Best way to heat a bath with electricity

[JohnW2]

.... then the water can also be generated with less electricity at an off peak time

Sorry, but I couldn't help observing that ....

I'm not sure that it's very easy to "generate water" with any amount of electricity, at any time

Kind Regards, John

 

[SimonH2]

Agree; swap them out for heat pump driven well insulated tanks then the hot water can also be generated with less electricity at an off peak time

If only. Personally I think it could be "interesting" in the next few years to see if legionella becomes a big thing. AIUI anything under about 45˚C is fertile breeding ground, above 50˚C will kill it over time, and over something like 55˚C is needed to be sure there's none left. Just out of curiosity, I looked at the specs for a (air-water) heat pump currently on the UK market for domestic use, from a well known manufacturer, and the maximum it could be programmed to for heating a DHW tank was 55˚C - note that's the flow through the HE coil in the tank, not the actual tank water temperature. And I'm fairly certain that it could not reach this without an unusually warm outside air temp, or using the direct electric heater.
The fact that the systems have disinfecting routines (e.g. weekly heat the DHW tank up to some higher temperature) probably tells you all you need to know.

I also noted that the seasonal efficiency rating was only about 1.8 - very much not the "around 3 for a rule of thumb" figures often quoted (including my myself in the past). That would be taking into account the reduction in efficiency when the outside air is cold (i.e. when the heating is most needed). What I couldn't find, and I have seen in the past when looking into A/C systems for work, were any tables/charts showing performance under various operating conditions.
So it's easy to see where the stories along the lines of "I had a heat pump installed because it was supposed to be better and my billed rocketed" come from. Hook one of these heat pumps up to an existing heating system needing (say) 50˚C in cold weather and it's going to be using it's direct electric booster heater a lot - i.e. direct electric heating.

 

[ericmark]

Interesting what @SimonH2 says, I do worry about my own DHW, it is heated by setting the time the boiler runs for, in winter with C Plan it is very hot, but summer it can be just nice at the taps, which likely means too cool to kill legionnaires, but also too cool to fill a bath, the bath uses more than is in the tank, and if the water is around the 75ºC when the bath is run, we clearly need to cool it with some cold, so can still get a full bath. But if only at 40ºC then it would not fill the bath, and the whole point it seems of this thread is to fill a bath, not run a shower, and to do it on the cheap, so a heat pump was a non starter.

To run an EV car, fit solar panels, install heat stores, and all the other ideas floated around may help reduce the use of fossil fuels, but they are not economic, the interest on the money either loss or paid be it by the individual or government, there is no such thing as free fitting, some one pays, means they don't save money. But what seems is forgotten is the on cost, be is a larger hot water tank, reinforcing the beams to carry the hot water tank, the cost of repairing the roof, or reinforcing the roof, even swapping type of RCD protection, all well and good if new build, but retro fit all depends what else needs changing.

I am considering my self a timer for the immersion heater, likely cheaper and easier option to fitting a thermostat connected to boiler controls, if set to switch on just after the boiler has run, once a week then it can boost the tank temperature once a week to remove the legionnaires problem, and likely it would not need to run for long, but likely the wifi option would be better, just more expensive to install.

OK I can afford it, but this guy has already said at the start heat pump is out, I note the Welsh government has a scheme for fitting heat pumps, but they don't pay the extra fuel bills when they don't work, which they are not likely to work, as they are replacing a boiler that heated water to 75ºC with a heat pump at 45ºC so it also needs new pipe work and larger output radiators which unless all walls covered with radiators means using fan assisted types, which really need to be plumbed in series not parallel as output controlled by fan speed not how fast the water travels through them, they do not restrict water flow in any way. So to work it is a complete rip out of old and fit new, and it can't be reverted to old type boiler very easy as a completely different system.

 

[robinbanks]

And I'm fairly certain that it could not reach this without an unusually warm outside air temp

My ASHP does 60 degrees even in sub zero temps; it takes longer but outside air temp doesn't impact the final tank temp.

You're looking for coefficient of performance charts I think; most ASHP are given a rating of how much heat they produce for the electrical energy they consume and the typical test conditions are to have the pump produce 35 degree water at an ambient temp of 7 degrees. At these a typical efficiency is at least 3x, which is on par cost-wise with a gas boiler

These "I switched from a gas boiler to an ashp and my electricity bill rocketed" stories are nearly universally caused by completely unsuitable properties. An ASHP is not a drop-in replacement for a gas boiler and the vast majority of properties need significant upgrade before an ASHP makes sense as a heat source

 

[ericmark]

An ASHP is not a drop-in replacement for a gas boiler and the vast majority of properties

That is what I was trying to say, the problem is people see the advert for a free heat pump, and go for it, but the re-plumbing to use a heat pump it seems is not included, the installer does what he is told to do by the government, as the government is the client, not what the home owner wants.

 

[JohnW2]

Personally I think it could be "interesting" in the next few years to see if legionella becomes a big thing. AIUI anything under about 45˚C is fertile breeding ground, above 50˚C will kill it over time, and over something like 55˚C is needed to be sure there's none left.

I personally doubt it and, if it were to become 'a big thing', it would probably be much wider than just Legionella, since many/most bacteria have similar ';environmental' requirements (for survival and/or proliferation).

One thing I've never really understood is how Legionella (or any other bacteria) can seemingly manage to thrive and breed in water which contains minimal nutrients.

Just out of curiosity, I looked at the specs for a (air-water) heat pump currently on the UK market for domestic use, from a well known manufacturer, and the maximum it could be programmed to for heating a DHW tank was 55˚C - note that's the flow through the HE coil in the tank, not the actual tank water temperature. .... I also noted that the seasonal efficiency rating was only about 1.8 - very much not the "around 3 for a rule of thumb" figures often quoted (including my myself in the past). That would be taking into account the reduction in efficiency when the outside air is cold

I would have thought that there would be no theoretical limit to what temperature could be achieved, regardless of outside temp. Is it not the case that, given an appropriate choice of 'refrigerant'; and pressures, one could theoretically achieve any temp, provided only that the outside temp was above absolute zero? What I don't know is what happens to efficiency when the outside temp gets low, and that may possibly be 'the crunch'.

Kind Regards, John

 

[flameport]

Legionella is rarely a problem in domestic hot water, as it's not just about temperature - to grow it also needs static or recirculated water, neither of which is likely.
Heating to what would be considered 'low' temperatures such as 50 or 55 rather than 60+ doesn't make much difference if the water is being used regularly as the cylinder is constantly being refilled with clean water.

 

[JohnW2]

Legionella is rarely a problem in domestic hot water, as it's not just about temperature - to grow it also needs static or recirculated water, neither of which is likely.

Indeed - and, as I added, it (or any other pathogen) also needs nutrients, and I'm not sure where they are coming from in ('clean') domestic water.

Kind Regards, John

 

[SimonH2]

My ASHP does 60 degrees even in sub zero temps; it takes longer but outside air temp doesn't impact the final tank temp.

And have you looked into how much lecky it uses when it's sub-zero outside ?
As mentioned, these heat pump systems tend to fall off in performance and at some point will fall back on an in-line immersion heater.

You're looking for coefficient of performance charts I think; most ASHP are given a rating of how much heat they produce for the electrical energy they consume and the typical test conditions are to have the pump produce 35 degree water at an ambient temp of 7 degrees. At these a typical efficiency is at least 3x, which is on par cost-wise with a gas boiler

Yes, those seem to ring a bell. But in most retrofit environments you aren't going to any much heat of the rads with a 35˚C flow. Crank it up to (say) 45˚C and the CoP drops off significantly. As I mentioned above, for the model I looked at the seasonable rating was around 1.85 - a long way from 3, and a lot more lecky used than you might think.

These "I switched from a gas boiler to an ashp and my electricity bill rocketed" stories are nearly universally caused by completely unsuitable properties. An ASHP is not a drop-in replacement for a gas boiler and the vast majority of properties need significant upgrade before an ASHP makes sense as a heat source

Much as already suggested. Except they are being pushed (in many cases) as simple replacements - before the installer rides off on his horse.

One thing I've never really understood is how Legionella (or any other bacteria) can seemingly manage to thrive and breed in water which contains minimal nutrients.

Presumably they manage to absorb these minimal nutrients.

We did have a legionella outbreak near here some years ago - in that case, "cooling tower" A/C system and contaminated droplets affected members of the public walking past the building. Several people died, and as you might expect, a scapegoat got thrown under the bus while the guilty parties shrugged their shoulders. The maintenance guy where I worked at the time told me he never had problems getting orders signed for quarterly tests/dosing of our process systems - he'd just reference the very public case a few years earlier if there was any reluctance to pay and the pen came out very quickly.

 

[robinbanks]

And have you looked into how much lecky it uses when it's sub-zero outside ?
As mentioned, these heat pump systems tend to fall off in performance and at some point will fall back on an in-line immersion heater.

Don't need to; I know that it doesn't use an inline resistive heater, because it hasn't got one, and it isn't in control of the tank immersion. COP is lower (note: I never said it wasn't) but it's still higher than an immersion (COP 1)

in most retrofit environments you aren't going to any much heat of the rads with a 35˚C flow

True, but if the installer hasn't upsized the rads to be useful with a low flow temp then it's probably because his horse couldn't carry them

for the model I looked at the seasonable rating was around 1.85

Hmm, it seems one ought to be making improvements elsewhere so that comes up; the reasonable payback period for the (imho excessive) cost of the pump over simple resistive heating at a sub-2 COP is lengthy

and a lot more lecky used than you might think

I know the equation/have no issue with the maths side of it, so I can appreciate ahead of time the power use.. But in a nutshell, my ASHP does OK; it was cheap to buy, qualified for the RHI (though not at the best rate but I made up for that in other ways) and adequately heats the house without being flogged. It runs for about 250 hours a year in CH mode producing 26 degree water so hopefully has a fairly easy time of it. Service life wise I'm expecting better longevity than a combi

 

[JohnW2]

Presumably they manage to absorb these minimal nutrients.

They presumably must do. However, the outbreaks of Legionella one hears about are not in domestic premises(where the water is pretty clean), so maybe they generally happen in situations where the water is much more contaminated with what some bugs may be able to utilise as 'nutrients'.

There has to be something, since one has the Laws of Physics, as well as biology to satisfy. Bacteria have mass, and when there proliferate by the multi-millions, the total mass is presumably not insignificant, must have 'come from somewhere,which cannot possibly have been just from H2O - quite apart from the fact that they also need all sorts of atoms which do not exist in 'pure water'.

We did have a legionella outbreak near here some years ago - in that case, "cooling tower" A/C system ...

As I understand it, a high proportion of substantial Legionella outbreaks have been related to 'cooling towers' of one sort or another. Although one quite often sees concern expressed about 'the risk' in domestic environments (as per your comments), I have no idea as to how common actual 'incidents' in domestic environments are.

Kind Regards, John

 

[SimonH2]

Tap water is far from pure - I recall reading/hearing some time ago that excessively pure tap water causes complaints (to the water companies) about it's poor taste.

The two risk factors with legionella are standing water at a healthy breeding temperature, and a process that creates airborne droplets. The airborne droplets get breathed in and so find an entry route via respiratory tract.

Clearly a cooling tower system meets both of these with lukewarm water dropping through the slats creating a lot of droplets which are carried upwards in the airflow.

In a domestic setting, header tanks in warm attics - particularly if the property is empty - are a prime suspect. Then add a shower to create plenty of droplets ... Normally, the DHW cylinder is kept above 50°C so not a problem. I can't help thinking a lot will be kept less hot with heat pumps which then intruduces a risk.
As suggested, in "clean" water, growth rate is slow, hence not generally a problem where systems are in regular use.

Also, domestic systems are generally copper which has natural antiseptic properties. I gather stainless steel cylinders are commonly used with heat pump installations, and SS doesn't have such antiseptic qualities.

 

[JohnW2]

Tap water is far from pure - I recall reading/hearing some time ago that excessively pure tap water causes complaints (to the water companies) about it's poor taste.

I'm sure that's true. However, as far as I am aware, most of the 'impurities' are minerals, whereas most living organisms (maybe Legionella is an exception {as, apparently, are some very deep-see a creatures} I'll try to discover) need organic nutrients, which I imagine are pretty lacking in most tap water (but, again, I may be wrong).

Whatever, as I said, I imagine that there must be something 'special' about Legionella (unless it's all 'selective media hype'!) (maybe per above) since otherwise the same problem would exist with a wide range of pathogens.

The two risk factors with legionella are standing water at a healthy breeding temperature, and a process that creates airborne droplets. The airborne droplets get breathed in and so find an entry route via respiratory tract. Clearly a cooling tower system meets both of these with lukewarm water dropping through the slats creating a lot of droplets which are carried upwards in the airflow.

Indeed so. As I said, the majority of well-publicised outbreaks seem to be related to cooling towers.

.... In a domestic setting, header tanks in warm attics - particularly if the property is empty - are a prime suspect. Then add a shower to create plenty of droplets ... Normally, the DHW cylinder is kept above 50°C so not a problem. I can't help thinking a lot will be kept less hot with heat pumps which then intruduces a risk.
As suggested, in "clean" water, growth rate is slow, hence not generally a problem where systems are in regular use.

Who knows? We certainly hear very little about domestic Legionella issues at the moment, we don't know whether your suspicions that DHW temps will fall will prove to be correct and we don't know whether, in an 'occupied' domestic environment it would result in a problem, anyway

Also, domestic systems are generally copper which has natural antiseptic properties. I gather stainless steel cylinders are commonly used with heat pump installations, and SS doesn't have such antiseptic qualities.

Very true. Copper is very toxic to micro-organisms (and also higher forms of life, like humans, at adequate levels of exposure). Given your 'predictions of doom', I'm a little surprised that you have not noted that copper pipework is gradually being replaced by plastic!

Kind Regards, John

 

[flameport]

In a domestic setting, header tanks in warm attics

Such things should have gone away a long time ago, and any new system with a heat pump would use an unvented cylinder.

 

[robinbanks]

I can't help thinking a lot will be kept less hot with heat pumps

The pump DHW controllers have built in control for this; once a fortnight both the ASHP I have take the tank to 60 degrees