Central heating control, any errors?

[ericmark]

Well latter today the plumber is due to arrive to complete the work, the gap has been good as been able to iron out some problems. If using two pumps this

diagram was to be used, however with a single pump this

this actually works better giving full central heating control in the flat.
Also the problem with random boiler firing has been solved, it was not a fault, why the help lines did not point this out I don't know? However I in the end found the following

How the Nest thermostat helps prevent Legionella bacteria
Legionnaires’ disease is a potentially fatal type of pneumonia that’s contracted by inhaling water droplets that contain Legionella bacteria. The 3rd gen Google Nest Thermostat’s Bacteria Prevention feature works with both On/Off hot water systems and OpenTherm hot water system boilers. To help prevent bacterial growth, it will heat your hot water tank even if your home is in Away mode.

Legionella bacteria can develop in water that has been exposed to impurities like rust, algae and limescale, in temperatures between 20 and 45 °C. The bacteria are dormant below 20 °C and do not survive above 60 °C (Source: HSE UK).

Bacteria Prevention is an exclusive feature of 3rd gen Nest Learning Thermostats. Bacteria Prevention mode will activate if your domestic water hasn’t been heated for at least 2 consecutive hours in the last 48 hours. This can happen if there is a gap of more than 48 hours in your hot water heating schedule or if you are away from home for longer than 48 hours. For instance, if you have a second home or holiday home and only need hot water for weekends, you can create a hot water schedule for Saturday and Sunday, and the Bacteria Prevention mode will automatically heat the hot water tank during the week.

">Learn about Home/Away Assist >

">How to switch your home to Away or Home mode manually >

Your Nest thermostat will let you know Bacteria Prevention mode is running by showing you a water droplet inside of a circle

on the temperature screen.

Note: The Bacteria Prevention icon

(water drop with circle) is different than the hot water boost icon

(no circle). To learn about these or any other icons that appear on your thermostat screen, visit our article about what you'll see on your Nest thermostat.

so it seems no fault, I have turned off the feature to prove that is why it random fired up, but I am reasonably sure that was the cause of what I saw as a problem.

During winter there is no problem, however in summer maybe it needs an extra boost every so often to stop legionnaires? but not sure about every 48 hours? Tried to read through HSE web site, and it does talk about temperatures, however a standard (not Irish) system heat the water at the top, but not bottom of cistern, so there will always be a layer where the water is between 20°C and 50°C and no amount of running boiler or immersion heater will ensure whole tank at over 50°C so unless top of tank always above 50°C then one can never be sure.

 

[JohnW2]

During winter there is no problem, however in summer maybe it needs an extra boost every so often to stop legionnaires? but not sure about every 48 hours? Tried to read through HSE web site, and it does talk about temperatures, however a standard (not Irish) system heat the water at the top, but not bottom of cistern, so there will always be a layer where the water is between 20°C and 50°C and no amount of running boiler or immersion heater will ensure whole tank at over 50°C so unless top of tank always above 50°C then one can never be sure.

Interesting points/questions.

For a start, I'm not sure that heating just the top of a DHW cylinder is particularly 'standard', even in UK - that tends to be used as a 'boost' facility. Apart from anything else, a cylinder heated only at the top would not produce enough hot water to fill a bath. I've certainly never had a DHW cylinder which was not primarily heated low down.

Anyway, a bit of thinking aloud about the situation when it is 'top heating' ...

Water is, of course, drawn from the top of the cylinder, so is likely to be water that is, or has recently been, at a temp high enough to kill the bacteria.

Furthermore, I suspect that, even with top heating, there is probably enough circulation of water within the cylinder to bring bacteria into contact with water hot enough to kill them. In particular, although I haven't clue as to whether this is actually true, since a fairly high proportion of the structure of a bacterium consists of lipids, I suspect that they are quite probably less dense than water, and hence would tend to 'float' to the top of the tank.

... just a few thoughts!

Kind Regards, John

 

[SimonH2]

Standard is for the heat coil to be at the bottom of the DHW cylinder - probably occupying around the bottom 1/4 of the cylinder height (depending on cylinder size etc). Water heated by the coil then rises and is replaced by cooler water - there will be some circulation in the tank. Since the rising hot water will mix with the descending cooler water, it's not as simple as the cylinder heating up to boiler flow temp from the top down. Also, depending on capacity of boiler and cylinder coil, boiler flow temp could be depressed while the bottom of the cylinder is cold - getting hotter (until it reaches the boiler setpoint) as the cylinder heats up and heat transfer reduces. Leave the boiler running for long enough, and you'll get roughly uniform temperature all the way down to the bottom of the coil - with just a small space at the bottom that's still cool.
When you draw off hot water, unless you draw off enough to "empty" the tank, then the hot water will rise and be replaced by cold water in the bottom. As long as the cylinder gets re-heated before too much water is drawn off, then the water being drawn off will always be hot - whatever the boiler & controls heat the cylinder to. Thus there shouldn't be any cause for concern.

Also note that copper is naturally anti-septic which is one reason it's so widely used for cylinders and pipework for potable water. So clean cylinders and pipes won't harbour bacteria even if they aren't at 50˚C or above. Of course, if you live ina hard water area, then you may well have a layer of limescale on the insides - and that will act as a breeding ground for the bugs.

 

[ericmark]

I would guess 26 gallons or 120 litres in tank and bottom couple of inches not heated, so it says a 3kW immersion heater takes around 2.5 hours to heat the tank, so at 18-25 kW around 40 minutes. So since no option of 40 minutes has to be 30 minutes or 60 minutes, I tried once a day and water very hot, so every other day ½ hour and runs both Sunday and Monday so Monday it should heat whole tank to over 50°C however when I found it running without schedule I turned whole system off, so not sure what temperature is reached, but have noted after 20 minutes the boiler turns off, then starts to cycle, so would assume water is as hot as it's going to get?

 

[SimonH2]

I would guess 26 gallons or 120 litres in tank and bottom couple of inches not heated, so it says a 3kW immersion heater takes around 2.5 hours to heat the tank

The calculation is that 4.2kW will heat one kg (1 litre) of water through 1˚C in one second. (source: https://www.engineeringtoolbox.com/water-thermal-properties-d_162.html)
So 3kW will heat (say) 100l of water at a rate of 3.0/4.2 * 1/100 = 0.0071 ˚C/s or 0.43˚C/minute. So if it comes in at (say) 10˚C which it might do in this weather, and you heat it to 60˚C, then that's 50˚ @ 0.43˚/m, or 116 minutes - near enough 2 hours. In winter, the water will be colder, so it'll take longer.
And this is the bit that often gets argued about. If you draw off (say) 50l, then it does not take another 2 hours to reheat - it'll only take an hour. I've had people swear blind that heating the whole tank wastes heat(money) because it's heating all that water every time

But I note that the page you link to repeats the same old wives tale. IF your cylinder and pipework are well lagged, and especially if it's in a closed space such as an airing cupboard, then leaving the immersion on all the time will not waste significant energy. It will just heat the cylinder up until the thermostat turns the immersion off. No need for time clocks, countdown timers, any of that faffing around - just leave it on and it will keep the cylinder hot without significant waste. There is some merit in having two immersions and only keeping the top half of the tank hot all the time as this will reduce heat losses, but other than that, the days of "I'll put the hot water on so we can have a bath later" went out half a century ago

, so at 18-25 kW around 40 minutes. So since no option of 40 minutes has to be 30 minutes or 60 minutes, I tried once a day and water very hot, so every other day ½ hour and runs both Sunday and Monday so Monday it should heat whole tank to over 50°C however when I found it running without schedule I turned whole system off, so not sure what temperature is reached

Personally I'd consider one or both of two things here.
Firstly, put a cylinder stat on the cylinder and let that control the boiler when not using the heating. See later, but consider putting it (say) half way up so you only explicitly heat (say) half the tank - enough for regular usage when the heating is off.
Secondly, consider adding a blending valve as is normal on thermal stores. If the tank is very hot, then it'll blend in cold water to reduce the temperature to a more tolerable level. You can do this either at the tank to control all hot water, or individually at each sink - the latter will give a very slightly quicker time till hot water comes out the tap.
Again something I've had people argue against - it does not waste heat to add a blending valve. I've had people argue that thermal stores are inefficient because the first thing they do is "throw away" heat by mixing the hot water with cold to get the desired flow temperature

but have noted after 20 minutes the boiler turns off, then starts to cycle, so would assume water is as hot as it's going to get?

As per my earlier post, as the water in the cylinder warms up, it doesn't top a completely "top down" reheat. The rising hot water off the coil will mix with the descending cooler water, and so you'll have a steady temperature gradient down the cylinder. As the water round the coil warms up, the rate of heat transfer reduces - it sounds like after around 20 minutes, this reaches the output capacity of the boiler and so the boiler has to back off. As yours doesn't modulate, it can only start to cycle in order to reduce mean output power.
This doesn't mean the water is as hot as it will get - it just means that the water at the bottom of the tank is warm enough that the heat transfer is reduced to the point of the boiler cycling. The cylinder will continue heating up, with the boiler cycling at a reducing on/off ratio as it does so. If you leave it, the boiler would eventually stop cycling altogether when the temperature in the cylinder reaches the setpoint of the boiler and thermo-syphoning no longer carries heat from the boiler to the cylinder - but this would be inefficient and you would be better having the cylinder stat turn the boiler off when the water is hot enough for your needs.

Now, the "see later" bit ...
What did your plumber suggest regarding bypass valve vs modulating pump ?
My feeling is that you would be better (for your setup) to just fit a modulating pump - it'll probably cost less to buy that than the cost of time and materials for the plumber to add a bypass. This means that much of the time the heating is on, there will be "spare" capacity that goes into heating the hot water tank - so you'll end up with very hot water again (see above about blending valve). Short of adding a 3rd motorised valve to control it, this is going to be a natural situation for your setup.
So when the heating is one, you'll mostly have a full tank of very hot water - but the cylinder stat will take over when the heating is off an ensure the boiler runs just enough to keep some hot in summer.


As an aside, the house we lived in when I was "somewhat younger" had an oil boiler (no gas in the village). As built, there were no controls other than a time clock - so my dad added a wall thermostat, something of a novelty in the late 60s/early 70s. This stopped the long periods of boiler cycling that would otherwise have happened with the heat basically being controlled by the boiler stat. When the chap came to service it every year, he would comment on how clean it was inside - which was down to this reduction in cycling.

And yet, half a century later, there are still "plumbers" fitting a wall stat "because the regs require it" rather than because it makes no sense not to. At our last house, when the boiler was replaced (before I met my now wife), the plumber had fitted a wall stat next to the boiler "because, regs" - it was almost a waste of time, being in an unheated room, with single glazing, and next to a draughty back door

 

[JohnD]

In "easy go remember" terms, an immersion heats the cylinder at the rate of about 1 litre per minute. A bath contains around 100 litres, depending how fat you are. In winter you will not want to add much cold. IMO a 60C bath is too hot. Legionella do not multiply above 45C and most are dead within 2 hours at 50C. It is more likely to live in your loft tank in summer than in your cylinder.

Immersions don't usually reach the bottom of the cylinder. Depending how much hot water it already holds, around an hour and a half will heat a bathful.

 

[JohnW2]

But I note that the page you link to repeats the same old wives tale. IF your cylinder and pipework are well lagged, and especially if it's in a closed space such as an airing cupboard, then leaving the immersion on all the time will not waste significant energy. It will just heat the cylinder up until the thermostat turns the immersion off. No need for time clocks, countdown timers, any of that faffing around - just leave it on and it will keep the cylinder hot without significant waste.

I suppose it depends upon what one means by "well lagged" and "wasting significant energy". Mine is obviously a very different situation, but in terms of heat losses it is equivalent ...

As I've described before, I have a 140 litre DHW cylinder heated by E7 electricity for 7 hours per day. It is in a large 'airing cupboard' in a relatively unheated part of the house (so appreciable seasonal variations in ambient temperature). The cylinder was always fairly well insulated/lagged - initially with the built-on foam insulation it came with, supplemented by 200-300mm of thermal insulation (all around, including top and bottom). However, in mid-May 2017 I fairly dramatically increased the amount insulation, to a minimum of 500-600mm all around (including top and bottom). Give or take the inevitable day-to-day variation, our usage of DHW is pretty consistent throughout the year. The amount of energy required for the 7 hours of E7 heating (such that the cylinder is 'up to temp at the end of the 7-hour heating period) is therefore equal to the DHW usage during the preceding 24 hours plus the heat losses during the preceding 17 non-heated hours

As the graph below shows, even with the lagging (even the considerably enhanced lagging), there is considerable season variation in energy usage which mirrors the changes in mean daily temp (Met Office data), with something like 2kWh per day more being used in mid-winter as compared with mid-summer. Furthermore, as can be seen, when I increased the lagging (to a pretty unusual degree) (dotted vertical line on graph), there was a systematic reduction of 1.5 - 2 kWh in daily energy usage ...

Presented in a slightly different way, the total 'on duration' of the (thermostatically controlled) night-time heating obviously shows the same pattern ...

The bottom line I am intending to illustrate is that, even with abnormally large amounts of lagging, losses from a DHW cylinder can be quite appreciable.

Kind Regards, John

 

[JohnD]

I suppose it depends upon what one means by "well lagged"

does it have any pipes coming out of the top?

It would be interesting to see the energy the cylinder takes when you are away for a few days and there no hot taps in use (or leaks)

 

[JohnW2]

does it have any pipes coming out of the top?

Well, yes, of course, there is the 'draw off'/vent pipe coming out of the top. They are lagged (as best pipe lagging ever is!) but, except when hot water is being used, both are cold within a small number of feet from the cylinder. There is also the cold feed pipe to the bottom of the cylinder, but that is lagged for the first few feet and generally remains pretty cold.

It would be interesting to see the energy the cylinder takes when you are away for a few days and there no hot taps in use (or leaks)

Yes, I've looked at that. Just in terms of a single day, on Saturday we were out for virtually the entirety of the non-E7 period, hence used no hot water (and there are no leaks of which I am aware). During the following heating period (Sunday early hours), it used 3.330 kWh (i.e 3 kW for about 67 minutes) - but that will include a (small) bit of energy to replace the HW we used during the heating period (soon after our return) (we did not wake up and use any more HW until after the end of the heating period).

One problem, of course, is that if we go away for several days, we will not usually leave the HW heating on (even in Winter, will would only programme short bursts of heating to stop things 'freezing'!). The data I have therefore only relates to non-occupancy periods of a day or two at most - and, since I increased the cylinder insulation, the lowest daily usage I've seen on any night (when heating was enabled) is around 2.5 kWh.

Kind Regards, John

 

[ericmark]

If you leave it, the boiler would eventually stop cycling altogether when the temperature in the cylinder reaches the setpoint of the boiler and thermo-syphoning no longer carries heat from the boiler to the cylinder - but this would be inefficient and you would be better having the cylinder stat turn the boiler off when the water is hot enough for your needs.

That would only happen if the pipes from boiler to storage tank and the boiler were very well lagged, once off the boiler will cool, so in a relative short time it will switch on again even if the storage tank is full of really hot water.

There is no practical way to get wires from the storage tank to the boiler or heat link, looked for wireless but could not find one under £100 add to that the cost of the motorised valve looking at £150 for parts, so it seems two methods.
1) Use the immersion heater instead of oil.
2) Pay around £200 or more to fit better controls for oil.
My first idea was to fit better controls, however the plumber says boiler is designed to be cooled by permanent open thermo syphon and putting a motorised valve on the supply to cistern could cause over heating of boiler and damage, with newer boilers it has both switched line and permanent line supply so when switched off it can still run fans etc, and cool down, mine does not have a permanent line supply so once switched off the boiler can't purge.

So simple timed for summer hot water seems reasonable, ½ hour per day means around 20 minutes at 18-25 kW so 6 to 8.3 kWh to heat water, so would be equivalent to 2 hours or more with an immersion heater.

 

[SimonH2]

I suppose it depends upon what one means by "well lagged" and "wasting significant energy".

Clearly a very much YMMV area !
I tested the standing losses when I put the thermal store in the flat - partly because of all those "storing hot water wastes lots and lots of heat, only a combi makes sense" comments aimed at thermal stores by the "I only ever fit or recommend a combi" brigade. Test method was to shut down the boiler, turn off the valves to prevent thermo-syphoning round the heating, turn on the immersion heater and leave for a day. Read the lecky meter, leave for a couple of days, read the meter again, do energy/time to arrive at .... 80W, which is under 2kWHr/day. This was done during that prolonged cold spell (barely above freezing during the day) of Dec 2010.
By contrast, when I did the same test in the house I'd just bought, the combi was using the equivalent of about 160W of gas in standby mode This was with eco mode off, so it fires up every 20 minutes to keep the DHW heat exchanger warm to reduce the "tun on tap, waste lots of water while the boiler heats up" time. With eco mode on, it more or less eliminates the standing losses - but then could take up to a full minute to output hot water from cold

The lagging on the store was the rockwool (or similar) jacket it came pre-fitted with, about 2" of glass fibre tucked between it and the two walls, and about 3" of glass fibre in the doors I made for the enclosure. So not "amazingly large" quantities of insulation. But it did eliminate the boiler running all the time when heating called for, short cycling all the time as it's lowest power output was something like 5 times the max heating load of the flat (about 2kW during said cold spell, measured in the same "run it off the immersion and watch the meter" manner).

Definitely a case of YMMV
Obviously, having an essentially uncontrolled boiler overheating the DHW cylinder is almost certainly going to involve higher standing losses.

 

[ericmark]

When I open airing cupboard door the heat does not hit me, so although intend to fit more insulation, the standard insulation is clearly doing a reasonable job, but time from turning on tap to getting hot water is not that fast, so in the main end up washing hands in cold water as it simply has not reached tap by time I have finished. so may be I should not bother heating the water to start with? With the dish washer using cold, and washing machine uses cold, I think in spite of oil being cheaper than electric, a under sink water heater is likely going to save money, but that's £70 plus loss of cupboard space. So likely I will do nothing.

 

[JohnW2]

Clearly a very much YMMV area !

Up to a point - but if one's mileage appears to be varying unexpectedly, that's presumably due to a lack of understanding either of the laws of physics and/or of the (many) factors which need be taken into consideration.

... Test method was to shut down the boiler, turn off the valves to prevent thermo-syphoning round the heating, turn on the immersion heater and leave for a day. Read the lecky meter, leave for a couple of days, read the meter again, do energy/time to arrive at .... 80W, which is under 2kWHr/day.

That's not far off the figure I mentioned for the lowest per-day 'heat loss' figures I see in my records (which presumably relate to days on which there is little/no hot water usage).

The lagging on the store was the rockwool (or similar) jacket it came pre-fitted with, about 2" of glass fibre tucked between it and the two walls, and about 3" of glass fibre in the doors I made for the enclosure. So not "amazingly large" quantities of insulation.

Indeed. That's obviously 'peanuts' in comparison with the amount of insulation I now have.

Certainly during the 7-hour cheap E7 period during which my immersion is powered, after the initial period of heating, the thermostat only has to fire it up very occasionally, and for very brief periods, to keep it 'up to temp' ....

Kind Regards, John

 

[JohnW2]

With the dish washer using cold, and washing machine uses cold, I think in spite of oil being cheaper than electric, a under sink water heater is likely going to save money, but that's £70 plus loss of cupboard space.

I may be wrong, but I think you'll find that unless (unusually), they are shower-sized in terms of their power (- i.e. ≥7.5 kW, with all the supply and wiring issues that could invoke), most 'under-sink water heaters' are not actually "instant ('as required') heaters" but, rather maintain a reservoir of hot water below the sink - with all the heat and energy losses that entails.

Kind Regards, John

 

[ericmark]

I may be wrong, but I think you'll find that unless (unusually), they are shower-sized in terms of their power (- i.e. ≥7.5 kW, with all the supply and wiring issues that could invoke), most 'under-sink water heaters' are not actually "instant ('as required') heaters" but, rather maintain a reservoir of hot water below the sink - with all the heat and energy losses that entails.

Kind Regards, John

Yes fitted one in an old caravan, the loses were very small due to having so much insulation, had a 1 kW heater, and held 7 litres of water, never seemed to run out of hot water, and no waiting, but only supplied one sink. Special tap when in a house, it turned on cold supply to it which forced hot water out.