Hot water - colder and use more, or hotter and use less?
Discussion
Just back into our renovated house and have all the bells and whistles Viessman install with a 200W boiler and 300 litre unvented Viessman cylinder. All working really well - with hot water priority it reheats the cylinder at 70 degree flow really fast, but otherwise runs on weather compensation down at around 40 degrees.
Heating engineer suggested that storing hot water at 50 degrees was the right thing to do as it would reduce losses sitting in the tank. Makes sense to me. But, when showering if you have 50 degree hot water, you are going to use more of it than if you had 60 degree hot water - since you'll be mixing less cold into it to get it down to a sensible temp. That means heating up more water to get the cylinder back up to temp.
I guess, all being equal, you are taking the same energy out of the cylinder to have the same temp shower so it should equal out - with some benefit from the lower temp reducing sitting losses. If we had lots of people to stay we'd probably then bump up the temp in the cylinder to ensure there's enough hot water.
Am I missing anything?
Heating engineer suggested that storing hot water at 50 degrees was the right thing to do as it would reduce losses sitting in the tank. Makes sense to me. But, when showering if you have 50 degree hot water, you are going to use more of it than if you had 60 degree hot water - since you'll be mixing less cold into it to get it down to a sensible temp. That means heating up more water to get the cylinder back up to temp.
I guess, all being equal, you are taking the same energy out of the cylinder to have the same temp shower so it should equal out - with some benefit from the lower temp reducing sitting losses. If we had lots of people to stay we'd probably then bump up the temp in the cylinder to ensure there's enough hot water.
Am I missing anything?
Mr Pointy said:
Well the HSE say water storage needs to be at 60°C minimum to prevent Legionnaire's being a potential issue so I'd go for that at least.
^This^ 60 degree C minimum storage is recommended, absolute minimum 55. (This is not just PH 'nice to have' advice, this is readily available online in various guides). The difference may be that your boiler controller runs a high temperature 'anti legionella' cycle every so often, but nonetheless, I would personally store at 60 degrees to offer improved outlet temps / flow rates / fill time. (i.e.: my wife likes a hot shower, kids are always having baths etc).As you've eluded to yourself, I would say that standing losses will be minimal from a well insulated modern cylinder and hence energy differences between storing cooler water / using more of contents for the same 'net' result is a fair assessment, but energy consumption difference overall will be negligible.
Regarding your 200W boiler....should that read 20kW? Suspect 200 Watts won't get you very far.
Edited by MattyD803 on Wednesday 27th November 10:16
With 70degC in the tank, you waste less water running a sink or a bath, because the hot water will raise the average temp in the sink.
With 40degC water, you have to let it run to full temp and you can't 'top it up with hot'.
Maybe there's an optimum i nthe middle?
A well insulated tank should not lose much, and it should be possible to quantify that.
There are complex options involving heating it hotter on cheap rate or solar.
Heat pumps will mean cooler is cheaper due to the efficiency/cop of the heat pump improving.
Some people (thiose with mega power showers mainly?) spend a lot on hot water, for many of us it's not a big expense, payback on a bigger tank would be slow. I suspect getting ready for ASHP is a force in the market?
With 40degC water, you have to let it run to full temp and you can't 'top it up with hot'.
Maybe there's an optimum i nthe middle?
A well insulated tank should not lose much, and it should be possible to quantify that.
There are complex options involving heating it hotter on cheap rate or solar.
Heat pumps will mean cooler is cheaper due to the efficiency/cop of the heat pump improving.
Some people (thiose with mega power showers mainly?) spend a lot on hot water, for many of us it's not a big expense, payback on a bigger tank would be slow. I suspect getting ready for ASHP is a force in the market?
OutInTheShed said:
...
A well insulated tank should not lose much, and it should be possible to quantify that.
There is an argument that I often have with myself... the 'losses' in the tank aren't really lost - they go in some way to heating the fabric of the house. It depends - if your tank is outside in a garage or in a loft, then of course, those losses are .. err.. lost.A well insulated tank should not lose much, and it should be possible to quantify that.
Thanks all. Yes Legionella is a known issue, as mentioned by another poster above the boiler does indeed do a Legionella cycle every week to deal with that and I'm happy relying on that. Appreciate others may not want to take that view, but I'm ok with it - with a wife and two tweenagers in the house, hot water doesn't sit around for long...
One thing that I will do is monitor heat losses from the cylinder in terms of speed of loss at 50 vs 60, when we are next on holiday - I can change the hot water stored temp on the app, and it gives me live read-out of the temp in the cylinder, so shouldn't be hard to do if no-one is using the hot water. I'd imagine them to be very similar in reality making the whole thing a bit of a non-issue.
One thing that I will do is monitor heat losses from the cylinder in terms of speed of loss at 50 vs 60, when we are next on holiday - I can change the hot water stored temp on the app, and it gives me live read-out of the temp in the cylinder, so shouldn't be hard to do if no-one is using the hot water. I'd imagine them to be very similar in reality making the whole thing a bit of a non-issue.
We run a hot water priority system and programme to heat the tank to 50c, which is plenty for the 2 of us with a well insulated 210ltr tank.
The heating side is load and weather compensated so average days it pumps out around 40c to the CH and goes up to about 55c on below freezing days for a short while.
My system is Vaillant one with full Vaillant controls and it allows for programming an anti-legionnaire cycle day once a week where it brings the tank up to about 67c. That cycle is on a Friday and you can really tell the difference, being retired now it’s a good way of finding out its actually Friday
The heating side is load and weather compensated so average days it pumps out around 40c to the CH and goes up to about 55c on below freezing days for a short while.
My system is Vaillant one with full Vaillant controls and it allows for programming an anti-legionnaire cycle day once a week where it brings the tank up to about 67c. That cycle is on a Friday and you can really tell the difference, being retired now it’s a good way of finding out its actually Friday
MattyD803 said:
dhutch said:
Viessmann 'Vitodens 200-W' is a model of boiler, rated up to 32kW.
Got it, not familiar with Viessman range, but I hear they are very good...We had a 100W before as a temporary solution (since the 200W took ages on lead time) and that was good but didn't have anything like the features the 200W has.
Calza said:
I don't think the 60degree thing is as clear cut as that..
Within educational, healthcare, commercial and industrial settings....I can assure you that minimum water storage temperatures to minimise legionella risk is very clear cut, to the point of being very clearly referenced with HSE ACOP L8 documentation, CIBSE guidance, HTMs and supported by manufacturers literature. To go lower would need risk assessment by the designer / maintainer etc.Within residential settings, there is naturally a little more scope for variability due to the reduced associated risk and much lower storage volumes (and as I mentioned, anti legionella purge / cycle), but its a brave designer/installer/maintainer that decides to venture too far off-piste unnecessarily.
The other thing to take into account is that you can get away with a smaller and cheaper tank if you heat it up hotter, whilst still being able to run the same number of showers or sinks of water. Ours is quite small (only about 125l, unvented cylinder) which is 'not large' for a family of four, but in reality it's OK as long as no more than two consecutive showers are taken with no reheat time in between. Our 24kW boiler heats it up fairly quickly as it's small.
As the OP already said, keeping the tank at 65 means you simply use less of it as it gets diluted down more with extra cold - and zero chance of legionnaire's anywhere. You do really need mixer taps at that sort of temperature though, but since hot and cold are at essentially the same pressure they work nicely. The outside of the tank always feels cool to the touch (and since it's in a garage with no heating you'd certainly notice it, so I don't think we're losing much heat that way.
As the OP already said, keeping the tank at 65 means you simply use less of it as it gets diluted down more with extra cold - and zero chance of legionnaire's anywhere. You do really need mixer taps at that sort of temperature though, but since hot and cold are at essentially the same pressure they work nicely. The outside of the tank always feels cool to the touch (and since it's in a garage with no heating you'd certainly notice it, so I don't think we're losing much heat that way.
OP, you've got the jist of it, the things your missing are the Legionella risk (which others have covered) and what you're actually saving. So let's chuck some numbers at this
Viesmann will quote a standing heat loss for your cylinder - I don't know exactly what you've got, but I've found specs for one of their 300l cylinders @ around 1.2 kWh/day. This should be at standard ERP condition of 65°C stored water in 20°C room.
I don't know what your gas rate is, let's say 6p/kWh
6p x 365 x 1.2 kWh/day = £26 a year. That's what it's costing you in standing losses (roughly... your average cylinder temperature will be less due to draw off and replenishment, but let's go with that)
The heat loss from the cylinder is directly proportional to the temperature difference between the stored water temperature and air temperature of the room the cylinder is in ('delta T', or dT). Stored at 65 and room at 20, the dT is 45°C. Stored at 50°C, the dT is 30.
As we know heat loss from the cylinder is directly proportional to dT, and cost is directly proportional to the heat loss, we can simple take the £26 pa and multiply by 30/45, = £17 - that's what your standing losses cost per annum at 50°C stored temp
So you're saving £9 a year. I'd stick with the higher temperature....
(If we were being very scientific, we'd need to factor in that with lower flow & return temps the boiler would be able to operate in condensing mode one you've got a return temp of below 57°C, but only just. And you'd get reduced heat loss from distribution losses. But I can't be bothered to work that out...)
Viesmann will quote a standing heat loss for your cylinder - I don't know exactly what you've got, but I've found specs for one of their 300l cylinders @ around 1.2 kWh/day. This should be at standard ERP condition of 65°C stored water in 20°C room.
I don't know what your gas rate is, let's say 6p/kWh
6p x 365 x 1.2 kWh/day = £26 a year. That's what it's costing you in standing losses (roughly... your average cylinder temperature will be less due to draw off and replenishment, but let's go with that)
The heat loss from the cylinder is directly proportional to the temperature difference between the stored water temperature and air temperature of the room the cylinder is in ('delta T', or dT). Stored at 65 and room at 20, the dT is 45°C. Stored at 50°C, the dT is 30.
As we know heat loss from the cylinder is directly proportional to dT, and cost is directly proportional to the heat loss, we can simple take the £26 pa and multiply by 30/45, = £17 - that's what your standing losses cost per annum at 50°C stored temp
So you're saving £9 a year. I'd stick with the higher temperature....
(If we were being very scientific, we'd need to factor in that with lower flow & return temps the boiler would be able to operate in condensing mode one you've got a return temp of below 57°C, but only just. And you'd get reduced heat loss from distribution losses. But I can't be bothered to work that out...)
Jamgy thanks for that - great to see the maths behind it. I think that is right re the standing losses on this cylinder, so your numbers are likely about right. £9 a year, I'm spending that in gas a day and more at the moment heating up the slab for the UFH so it's not really worth the pain I'd say.
also if the tank is emptied every couple of days then the chances go even lower
re flow temp one of the biggest issues you have is the temperature control on the showers not working if its too low alot require the flow temp to be 10c above the required temp the safety detente is 38c so have your dhw no lower than 50 really or you may find it flipping between scalding and freezing.
welcome to the 200w club
re flow temp one of the biggest issues you have is the temperature control on the showers not working if its too low alot require the flow temp to be 10c above the required temp the safety detente is 38c so have your dhw no lower than 50 really or you may find it flipping between scalding and freezing.
welcome to the 200w club
No ideas for a name said:
OutInTheShed said:
...
A well insulated tank should not lose much, and it should be possible to quantify that.
There is an argument that I often have with myself... the 'losses' in the tank aren't really lost - they go in some way to heating the fabric of the house. It depends - if your tank is outside in a garage or in a loft, then of course, those losses are .. err.. lost.A well insulated tank should not lose much, and it should be possible to quantify that.
In summer, it's potentially making your house warmer than you want, maybe even costing you in aircon.
But if the heat loss is small, it's a bit of a detail.
I ought to be able to work out how much HW we use. as we've got a combi, so the Summer use is just HW and a gas hob.
About 4.5 kWh per day for an average of two people. I guess that's mostly HW but we do mostly cook on the hob.
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