Central heating control, any errors?

[JohnW2]

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.

You rather surprise me, particularly given the fact that the amount of insulation possible in something that will fit under a sink is presumably pretty limited.

Does not keeping even just 7 litres of water at 60°+ for 24/7 use/waste an 'appreciable' (obviously a 'personal' definition!) amount of energy?

Kind Regards, John

 

[SimonH2]

Where I used to work they had 9kW instant heaters for the hot taps - truly horrible things, and I'd go as far as saying dangerous. Downsides of a combi - wait from cold for hot water. But variable temperature, and in particular you coukd get a slug of scalding hot water at turn on if it's been recently used - follwed by cold until the over-temp reset and it warmed up again
The storage type are much better. But as said, take up space under the sink.

 

[JohnW2]

Where I used to work they had 9kW instant heaters for the hot taps - truly horrible things, and I'd go as far as saying dangerous. Downsides of a combi - wait from cold for hot water. But variable temperature, and in particular you coukd get a slug of scalding hot water at turn on if it's been recently used - follwed by cold until the over-temp reset and it warmed up again

Ah - that brings back some (un)happy memories ....

.... many many moons ago, there was a hospital Intensive Care Unit which (far from uniquely) developed the problem that nasty (very nasty in an ITU) bacteria (Pseudomonas) were colonising the wastes and traps of the many hand-wash basins. They therefore installed metal traps and waste pipes, with built-in heating elements. Every so often, and without any warning, the water in the trap would be boiled to sterilise them - resulting in, at best, clouds of steam suddenly appearing but, at worst, geyser-like bursts of boiling hot water shooting vertically out of the basin and towards the ceiling!

The storage type are much better. But as said, take up space under the sink.

Indeed - but, as I have said, is there not an energy issue as well? If I suggested that a litre or two of water should be kept, 24/7, at, or close to, the temp needed for making tea or coffee, I'm sure that someone would have something to say about 'energy wastage', so is keeping 7 litres of water at, or near to, 'hand washing temp' for 24/7 much different, given how little 'true use' the hot water would probably get in any 24h period?

Kind Regards, John

 

[SimonH2]

Eventually you have to admit that it's all a compromise. Keeping (say) 7l of water hot under the sink could well be better than a central cylinder or instantaneous heater - have to consider not only energy usage (including losses from the hot pipes between uses) but also the effect of running water to waste. The wasted water could include the initial very very hot water from some instant heaters so wasting heat as well as water.

 

[JohnW2]

Eventually you have to admit that it's all a compromise. Keeping (say) 7l of water hot under the sink could well be better than a central cylinder ....

If such were alternatives (for the desired functionality), then keeping 7 litres hot is obviously preferable to keeping, say, 120 or 140 litres hot. It really all depends what one wants hot water for, and hence how much. If one has (cold fill) DW & DWM, an electrically-heated shower, and does not take baths, then the hot water may well be needed for little more than hand/face washing etc., so to keep 120/140/whatever litres hot 24/7 presumably would presumably make little sense.

Here, we virtuallyt never have baths (although visitors might once in a blue moon), but we do need the stored hot water for showers - so, without some major conceptual changes, could not do without a 'large' HW cylinder.

or instantaneous heater - have to consider not only energy usage (including losses from the hot pipes between uses) but also the effect of running water to waste. The wasted water could include the initial very very hot water from some instant heaters so wasting heat as well as water.

There are obviously many factors to be considered but, in the above scenario (primarily hand/face washing), I would doubt that keeping, say, 7 litres of water hot 24/7 would 'win' (in energy terms) over some sort of instant heater - although, as always, I might be wrong!

Kind Regards, John

 

[SimonH2]

There are obviously many factors to be considered but, in the above scenario (primarily hand/face washing), I would doubt that keeping, say, 7 litres of water hot 24/7 would 'win' (in energy terms) over some sort of instant heater - although, as always, I might be wrong!

I'd be inclined to agree if looking only at energy. But as I said, you have to consider other issues - water wastage, including dumping "too hot" water in some circumstances. So looking at all factors - a small stored water heater may be the "best" compromise, for whatever someone's definition of "best" is based on their preferences and requirements.
Clearly if minimising energy usage is the only criteria then only using cold water would be the "best" solution. But for most of us, that's not an acceptable compromise Each has their own priorities, so "best" will be different for different people.

 

[JohnW2]

I'd be inclined to agree if looking only at energy. But as I said, you have to consider other issues - water wastage, including dumping "too hot" water in some circumstances. So looking at all factors - a small stored water heater may be the "best" compromise, for whatever someone's definition of "best" is based on their preferences and requirements.

Possibly - although if it really were the best compromise, it suggests that the instant water heater one is considering is probably, itself, 'sub-optimal' (and therefore could be improved upon). In other words, I would suggest that an 'optimal' instant heater would probably win over any system utilising stored hot water.

Although you have mentioned one possible downside of a ('sub-optimal') instant heater, I'm struggling a bit to think of any others (except, perhaps, initial cost - although I'm far from sure about even that).

Kind Regards, John

 

[SimonH2]

... it suggests that the instant water heater one is considering is probably, itself, 'sub-optimal' (and therefore could be improved upon)

Without altering the laws of physics, I'm not sure it could.
Basic design: Pipe through which water flows, also containing a heating element. Element controlled by flow switch(es) and thermostat(s). User turns on tap, water flows, flow switch activates, and heating element turns on.
From cold, there is a finite time as element heats up and water gets hot - determined largely by the design of the heating element. The one's I've seen the insides of have the common resistance wire down the middle of a mineral insulated tube design, which has a very definite thermal mass and resistance. I have also seen on here references to a bare resistance wire directly in contact with the water - so minimal thermal mass and resistance, but "some safety concerns"
When the user is done, tap is turned off, water flow stops, element is turned off. The bare wire in direct contact with the water is probably OK, but the mineral insulated and metal sheathed element certainly is not - there's going to be a fair amount of heat still in the element assembly. So with the latter, the heat transfers to the small quantity of water, and hence the water gets very hot - hot enough to trigger the over-temp stat as well as being dangerous for users.
Assuming that the bare wire in contact with the water is not acceptable, then I'm not aware of any magic heating element which has high output, very low thermal mass, and very low thermal resistance - without which you will always have these two problems. And of course, there is a trade-off between water volume - lower volume=faster heat up, but also worse overheating at the end; larger volume=slower heat up; but less overheating at the end.

But I think we've gone off on something of a tangent

 

[ericmark]

The unit I had used a plastic tank in polystyrene and used very little power on stand-by. The unit had no pressure, water in forced water out, on the caravan today the water heated under the seat does not make the area hot so must be very well insulated, a standard cylinder cost around £180 but to get a well insulated one, the price jumps to around £420 and it seems the whole idea of the hot coil is reversed, the heat exchanger is much more efficient in the modern one, my brother in law had a house where water heated by wood burner, solar, and boiler was stored in these well insulated tanks and used to power central heating, yes it worked, but the installation cost was silly, and only really possible when installed as house is built, both because of room it needed and weight.

So only real thing we can do is add insulation to existing cistern.

 

[SimonH2]

That sounds a bit like you are describing a thermal store, a different beast altogether - and usually quite expensive for what they are (says I currently pricing one up and having to choose from a very small selection due to size contraints)

As an aside, I had considered whether one of those instantaneous water heaters might work as an "electric boiler" for a larger heat input into a thermal store than a 3kW immersion. Considering how little there is in them, electric boilers are ridiculously expensive.

 

[ericmark]

Are you talking about the Wilis system? this

fit at the side of the main cistern, can also be used with solar

it seems common in Ulster but never really caught on in mainland UK. The idea is it heats from top down so minutes after switch on you get piping hot water, and the longer it is left to run the more of the tank that is heated.

 

[SimonH2]

No, but I've heard of it (or equivalents). I was talking about the sort of instantaneous heater where you plumb the mains water to one end and the other end to the hot tap. The ones they had in the communal spaces at my last place were 9kW - so I had wondered what downside there mught be to adding a pump and using one to heat a thermal store (as backup) where 3kW wasn't enough to run CH as well as DHW.

This is the sort of thing. If it's the one I think it is, there's a foot or two of about 15mm copper pipe coiled inside with the heater inside the pipe. Very little water content.

 

[JohnW2]

Without altering the laws of physics, I'm not sure it could. ...

I think you may be over-playing the magnitude of the problem. Whilst everything you say is theoretically true, it's exactly the same as an electric shower, and one doesn't usually get scolded when one turns the flow on, even if it has only very recently been turned off.

Kind Regards, John

 

[JohnW2]

As an aside, I had considered whether one of those instantaneous water heaters might work as an "electric boiler" for a larger heat input into a thermal store than a 3kW immersion. ...

... indeed, or perhaps an electric shower. The issue presumably is that neither under-sink instantaneous heaters nor shower are designed/intended for service as a 'boiler' (rather, for only very occasional brief periods of operation), so I'm not sure how well they would fare in such service

Kind Regards, John

 

[ericmark]

In my case the immersion heater could not be used to heat house, as there is no pump on the hot coil it is thermo syphon, so there is no way to get water to circulate. However where there is a pumped supply to the hot coil, then I suppose the coil can cool the store just as easy as heat the store, it would be rather wasteful as it would be sending the hot water through the boiler, but if the boiler failed it would likely heat a radiator or two.

As two selecting radiators so only some rooms heated, not sure how it will work, be it from a immersion heater or main boiler, my previous houses were not well insulated room to room, they were good at stopping heat leaving the house, but it was noted you could turn bedroom radiators completely off, but with down stairs set to 20°C it was hard to get a bedroom below 17°C of course if the room below was not heated then yes bedroom could get very cold.

So today the programmable TRV is dropping in price, I have bought 5 programmable blue tooth heads for £15 each, so unused rooms will not be heated, already have 4 programmable wifi units which follow Nest, but these were a lot more expensive at around £40 each, with the existing as they close also boiler will switch off as everything linked, but where the TRV turns off and boiler is still running what happens to the heat? With modulating boiler OK, but simple off/on it will cycle, is this really better than the old throttling back of the lock shield valve?

I wonder if these energy saving organisations really know? or do the just thing if you have more control you must save energy without testing? We have seen so many daft statements, energy saving bulb for example, how much energy do they save, it will be dependent on how often the central heating has to run with them and without them, I don't use lights much in the summer, likely the lights are used when no central heating running for 10% per annum, rest of time any heat will not be wasted, OK not having to change bulbs every week, and fact that gas and oil is cheaper than electric I have fitted LED bulbs throughout the house, however all the energy saving organisations seem to use them in isolation, which in the home is rare. OK may have some out side lights, but most are inside.

So well look at one area where they have clearly got it wrong, and one asks can I believe anything they say? Some times one can use simple logic, if using a bus saves so much energy compared with private car, why are they so expensive? Trains must use more energy than buses as they cost more. OK there are exceptions, I pay £42 for a three year ticket on local train, likely average use it once a week, so rather cheap for 8 mile (12 mile by road) trip, however as to how much coal that train burns, not sure it's better for the environment, but I love travelling on a steam train.

Some times it not down to economy, it's simply what one likes, and in the main returning to central heating it's more about ensuring rooms not too hot for comfort than what it costs, and having hot water at turn of tap, and not having to wait for hot water to arrive.