Boiling Water

[SimonH2]

80W and 160W seems a heck of a lot of power, same heat output or more as no light as a bulb of same wattage, if I used even a small amount of insulation then that amount of heat would soon be boiling water.

Indeed, but then think about it ...
With the combi (or at least this one, a BG branded Worcester Bosch), every 20 mins it fires up. When it fires up it first blows cold air through the main heat exchanger, then it turns on the gas burner and heats everything up, then it turns off the gas and blows cold air through the main heat exchanger for a few seconds (purging POCs), and then shuts down. The main heat exchanger is designed to transfer heat effectively, so it's going to heat the air in the combustion chamber which is able to thermo-syphon out through the air intake (so truly wasted heat). The pipework and DHW heat exchanger are not lagged at all (AIUI some modern boilers to address this to some extent) and so will lose heat readily. All in all, a recipe for wasted heat.
With the thermal store, a large cylinder - so large surface area - and all the associated plumbing. Everything lagged as well as I could, and I even built an insulated cupboard around it. Even so, 80W of leakage doesn't sound too bad. And it's not wasted heat - at least in winter.

 

[mattylad]

While I cannot tell which is more efficient a method to use, I Know that I wont be going round Securespark's house for a brew - I'd die of thirst before it arrive...lol

Just put the kettle on with a cupful of water (per cup required) in.

 

[bernardgreen]

When it fires up it first blows cold air through the main heat exchanger, then it turns on the gas burner and heats everything up, then it turns off the gas and blows cold air through the main heat exchanger for a few seconds (purging POCs), and then shuts down. The main heat exchanger is designed to transfer heat effectively, so it's going to heat the air in the combustion chamber which is able to thermo-syphon out through the air intake (so truly wasted heat).

And this happens every time there is a call for hot water from a combi. If instead a well insulated heat store or hot water cylinder is used to provide hot water then this waste of heat only occurs once per tank full.

 

[JohnW2]

And this happens every time there is a call for hot water from a combi. If instead a well insulated heat store or hot water cylinder is used to provide hot water then this waste of heat only occurs once per tank full.

That would surely only be true if the water heating system was configured in a manner that I have never heard of being used for gas water heating - i.e. if one heats the water in the cylinder/thermal store in just one continuous 'session' per day, and then uses that stored water (gradually getting cooler, due to both losses and replacement with cold water) for the rest of the day (i.e. the same as with, say, E7-electrically-heated water).

With the usual system, in which the gas water heating will be 'on' continuously, if the cylinder thermostat had zero hysteresis, the situation would be exactly the same as with a combi - i.e. the cylinder/store would call for heat from the boiler (which therefore would have to 'fire up') every time any hot water was used. Thermostat hysteresis will obviously moderate that effect, but the boiler will still fire up multiple times, since it is aiming to keep the cylinder/store up to temp (not 'heat a full tank' and then use the water until it is cold).

Kind Regards, John

 

[bernardgreen]

With my set up a single burn session per day is more than adequate. When people are visiting a top up session may be needed if they are having a few showers.

But then my boiler is a heat only unit and not a combi.

 

[JohnW2]

With my set up a single burn session per day is more than adequate. When people are visiting a top up session may be needed if they are having a few showers. But then my boiler is a heat only unit and not a combi.

Fair enough - but it sounds as if you are using a gas boiler in an E7-like fashion - which, as I said, is something I've never heard of being done with domestic gas-fuelled water heating. Maybe it's more common than I thought!

Are you sure that the heat loss from your cylinder/store over 24h is less than the 'wasted energy' that would result from using real-time water heating (as with a combi)? With my E7 setup, I have an extremely well-lagged cylinder (600-900 mm of insulation on all sides and also above and below, in addition to the insulation which the cylinder comes with) and, even with that, I usually loose around 2kWh per day (i.e. if I use little/no hot water, a ~2kWh top-up is required at the end of 24h).

Kind Regards, John

 

[bernardgreen]

Are you sure that the heat loss from your cylinder/store over 24h is less than the 'wasted energy' that would result from using real-time water heating (as with a combi)?

Without measuring it I am not sure. Maybe the heat "loss" from the cylinders ( there are two ) is greater than the loss from a combi's purging cycles but the "loss" from the cylinders is not lost as it is retained in the bathroom and the kitchen where as the loss into the atmosphere from a combi really is lost.

Reason for having two cylinders is that the route for pipes between kitchen and bathroom is very long and convoluted. With a single cylinder in one room there would be a large amount of wasted cold water before hot water reached the taps in the other room. Circulating hot water in flow and return to and from the cylinders does not waste any water,

 

[JohnW2]

Without measuring it I am not sure. Maybe the heat "loss" from the cylinders ( there are two ) is greater than the loss from a combi's purging cycles but the "loss" from the cylinders is not lost as it is retained in the bathroom and the kitchen where as the loss into the atmosphere from a combi really is lost.

As I've said a number of times, "the 'loss' is not lost..." is only really true at times of year (primarily winter, but who knows with our climate?!) when space heating is required. At other times of year (primarily summer?) it really is lost/wasted (since it is unwanted) heat/energy.

Circulating hot water in flow and return to and from the cylinders does not waste any water,

I presume you mean heat/energy, rather than water (unless you have leaks!)! As above, any injection of heat into the house at times of the year when space heating is last thing that one wants surely represents 'wasted energy', doesn't it?

The cost considerations are obviously very different for people, like me, who use E7 electricity for hot water heating. The considerable difference in peak/off-peak electricity prices more than compensates for the 'waste' (during warm times of year) associated with stored hot water. However, your gas presumably costs the same at all times of day, meaning that the 'waste' (during...) associated with stored hot water becomes much more of an issue.

Kind Regards, John

 

[ericmark]

Indeed, but then think about it ...
With the combi (or at least this one, a BG branded Worcester Bosch), every 20 mins it fires up. When it fires up it first blows cold air through the main heat exchanger, then it turns on the gas burner and heats everything up, then it turns off the gas and blows cold air through the main heat exchanger for a few seconds (purging POCs), and then shuts down. The main heat exchanger is designed to transfer heat effectively, so it's going to heat the air in the combustion chamber which is able to thermo-syphon out through the air intake (so truly wasted heat). The pipework and DHW heat exchanger are not lagged at all (AIUI some modern boilers to address this to some extent) and so will lose heat readily. All in all, a recipe for wasted heat.
With the thermal store, a large cylinder - so large surface area - and all the associated plumbing. Everything lagged as well as I could, and I even built an insulated cupboard around it. Even so, 80W of leakage doesn't sound too bad. And it's not wasted heat - at least in winter.

This has been the whole argument for not using on/off thermostats with a boiler able to modulate it's output. Whole idea is to cycle the boiler as little as possible. So either a modulating thermostat or control the water through each radiator which in turn controls boiler output.

However the problem is each boiler is different, there is no "This is the best method" and it is so hard to get information as to how much heat is lost through the flue each time the boiler stops and starts.

I had all big ideas of doing it right, however my Bosch Worcester can only it seems use their own thermostat, think called Wave, and this does not connect to eTRV heads, so you can only control one room, and the whole idea of central heating is to heat whole house, so seems rather pointless.

This seems to be repeated though out the domestic central heating systems, it seems the right hand does not know what left hand is doing.

I looked at what seems the Roles Royce of radiators, the Myson ivector it seems to tick every box, until it comes to getting it to talk to the boiler, the building management system is super expensive, clearly not designed for domestic use, but they are it seems able to connect to both heating and cooling, so for an integrated system there is no option but the fan assisted radiator.

But then the big question, why do we need a fully automated system, is it so hard to turn heating on/off?

As any gas boiler gets bigger you can extract more and more energy from the gas, when gas is burnt it both gets hot and increases in volume, so any efficient system must use both,

I have two both within two miles of where I live, both are combined power and both are far more efficient to any domestic boiler, why two just 1 mile apart not a clue, 1,420 MW and 498 MW the first the steam turbines are on the end of the gas turbine shaft, the second the steam turbine is separate. But using electric power from these to boil the kettle is far more efficient than using gas direct. However that does not mean cheaper, and it is all down to a balance, including how much we want to automate, and how much we will do for our selves.

Instead of having a super quick boiling device for my coffee, I could take the kettle into the living room, and watch TV while boiling the water. Or even have a TV in the kitchen. If I have a house full of visitors all wanting coffee, then filling the two one cup boilers with warm water would speed things up, or I could take the filter coffee maker into living room with a jug of milk, and bowl of sugar and let everyone help them selves. Bound to be some one who wants sweetener, or caffeine free or some other weird request.

So eco hot drink making, why not an insulated coffee pot, or cup, or even kettle so no heat lost, I have considered a tea cosy for pans, with an induction hob no fire risk, so no reason why a simmering pan should not have insulated sides and lid. But then we also want it dish washer proof etc.

So today's method has stood the test of time, it has not broken so why change it.

 

[JohnW2]

With the thermal store, a large cylinder - so large surface area - and all the associated plumbing. Everything lagged as well as I could, and I even built an insulated cupboard around it. Even so, 80W of leakage doesn't sound too bad. And it's not wasted heat - at least in winter.

That sounds very similiar to my experience with my (E7-heated) primary hot water cylinder. It is roughly in the middle (vertically and horizontally) of a cupboard about 1.8m x 1.8m and nearly 3m tall. The entire cupboard is stuffed full of thermal insulation. With that, as I recently wrote, my 'losses' amount to around 2 kWh per 24hours - almost identical to your 80W. As you say, that 2 kWh/day is not wasted during winter, although it is in summer - costing me a bit under 16p/day at my current off-peak E7 rate - i.e. around £28 over a '6-month summer'.

Kind Regards, John

 

[SimonH2]

And this happens every time there is a call for hot water from a combi. If instead a well insulated heat store or hot water cylinder is used to provide hot water then this waste of heat only occurs once per tank full.

It depends on the setup.
With my setup in the flat I timed it as the boiler firing (roughly) once every 20 minutes for about 90s - while heating the flat in very cold weather. By using two stats and a relay, they can be configured to run the boiler until the upper stat is hot, and not re-fire it until the lower one is cold - thus leading to fewer & longer boiler firings.
But in either case, it's going to be better than the average combi boiler driving heating directly - where the boiler (by design) is grossly mismatched to the heating load and even on minimum power output cannot avoid frequent cycling. And also, for good measure, almost certainly not condensing for much of the time.

 

[John D v2.0]

If you are having showers as you main hot water usage, a Combi set to minimum will likely be more efficient. This is because the boiler will condense much better when you are heating the dhw to 35 degrees than if you are heating it to 60. If that, then you probably won't be using the eco/comfort mode anyway.

 

[securespark]

I Know that I wont be going round Securespark's house for a brew - I'd die of thirst before it arrive...lol

We do boil just a cupful when required I chose the quantity I did as I often boil a full kettle for cooking (boiling veg, making gravy, stuffing, etc...).

 

[ban-all-sheds]

If that were the case, then you could not use mixer taps which mix at base of tap in the kitchen, only those which mix at end of faucet, since the introduction of combi boilers we have also had kitchen taps that mix water at the base of the tap.

Why, if you have no HW flowing from the boiler into the mixer?

 

[ban-all-sheds]

"If" indeed.

Why do you think tap water that's been pasteurised shouldn't be consumed?

1) It's not been pasteurised.

2) That's not the issue.