Turn up boiler or top up DHW with immersion

[shu48]

I don't know if I'm best off turning down my boiler and using a bit of immersion, or running the boiler with a high flow temperature.

Just moved and new place has an old vented CH system. The cylinder is poorly insulated. The boiler is oil fired and is a relatively new, condensing type.

In order to hit 60c in the cylinder, I have to run the boiler hotter than is efficient. Due to strictly throttling my radiators with valves, I don't need to chuck a high temperature at my heating circuit. The temptation is to turn down the boiler so it operates more efficiently. However, that would mean it not heating the cylinder to 60c.

Due to poor insulation and not always using a lot of hot water, I don't heat the cylinder 24/7. I have been heating it for 1 hour, twice a day. Sometimes that's enough for a bath, and sometimes I manually switch it on for an hour or so if I know I've been using plenty of hot water. If I have a bath after it's been on twice that day and I haven't used much it's usually OK, and I still have enough after it's been on in the morning for washing up etc the next day, but I wouldn't get away with a bath in the evening and another in the morning, or if I've used a lot for cleaning.

Would I be best off having the boiler set to have a return temperature of 54c or lower and have the immersion come on once or twice a day, for say 30 mins, right after the boiler has done its thing for an hour, just to top up to 60c? Or am I better off having the boiler pumping out at more like 65/70c and not using the immersion?

It seems a shame to have the boiler set higher than is needed for my radiators, and higher than is efficient. But I also know from experience that immersion heaters aren't cheap to run either. My last place only had an immersion for the DHW and no CH at all.

I'd be glad to hear anyone's opinions. I really don't know which approach to take.

 

[wundaboy]

If the cylinder is poorly insulated, the first and by far the most effective step you can take is to improve the cylinder insulation. The immersion heater should exist merely as a backup in case the main heating system fails.

Everything else will follow from that. Your boiler should be chucking out the standard 82C flow temperature, this being used to run both your heating and hot water efficiently and then being throttled down at the cylinder by a thermostat linked to the motorised valve feeding it, and at the individual radiators by thermostatic valves.

Running at a 54C flow termperature raises the risk of legionella flourishing in the open parts of the system.

I don't understand why you think 54C is efficient? Traditional heating systems such as yours aren't designed to run that low, and recent (past 30+years) boilers are specifically designed to operate most efficiently at higher temperatures.

 

[flameport]

and recent (past 30+years) boilers are specifically designed to operate most efficiently at higher temperatures.

Condensing boilers should run at 55 or below.
Condensing boilers have been the standard item for 20+ years.
Like most people, @shu48 has a condensing boiler.

Your boiler should be chucking out the standard 82C flow temperature,

Absolutely not.
That is the kind of 'advice' which over the last couple of decades has cost millions of people vast amounts of money for gas or oil they didn't need to burn.

 

[flameport]

Would I be best off having the boiler set to have a return temperature of 54c or lower and have the immersion come on once or twice a day, for say 30 mins, right after the boiler has done its thing for an hour, just to top up to 60c? Or am I better off having the boiler pumping out at more like 65/70c and not using the immersion?

Ideally you would have controls which can have a lower flow temperature for the heating, and a higher one for the hot water.

If those are not available for your boiler, another option would be to set it at 55 all the time, and use the immersion on a timer once a week to heat to 65.

it not heating the cylinder to 60c.

60C isn't some magic number at which legionella ceases to exist.
At higher temperatures it's killed off more quickly.
Whether it's needed to be heated over 60C, how often and for how long depends on how much hot water is being used and what the source of the cold supply to the cylinder is.

If it's an unvented with a reasonable amount of hot water used every day, it may never need heating over 60C.
If it's an old open vent cylinder supplied from an open cistern with dead pigeons floating in it, and someone only does half a bowl of washing up per week, then no amount of heating may be sufficient.

 

[shu48]

If the cylinder is poorly insulated, the first and by far the most effective step you can take is to improve the cylinder insulation. The immersion heater should exist merely as a backup in case the main heating system fails.

Everything else will follow from that. Your boiler should be chucking out the standard 82C flow temperature, this being used to run both your heating and hot water efficiently and then being throttled down at the cylinder by a thermostat linked to the motorised valve feeding it, and at the individual radiators by thermostatic valves.

Running at a 54C flow termperature raises the risk of legionella flourishing in the open parts of the system.

I don't understand why you think 54C is efficient? Traditional heating systems such as yours aren't designed to run that low, and recent (past 30+years) boilers are specifically designed to operate most efficiently at higher temperatures.

I was talking about return temperature, not flow. My understanding is that the return is used to cool exhaust gasses. I'm pretty sure that as far as the heating circuit is concerned, the lower the flow temperature the better, so long as it's high enough to heat the home and satisfy the thermostats. I agree better cylinder insulation would be good, but you still need a flow higher than the temperature you want to achieve in the cylinder, even with the best insulation in the world. So my question still stands. Would I be better off running the boiler with a return temperature (not flow), low enough that it's condensing properly and make up the shortfall with the immersion, or raise the the thermostat on the boiler? I don't believe a flow of 80c would do my oil bill any favours though. The boiler has the usual markings on the thermostat that indicate where it runs efficiently. The top end of the scale is marked as not being so efficient, but I think they are only appreciate, because it has to do with the return temperature

 

[shu48]

Ideally you would have controls which can have a lower flow temperature for the heating, and a higher one for the hot water.

If those are not available for your boiler, another option would be to set it at 55 all the time, and use the immersion on a timer once a week to heat to 65.


60C isn't some magic number at which legionella ceases to exist.
At higher temperatures it's killed off more quickly.
Whether it's needed to be heated over 60C, how often and for how long depends on how much hot water is being used and what the source of the cold supply to the cylinder is.

If it's an unvented with a reasonable amount of hot water used every day, it may never need heating over 60C.
If it's an old open vent cylinder supplied from an open cistern with dead pigeons floating in it, and someone only does half a bowl of washing up per week, then no amount of heating may be sufficient.

That sounds plausible. I will experiment with an efficient return temperature. I might have to set the programmer to heat the cylinder for longer periods. Once a week or so, I can leave it on continuous for a good while before running the emersion for a while to hit 65. Alternatively, I could turn the boiler up once a week, but just a bit of a pain as you have to remove the boiler covers to get into the thermostat. An immersion heater timer is something I have yet to invest in. I've started off with smart TRVs and fixing leaks as my priority. Thanks for your advice