most) homes use solar energy & most solar HWS systems are of the storage type. It will be difficult to find the "glass tube" type solar HWS in Australia as they don't have a good lifespan & the tubes can develop problems. Glass tubes can also be shattered by hail. The Australian government also offer rebates for approved solar systems.

Instantaneous hot water heaters are cost effective in a small household (1 to 4 people). Any larger than this & I would be considering a storage type system. There are now ways to greatly minimise water wastage with a storage system. It involves valves & a slightly different pipe configuration. Methods such as these are now being employed in Australia since it is the driest inhabited continent on earth. ALL hot water pipes in Australia are insulated to minimise heat loss (plumbing Standard).

With regard to HWS's in Thailand, I would fork out the money for a decent solar HWS. Sola Hart & Rheem are two very good (& expensive) brands that will last a very long time if the water quality is good (low in metals - calcium, iron etc). If the water quality is poor (as generally in Thailand), I would choose a "glass lined" storage HWS (not solar).

With regard to Legionella, it is not a requirement or necessary in any way, to have the water temperature above 55 degrees Celsius. This bacteria lives everywhere (in the soil, in ponds, puddles etc) & is not a problem until water vapour is inhaled. It thrives at temperatures of 20 degrees Celsius but is destroyed at temperatures at or above 70 degrees Celcius.

Whilst one may ask, "My storage type HWS doesn't heat the water to above 70 degrees C. Can I have a problem with Legionella?"

The answer is NO. You will not have a problem. The reason being is that small storage systems (like the ones used for your home - less than 400 Litres) generally have a high turnover of water. This will not allow dangerously high coliform levels to exist.

The next reason is that Legionella bacteria need lots of oxygen, which is very limited in a storage system.

The long & the short of this is that Legionella bacteria is not a problem in domestic hot water storage systems. Of course, if you want to wash in a "cooling tower", you are asking for trouble.

Thailand does have a off peak pricing for large 3 phase meters used for projects and hotels. 3 prices during 24 hours. From 3 baht/kwh

all included to 6 baht. I have a house in such a project (15 houses/one meter PEA) with individual private meters for each house. Average kwh price in one year is 3,3 baht/kwh compared to other houses I have with individual PEA meters, costing 4-4,10 baht/kwh.

Size of water heater tank depends very much on the set temp. set at 65C large tank is needed for 6 people, set at 85C same tank can serve 12 people just fine.

Guess different governments have different recommondations regarding Legionella. Back home we where advised to set the watertanks at 60C to save power. 2 years later the recommondation was to increase to 65 or 70C (forget) to avoid Legionella.

[monty]

hailand does have a off peak pricing for large 3 phase meters used for projects and hotels. 3 prices during 24 hours. From 3 baht/kwh

all included to 6 baht. I have a house in such a project (15 houses/one meter PEA) with individual private meters for each house. Average kwh price in one year is 3,3 baht/kwh compared to other houses I have with individual PEA meters, costing 4-4,10 baht/kwh.

I think your prices are way of. Nobody, whatever payment scheme/meter they use ever pays 6 Baht/unit.

I have 2 such meters as well, and we seem to pay according to below.

http://www.pea.co.th/th/eng/page.php?name=...mGeneralService

Look at the above website of the PEA.

Our pricing scheme is 3.2.2, being a TOU meter hooked up to atransformer on a 24 KV supply where you will see only 2 rates.

Peak rate is 2.65 Baht/unit and off peak rate is 1.1914 Baht/unit.

On this rates the FT charge gets added, working out to about 3.3 Baht peak and 1,4 Baht off peak.

Peak time is only on weekdays from 9 AM till 10 PM.

Off peak is 10 PM till 9 AM on weekdays, and 10 PM Friday all the way through to 9 AM Monday.

So one week sees 65 peak hours and 103 off-peak hours.

This for both a 50 KVA transfomer and a 100 KVA transformer.

If you're on a smaller transformer (30 KVA) you'll pay following rates:

http://www.pea.co.th/th/eng/page.php?name=...lGeneralService

The peak rates are higher (works out to 4.5 Baht something per unit, FT included), but the off peak rates are the same as for the bigger consumers.

[katabeachbum]

hailand does have a off peak pricing for large 3 phase meters used for projects and hotels. 3 prices during 24 hours. From 3 baht/kwh

all included to 6 baht. I have a house in such a project (15 houses/one meter PEA) with individual private meters for each house. Average kwh price in one year is 3,3 baht/kwh compared to other houses I have with individual PEA meters, costing 4-4,10 baht/kwh.

I think your prices are way of. Nobody, whatever payment scheme/meter they use ever pays 6 Baht/unit.

I have 2 such meters as well, and we seem to pay according to below.

http://www.pea.co.th/th/eng/page.php?name=...mGeneralService

Look at the above website of the PEA.

Our pricing scheme is 3.2.2, being a TOU meter hooked up to atransformer on a 24 KV supply where you will see only 2 rates.

Peak rate is 2.65 Baht/unit and off peak rate is 1.1914 Baht/unit.

On this rates the FT charge gets added, working out to about 3.3 Baht peak and 1,4 Baht off peak.

Peak time is only on weekdays from 9 AM till 10 PM.

Off peak is 10 PM till 9 AM on weekdays, and 10 PM Friday all the way through to 9 AM Monday.

So one week sees 65 peak hours and 103 off-peak hours.

This for both a 50 KVA transfomer and a 100 KVA transformer.

If you're on a smaller transformer (30 KVA) you'll pay following rates:

http://www.pea.co.th/th/eng/page.php?name=...lGeneralService

The peak rates are higher (works out to 4.5 Baht something per unit, FT included), but the off peak rates are the same as for the bigger consumers.

I think you need to calculate

kwh, peak 2,65

ft, peak 3,30

= 6,37

incl 7% vat 6,82 baht/kwh all included is what you pay peak rate according to your prices above

to bad I cant display the PEA invoice

[monty]

Why so much guess work!

The current Ft charge is found on your bill, and currently 0,9255 Baht/unit.

I'll attach the my bill of this month where you can clearly see this.

So my actual consumption on that bill costs me just under 30,000 Baht, and after adding the FT charge it comes to 41,000 something, add VAT and I'm at a final 44,000 something Baht.

I obviously was wrong on the net cost as it would be 3,6 Baht/unit peak and 2,1 something Baht/unit off peak when you include the Ft charge. To this VAT would be added.

[monty]

And if you look, at my consumption (12672 units) I paid about 3,3 Baht/unit on average, including Ft charge and VAT...

At the regular rate (non TOU) this would be about 4,2 Baht/unit (including VAT and Ft), so my bill would be about 11,500 Baht higher (or about 25% more).

This was with a low occupancy rate (bill is from a resort), with high occupancy the difference is even much more pronounced as the night time consumption shoots up (A/C's in the room), so I use much more units in off peak times compared to the units used in peak time...

At 80% occupancy my savings are close to 40% compared to having a normal meter...