Huge hot water cylinder heating more water than needed

Flight3287462

Have a look at these

https://sunamp.com/residential/

[Deleted User]

Reed_Richards said:

Deleted_User said:

I always wonder where the steam goes in an immersion tank as surely using some hot water and it topping itself up with cold water would create steam, a bit like topping up a boiled kettle. How does the tank not swell up and explode?

There will not be any steam but hot water expands and this could pressurise the cylinder.  In a "vented" system the cylinder is topped up by a header tank (usually in the loft).  This is open to the air so any expansion merely causes the water level in the header tank to rise slightly.  This is the traditional system but it has the disadvantage that the hot water pressure is limited by the height of the header tank above the tap or shower head.

More recently we have  had "unvented" cylinders where the water pressure in the tank is regulated and set to nearly the same pressure as the mains pressure.  In combination with "mains pressure" cold water this makes the hot and cold water pressures much more even so it is much easier to set the desired temperature on a mixer tap or shower and you get better flow rates near the top of the house.  Expansion in the tank is taken up by a "pressure vessel" -which is essentially like a balloon in a metal case which is compressed when the water in the tank expands.  If this fails you will see water discharged through the tundish whenever the tank is heated.

Thanks for answering. I did see another tank up there but didn’t know what it was for.

[Deleted User]

Shedman said:

[Deleted User] said:

We have a 250L unvented cylinder with just two occupants in the home. We use on average about 3.5kWh of electricity a day to re-heat the cylinder. The timer is set for 2 hours a day. We never run out of hot water and my wife prefers to have a bath rather than shower. She is also not a great fan of dishwashers. I suspect that a dual immersion heater might save us a bit of money but it is not worth the cost of fitting a new HW cylinder.

Is yours a top mounted immersion heater element and, if so, does it heat the whole of the tank as I read that, as the elements are normally only about 27" long, they only heat the top section of the tank.  

Interested to know real life example as wondering whether to use the immersion heater on ours instead of gas  (esp during summer when GCH is off) as on a fixed 19.5p kWh electricity tariff for next 18 months vs 7.5p gas until end Jan (and god knows what thereafter) so thinking it might actually be cheaper to use electric esp for the evening boost

I have a 250L Worcester Bosch Greenstar unvented cylinder with a bottom mounted immersion heater and thermostat. It has a separate heating coil with its own thermostat. The thermostats are at the same level in the tank. When comparing gas and electric re-heating usage and costs you are comparing apples and oranges.

Modern gas boilers have what is known as a TMaxSet temperature which is normally set by the installer. When there is a demand for gas heating of a HW cylinder, the boiler will heat up to TMaxSet to give the shortest cylinder reheating time possible. If follows with a flow temperature of 80C ( the default TMax Set) for a lot boilers, the boiler return flow will never condense. Also, when cylinder reheating starts the water flowing through the cylinder heating coil wIll extract heat from the cylinder until the temperature of the boiler flow exceeds the cylinder water temperature. There are also pipe heat loses to take into account.

All that said, with a price differential of c.3:1 between electricity and gas then even with the inefficiencies associated with gas, you will be better off sticking with gas for your cylinder reheating. 

Shedman

@[Deleted User]  Thanks for that.  Your cylinder is clearly quite different as ours is a standard 162litre vented with top immersion element so yours will no doubt be far more efficient and effective.   

I doubt our near 20 year old Potterton Suprima boiler has the setting you mention (certainly don't recall any mention of TMaxSet in operating or installers guides) as its a non condensing one.  But as you say probably best to stick with the gas for now.

BobT36

Flight3287462 said:

Have a look at these

https://sunamp.com/residential/

Isn't that just essentially a big electric shower heater, or am I missing something? 

QrizB

BobT36 said:

Flight3287462 said:

Have a look at these

https://sunamp.com/residential/

Isn't that just essentially a big electric shower heater, or am I missing something? 

It's more like a big storage heater. It's full of "stuff" that gets hot on cheap(er) rate electricity and which you then use instead of a boiler to heat your wet CH and DHW.

Reed_Richards

QrizB said:

BobT36 said:

Flight3287462 said:

Have a look at these

https://sunamp.com/residential/

Isn't that just essentially a big electric shower heater, or am I missing something? 

It's more like a big storage heater. It's full of "stuff" that gets hot on cheap(er) rate electricity and which you then use instead of a boiler to heat your wet CH and DHW.

Generically it is what is called a Thermal Store.  The "stuff" inside a Thermal Store is heated, you pass cold water through the heated "stuff" and it comes out hot.  You don't have to heat the "stuff" inside the Thermal Store electrically, Thermal Stores are often used where you have more than one heat source but you don't want complex plumbing.  The most commonly used "stuff" inside a Thermal Store is water but other types of "stuff" can store the same amount of energy in a smaller volume than water.  Sunamp use "stuff" that can achieve a higher energy storage density than water but you pay a price premium for it.    

markin

Sunamp also has very low heat loss it keeps the heat for months.

Standby Heat Loss (kWh/24h | [W]) 0.48  https://sunamp.com/wp-content/uploads/2021/03/Sunamp-Brochure-Mobile-Version-3.pdf

Standard water tank is in the range of 1.3 - 1.6KW /24h

 

Reed_Richards

markin said:

Sunamp also has very low heat loss it keeps the heat for months.

That is until you connect pipes to it!  I don't think any of the heat loss figures you read for hot water cylinders or thermal stores are worth a great deal because the tests are not made in a real-world scenario.  If you have a hot object and you connect copper pipes to it you will lose a lot of heat down the pipes no matter how well insulated it is otherwise.    

Section62

markin said:

Sunamp also has very low heat loss it keeps the heat for months.

Standby Heat Loss (kWh/24h | [W]) 0.48  https://sunamp.com/wp-content/uploads/2021/03/Sunamp-Brochure-Mobile-Version-3.pdf

Standard water tank is in the range of 1.3 - 1.6KW /24h

Months?

Back of a fag packet, a loss of 0.48 kWh per 24 hours would drain 3.5kWh in 7.3 days or with the larger 12kwh unit with a loss rate of 0.84 kWh/24h all stored heat would be gone in about 14 days, surely?  Presumably the rate of heat loss will vary a bit depending on total heat remaining, but for practical purposes I'd suggest the useful heat energy remaining is more important.

Essentially by installing one of these the OP would be spending a fair amount of money replacing one form of heat store (which they consider to be excessively sized) with a different form of heat store (with the same issue of getting the capacity right for different use patterns).

Adding more insulation to the existing HWC should still be the first consideration for seeking out the best return for investment.