Hot Water Programme

[soapstar]

Hi Folks,

 

We currently have a 300l tank heated by a Grant Aerona 17kw ASHP.

 

Given the size of the tank (designed as per the size of the house at 4 bedrooms) which is currently 'overkill' at the moment due to only the two of us in the house, what is the most efficient method to heat our water?

 

The hot water is controlled via a programmable timer which is set to come on every day (at noon as I guessed this is when the room heating is less likely to be required) for around 45 minutes. This time was just a 'rough guess' from when we moved in to our new build approximately 8 months ago. This has served us well and provided enough hot water for the both of us.

 

My question is do I continue to set the timer for the 45 minutes OR do I increase the time to allow the entire tank to heat to the set temperature on the tank thermostat at 50 degrees? What would be deemed more efficient? From what I can gather the heat loss for our tank is 2.7kw @ 60 degrees if that helps

 

[JohnMo]

What temperature does your tank get to in 45 mins?

 

If hitting the thermostat , I would be tempted to leave the timer as is and reduce set temperature of the thermostat.  Once the tank is up to temp, the heat pump will stop adding to the the tank.

 

You shower and wash up at about 37 deg, so you are always watering down the hot water.  The lower you can get the temp, the lower the heating cost and the lower the losses.

 

Just a matter of adjust and use for a couple days, once things aren't hot enough add a bit more back on.

[soapstar]

6 minutes ago, JohnMo said:

What temperature does your tank get to in 45 mins?

 

If hitting the thermostat , I would be tempted to leave the timer as is and reduce set temperature of the thermostat.  Once the tank is up to temp, the heat pump will stop adding to the the tank.

 

You shower and wash up at about 37 deg, so you are always watering down the hot water.  The lower you can get the temp, the lower the heating cost and the lower the losses.

 

Just a matter of adjust and use for a couple days, once things aren't hot enough add a bit more back on.

 

Thanks @JohnMo

 

Well this is what im trying to get my head around (plumbing confuses me slightly  ). The thermostat is located about 1/3rd of the way up from the bottom of the tank - does this mean the temp at that location is not actually what the temperature would be at the top/hot water outlet?

[JohnMo]

The temp at the top of the tank will be closer to the temperature your heat pump is set at the bottom at closer to cold.  Where the thermostat is you can assume the contents at and above the thermostat level will at or above the thermostat temperature.

 

So if you are delivering heat from the heat pump at 55 and your thermostat is set at 45, at 1/3 up will be at 45 and top close to 55

 

 

[ProDave]

No 1 it is very unlikely the ASHP will use the cylinder thermostat to set the temperature.  That will usually be by a temperature probe inserted into a pocket on the tank.  A conventional cylinder thermostat should also be connected and set to a higher temperature as a fail safe to close the motorised valve if the water gets too hot.

 

You have to decide what your hot water requirements are.  Do you always shower at the same time of day so only need hot water then and are prepared to accept possibly tepid water at other times?

 

I work on the principle that anyone might shower at any time of day so hot water must always be available.  I have the HW set to come on at 11AM and go off at 10PM.  The 11AM start is to ensure good self usage of solar PV power, it should be generating well by then.  the 10PM is based on nobody normally showers that late so there should be enough hot water left in the tank overnight for a morning shower.

[soapstar]

1 hour ago, JohnMo said:

The temp at the top of the tank will be closer to the temperature your heat pump is set at the bottom at closer to cold.  Where the thermostat is you can assume the contents at and above the thermostat level will at or above the thermostat temperature.

 

So if you are delivering heat from the heat pump at 55 and your thermostat is set at 45, at 1/3 up will be at 45 and top close to 55

 

 

Thanks that makes sense.

 

15 minutes ago, ProDave said:

No 1 it is very unlikely the ASHP will use the cylinder thermostat to set the temperature.  That will usually be by a temperature probe inserted into a pocket on the tank.  A conventional cylinder thermostat should also be connected and set to a higher temperature as a fail safe to close the motorised valve if the water gets too hot.

 

You have to decide what your hot water requirements are.  Do you always shower at the same time of day so only need hot water then and are prepared to accept possibly tepid water at other times?

 

I work on the principle that anyone might shower at any time of day so hot water must always be available.  I have the HW set to come on at 11AM and go off at 10PM.  The 11AM start is to ensure good self usage of solar PV power, it should be generating well by then.  the 10PM is based on nobody normally showers that late so there should be enough hot water left in the tank overnight for a morning shower.

 

Yes generally we shower in the evening after work.

 

Going back to the efficiency part, do you feel the cost difference would be negligible with us heating the entire tank (or in other words leave timer on long enough to reach this point) or continue to only heat a portion of the tanks capacity? The initial heat up would be expensive I assume given we technically have never had the entire tank fully heated, however I would assume this would level out after a period of time.

 

In theory if we currently have say 20% of the tank hot after our daily heat up, this will deplete before the next heating cycle after showering etc. Now the next day it would need to heat up 100% of cold water in the tank to give us 20% hot water - Would this be less efficient than the other option of heating the entire tank every day (in theory the ratio of hot to cold water would be higher before heating i.e. 80% hot water)...no idea if that makes any sense what so ever ?

 

I guess the only way to find out would be to try it...  

 

[ReedRichards]

Logically you should charge (heat-up) your hot water tank when the outside temperature is warmest so around noon should be a good time.

 

The hotter you keep your tank temperature then the more heat will be lost to the surroundings,  although in winter that may not be a terrible thing because the heat will be lost to the house.  The cooler you keep your tank temperature the more efficiently your heat pump can heat it but the more chance there is that the hot water will run tepid.

 

In my case my hot water is set to 50 C and it is on from 6:30 to 22:30 so it just recharges as necessary.  However the hysteresis setting is set to 10 C so it should not recharge until the tank temperature drops to 40 C or less.