How Long to Heat 170 Litre Hot Water Cylinder

[Dazzh]

Hi All,

Looking for some advice re how long it take to heat a Hot water Cylinder.
Daughter has moved into a house that has a Heatrae Sadia 170 litre Unvented Cylinder and a Potterton Promax SL Heat ErP boiler.
It's timed to come on Morning/Afternoon/Evening for abour 70 mins each time.
It will get used for a shower in the morning before she goes to work and bath in the evening when she comes home.
I've only ever had a combi boiler so I dont know how long it takes to heat up her water cylinder. I dont want it coming on when not required and running up a gas bill.
Any advice much appreciated.
Thanks

 

[Rodders53]

Depends:
On the boiler output and flow temperature and HW cylinder temperature and how much water--- read the manuals for it or the data sheets on Heatrae Sadia's website. (If you can find the precise model you have fitted).

Megaflo Eco – High performance unvented cylinder | Heatrae Sadia

The Megaflo Eco hot water unvented cylinder is easy to install & maintain and provides exceptional high levels of comfort and hot water performance. Visit our website for more details.

www.heatraesadia.com

Recovery time at 15L/min, 80ºC primary flow, from 15ºC to 60ºC is 22 minutes for the 170i... and 16 minutes if only 70% of the tank's HW is drawn off.

your cylinder may be a bit different and it'll take a bit longer with a lower flow temperature and less time if the CH boiler water flow rate is higher.

Don't forget that you are not really wasting energy by having a longer 'on' time - as soon as the HW cylinder thermostat is satisfied the boiler stops heating the tank. I'd be inclined to heat time for the bath (30 min or so before) and reheat after bath is drawn for a time, without a morning 'top up' cycle. It'll retain heat for the morning shower well enough in all probability.

 

[Dazzh]

Thanks @Rodders53.

 

[JonathanM]

and less time if the CH boiler water flow rate is higher

I've read this quite a few times now, so I'm sure it's right. But I still can't get my head around why increasing the flow rate makes the cylinder heat faster? Isn't the boiler putting the same amount of heat into the circuit, whether the flow rate is 15l/min or 30l/min? Be as brutal as you want!

 

[Rodders53]

Ask the cylinder makers??

My take on it is water flowing in the coil at 1.5 litres a minute can transfer X joules into the water... at double flow rate the joules transfer can be higher, albeit the return water temp will also likely be higher?

Why do circulating pumps have different speed settings if it isn't to move the hot water from A to B faster so allowing the boiler to modulate flame higher (or not short cycle as much)?

 

[JonathanM]

Ask the cylinder makers??

Do you think they will know? I've not had much luck asking sciency type questions.

 

[JonathanM]

My take on it is water flowing in the coil at 1.5 litres a minute can transfer X joules into the water... at double flow rate the joules transfer can be higher, albeit the return water temp will also likely be higher?

I was trying to think of the cylinder like a really big radiator. With radiators, if you speed up the pump, you narrow the dT, so as you say, the return temperature is higher. But the temperature increase of the water passing through the boiler is also smaller, and in the end it all seems to work out the same overall. But I find it hard!!

 

[fixitflav]

I've read this quite a few times now, so I'm sure it's right. But I still can't get my head around why increasing the flow rate makes the cylinder heat faster? Isn't the boiler putting the same amount of heat into the circuit, whether the flow rate is 15l/min or 30l/min? Be as brutal as you want!

If the flow is low enough, the boiler flow temperature rises to control-stat setting and the boiler stops, and restarts a bit later. So the average heat input to the cylinder is below the boiler rating. If the flow rate is increased the flow temperature falls and the return temperature rises. So control-stat setting is reached later (if at all) and average heat transfer rises.
Also it depends on the system, if the cylinder is old and the coil furred-up (like mine) the above applies. If heat transfer is high so the cylinder can take all the heat the boiler can give it, flow makes no difference. Might be a bit different for a modulating boiler, but I think the principle still holds.

 

[MickyP10]

Maybe do a timed test. I have an identical tank size and similar system and mine's about 40-45 mins to heat it up. You could easily get a couple of showers out of that one session, but the bath, obvs, is quite wasteful...

 

[JonathanM]

If the flow is low enough, the boiler flow temperature rises to control-stat setting and the boiler stops, and restarts a bit later. So the average heat input to the cylinder is below the boiler rating. If the flow rate is increased the flow temperature falls and the return temperature rises. So control-stat setting is reached later (if at all) and average heat transfer rises.
Also it depends on the system, if the cylinder is old and the coil furred-up (like mine) the above applies. If heat transfer is high so the cylinder can take all the heat the boiler can give it, flow makes no difference. Might be a bit different for a modulating boiler, but I think the principle still holds.

So, it sounds like it's simply about keeping the boiler burner on as much as possible, so that it puts more heat into the circuit in a given time period. I can see that, now. I've noticed the issue of flow rate affecting cylinder heating time mentioned a few times, and I'd wondered whether there was a fundamental scientific principle I was misunderstanding (that wouldn't be unusual).