Most efficient way to set HW timing with UV cylinder?

[indus]

Hi
I have a system boiler with a 300L UV cylinder and honeywell ST9400C controller. If it makes a difference I have TWS mainsboost as well.

I'm just trying to figure out the most energy efficient way of setting the controller for HW. At the moment it is set from 0500 to 0800 and then 1700 to 2000hrs.

With these settings there is never a shortage of hot water whether showering in the morning/late morning or in the evenings after work for the adults or after school sports clubs for the kids.

I'm wondering whether I should try and reduce the hours even further or whether that would be counterproductive. ie would the boiler have to do more work if the stored water cools down more.

I hope my question makes sense. Thanks for your help.

 

[Harry Bloomfield]

I'm wondering whether I should try and reduce the hours even further or whether that would be counterproductive. ie would the boiler have to do more work if the stored water cools down more.

To answer that question - no. The hotter the water is kept, the more heat will be lost, the more it will cost to keep it hot. Only having the hot water hot, when you intend to actually make use of it - is the cheapest way, not that it would cost much to just keep it hot.

I have my system timed to come on in time to have hot water in time for dinner, just once per day, it is hot enough the rest of the day unless a bath is needed - in which case there is a one shot to heat a cylinder full. Normally I take a shower on a morning, which is electric.

 

[flameport]

Modern hot water cylinders have tiny heat losses, in the order of pennies per day.
Attempting to make savings on that will achieve nothing.

Most losses are from the pipework connected to the cylinder, so add decent insulation to that if it doesn't have any.
Otherwise, if it provides all of the hot water you need when you need it, keep the heating times as they are.

 

[JohnD]

You have a 300l cylinder.

I bet if you heat it up ready for morning, that will do all your morning showers, with enough left over for incidental handwashing

Although the cylinder is well-insulated, the boiler is not, and it is wasteful to let it start up every time you run a tap. You can avoid that by timing the boiler to run before and during your shower periods, and off the rest of the time.

There is no strain on the boiler in making it heat a full cylinder from cold. It will do that with ease in an hour or so if necessary. Starting and stopping twenty times a day are worse.

 

[indus]

Thank very much for your replies. I forgot to mention this system provides HW and CH to the first and second floors. The ground floor (inc kitchen) is run on a combi boiler.

I think I've understood what you have all said, however I note it was mentioned that a cylinder of water can be heated in a hour. If that is the case should the hw be set for just about one hour before the earliest morning and evening shower?

Thanks for the advice on the pipework leaving the cylinder, it is not lagged, I will get that done.

 

[Harry Bloomfield]

I think I've understood what you have all said, however I note it was mentioned that a cylinder of water can be heated in a hour. If that is the case should the hw be set for just about one hour before the earliest morning and evening shower?

If it does take one hour, then you can get away with it coming on one hour before needed. Worth testing how long it takes from a cylinder fully cold, to boiler satisfying the demand. Remember, it will take longer, if there is a demand for central heating at the same time.

 

[indus]

If it does take one hour, then you can get away with it coming on one hour before needed. Worth testing how long it takes from a cylinder fully cold, to boiler satisfying the demand. Remember, it will take longer, if there is a demand for central heating at the same time.

Thanks. I'll pay around with it.

 

[cdbe]

We only have a 145l cylinder, 4 people, 2 showers. I did try and second guess our usage pattern and minimise the on times but we (fairly) regularly ran out of hot water so in the end I gave up and put it on from 5pm to midnight (when we have showers) and half an hour in the morning. We never run out any more, but it does irk me to hear the boiler running after I've washed my hands or a few pots; I believe the issue is the hysteresis of the thermostat and given that we have a fairly old cast iron boiler it can't be very efficient to fire up for 5 minutes to top up a few litres of hot water.
I guess the most efficient way to run it (with the controls I have) would be to have it timed for one hour a day a day (say 30 minutes morning and 30 minutes at 5pm) and then press the +1 hour boost before each shower. But, it's well insulated, the heat loss in the winter just goes into the house and the actual cash saving is probably tiny.

 

[ericmark]

First a question, is there a thermostat on the tank, or is it purely timed? If there is a thermostat then there is no problem running the DWH for 24 hours as it will only switch on if the water is too cool. If however there is no thermostat then time is important.

Clearly you don't want to lose heat from the pipework, and with no thermostat the boiler will continue to cycle, and the pump if there is a pump will circulate taking heat out of the cylinder, so it does matter how it is controlled.

However I also have a 40 gallon tank heated from the central heating boiler, unlike yours my boiler will not modulate well at least not much, so with no central heating running, it will run for ½ hour, after that the hot coil can't absorb all the heat the boiler produces so boiler starts to cycle. As to how long it needs to run to transfer enough heat to heat the cylinder to over 60°C I don't know, running ½ hour every other day keeps the water hot enough for hand washing, but I would say once a week needs 3 hours to ensure over the 60°C needed to stop legionnaires.

With old cylinders with not much insulation likely do need heating twice a day, but most cylinders once a day is enough, however there is only one way to work it all out as too many variables, reduce time until not warm enough, then increase it a little again.

 

[cdbe]

Just for info, I've checked my gas bill - for the month covering June to July this year (I'm on a tariff where you have to submit monthly readings) and my gas consumption was 343 kWH which, excluding standing charge, cost me £8.75 ex. Vat - so this is hot water (heated by a band B boiler), a gas hob and a gas tumble dryer - even for a tight arse like me it's hardly worth worrying about.

 

[wannocks]

The only advice I give is to set the HW to come on 1/2 hour to an hour before the heating for a faster response when the heating comes on as it's not doing both at the same time.

 

[ericmark]

You have not answered the big question, is there a thermostat on the tank?

The only advice I give is to set the HW to come on 1/2 hour to an hour before the heating for a faster response when the heating comes on as it's not doing both at the same time.

That may not work, it all depends on how good the hot coil is in the cylinder, with my set up, the boiler produces more heat than the hot coil can transfer so it works better when central heating is running, but the main thing is does the boiler modulate? There are two completely different ways to control a boiler, the old way was on/off but to let the boiler condense the water in the flue gases the temperature of the return water is important, so to gain that latent heat the boiler must modulate (have a variable output) there are two way to control the modulation one is return water temperature, the other is direct connection to the ebus.

So the idea is the boiler slowly turns down as the demand drops until it can't turn down any more, at which point it starts to cycle off/on, but before cycling off/on the water temperature is lower so when it switches off less heat is lost out of the flue.

The problem is every boiler will have a sweet point where it runs most efficient, this could be when flat out, or mid way or just before it starts to cycle, the problem is we don't know, the problem using the boiler as a system boiler is the return water from DHW will likely be over 60°C but water from the CH could be as low as 40°C if the two mix giving a return water temperature of 50°C that is likely better than the much hotter return water if the DHW returns on it's own.

So the DHW and CH starting at same time will likely mean higher output water temperature so faster heating time for DHW because the boiler has not modulated, and also more economic, of course using an OpenTherm system where the wall thermostat/hub controls with algorithms the output of boiler to best match demand should work best.

But we don't have a clue if DWH has a thermostat controlling a motorised valve or if the boiler modulates, so there is really no answer to the question.