Pressurised tank + instantaneous water heater?

[tayloa17]

Is it possible to use some kind of pressurised tank in conjunction with an instantaneous water heater in order to improve hot water temperature in winter, by raising the cold mains temperature to room temperature?

I'm looking at options on hot water for a small flat conversion. Working to small budget.

Thanks for any suggestions.

 

[Mike Jackson]

No. Look at using a small unvented water heater.

 

[BLOD]

No. Look at using a small unvented water heater.

:iagree:

 

[AWheating]

cold water storage tanks need to be kept cold to avoid increased risk of legionella growth.

 

[jtsplumbing]

Are you talking about Elec or gas heater ? is it to do all of flat ?

 

[tayloa17]

Are you talking about Elec or gas heater ? is it to do all of flat ?

Electric, for shower, basin, and kitchen sink.

Thanks all for your valuable comments. Any opinions on the flow rate of a 10.8kw redring heater for shower use in winter? I guess at least it's better than a 7kw shower (so long as the basin, sink are not in use), but is it noticeably flaky compared to other options (unvented cylinder or combi boiler)?

 

[Mike Jackson]

The small instantaneous heaters are still fairly poor, even the 10.8Kw ones. I personally wouldn't consider them for anything larger than a single person bedsit. Perhaps look at a small handwash unit over the basin, an electric shower and an undersink water heater in the kitchen. Preferably try and find room for a small unvented cylinder in the flat.

 

[tayloa17]

The small instantaneous heaters are still fairly poor, even the 10.8Kw ones. I personally wouldn't consider them for anything larger than a single person bedsit. Perhaps look at a small handwash unit over the basin, an electric shower and an undersink water heater in the kitchen. Preferably try and find room for a small unvented cylinder in the flat.

Any idea why, and can you quantity "fairly poor"? i.e how does a 10.8kw instantaneous heater differ in performance from, say, a top-of-the-range 10.8kw electric shower? I assume heat exchange of either must be pretty efficient or the unit would overheat. So surely that just leaves heat loss through pipework - is this the main problem?

As this is for a studio flat, I'm assuming 1 person using hot water at any one time, and the kitchen/bathroom are adjacent so pipework lengths are short (and are insulated). Is this reasonable?

 

[Mike Jackson]

It's the way it's implemented. I used to maintain some properties that had powerstream 10.8 Kw instantaneous heaters running all of the hot water for the property. The dedicated showers are a mixer shower with a restriction in the hot side. To use it the hot is turned on fully and the cold adjusted until the correct temperature is achieved. Most of the tenants used to struggle getting the temperature right and we would regularly get calls to look at faulty units which were user error.

The flow rate of these units is somewhere around 4l/min for the 10.8 unit for a decent temperature rise so even a small sink would take an age to fill and we were very soon disconnecting the kitchen sinks and fitting under sink heaters. We had the same issue with the basins because standard taps had been fitted. Fitting the Redring spray tap improved matters a bit because the flow looked better but it still took abn age to fill a basin.

If you decide to proceed with these units don't try and feed the kitchen sink with one and expect call backs from tenants unable to work the shower.

 

[tayloa17]

Thanks a lot, this is interesting and useful as i'm doing a number of units, manage others mostly via agents, and researching for new products to market.

Your figure of 4L/min for a 10.8kW instant water heater represents about 30% loss of heat (assuming cold input is 10deg) which corresponds with what i experience with my own gas combi - about 25% loss over 7m of 15mm uninsulated pipe (rough tests only, but i'm shocked and will investigate further)

To match the current performance of, say, my gas combi, which delivers a reasonable shower at a flow of 8L/min, I would need a 15kW electric shower (no such thing i think), but a 10.8kW would also match this if the cold input were pre-"heated" to room temp, say 18deg. Hence my initial reference to a pressurised tank to hold room-temperature water to feed the heater.


I notice that unvented water heaters have a temperature control from 20 degrees upwards - does there exist an unvented "heater" that only heats upto 20degrees, that could be used as input to an instantaneous heater? or an instantaneous heater that has a tank holding say 50L of room temperature water?

Re the comment about legionella growth, does this apply to sealed units, and are there any regulations around this? I will check part G etc, but if you know off hand...

 

[tayloa17]

Govt info on Legionella: [DLMURL]http://www.hse.gov.uk/pubns/indg376.pdf[/DLMURL]

 

[tayloa17]

Such a set-up might even help reduce the risk of legionella, by delivering hotter water at 55-60 degrees from a 10.8kW unit, which kills this bacteria..?