[vwlowen]

@DebF_EONNext The domestic hot water side always seems to be conveniently forgotten about when discussing heat pumps. Personally, I don't want to go back to a separate hot water cylinder and power hungry immersion heaater. (most modern homes don't have space for one anyway..

[retrotecchie]

@wizzo227

2kWh of gas at 100% efficient transfer of energy to the water would heat 50 litres of water from inlet mains temperature to approximately 34°C.

Efficiency of water heating in a combi is no better than 86% at best. Either you like a cold shallow bath or you are using more than 2kWh of gas.

[retrotecchie]

Originally Posted by vwlowen

@DebF_EONNext The domestic hot water side always seems to be conveniently forgotten about when discussing heat pumps. Personally, I don't want to go back to a separate hot water cylinder and power hungry immersion heaater. (most modern homes don't have space for one anyway..

The pressurised high efficiency thermal store for a heat pump DHW system is a lot bigger and more complex than a conventional water cylinder.. It's a hell of a lot more expensive too!

[meldrewreborn]

@wizzo227

I can't provide figures for the consumption in heating bath water because I still have a hot water tank, heated by my solar thermal in summer (joy of joys) and gas system boiler in winter - but not separately measurable.

The problem with assessing heat pumps in the domestic situation is that the properties are not homogeneous, so how they perform in practice is always debateable. Strangely we don't seem to have the same problem with a gas boiler. Why so? I think its because the boiler is the standard option and the heat pump the new unknown kid on the block. The cost of a heat pump is considerable and the internal changes (like larger radiators) quite different for each property.

At current prices EDF put my gas consumption (heating, water, oven hob and flu less gas fire) at £1100 per annum. A new boiler fitted would set me back £2,500 to £3,000. A Heat Pump system perhaps £15,000. Unless the heat pump efficiency can off set the difference in fuel price (roughly gas is one quarter the unit price of electricity) It would cost me more to run than my current arrangement. And if I did need to replace my boiler I could run it for more than 10 years before I had expended what the Heat Pump Capital Cost would have been.

I fully expect Government (any, with any pretentions of hitting our Net Zero commitment) to tax carbon based fuels more heavily in a gradual fashion in the future to encourage (!) the switch to supposedly clean (but currently expensive) electricity. I'll sit on my hands for the moment - it'll keep them warm at least!!

[wizzo227]

Originally Posted by retrotecchie

@wizzo227

2kWh of gas at 100% efficient transfer of energy to the water would heat 50 litres of water from inlet mains temperature to approximately 34°C.

Efficiency of water heating in a combi is no better than 86% at best. Either you like a cold shallow bath or you are using more than 2kWh of gas.

Turns out that I use that boiler for longer than I'd thought; 12-15 minutes using 3 to 4 kWh of gas so 2.5 to 3-ish kWh of heat. What I think happened is that I was getting and using prewarming from burning an extra kWh of gas before running the bath; there are all sorts of laggy timescales to do with the weight of stuff which needs warming up in that boiler before running a bath. Anyway, you were right and I was wrong in overstating my accomplishment. As 5kWh ought to boil 50 litres of water from tap temperature, if that works exactly like my small kettle, the heat losses to the walls in the long pipe run must be a lot because bath temperature is less than halfway to boiling.

[retrotecchie]

@wizzo227

50 litres of water from tap (approx 10°C) to boiling is 90°C change x 50kg x 4200 Joules of energy, or 18.9MJ. That is equal to 5.25kWh but at an efficiency of 86% you would need nearer 6.1kWh of gas, not allowing for temperature losses in the pipe. So nearer 7kWh, most likely.

So if you heated your bathwater to 55°C, that's half the energy. Call it 3.5kWh, or about 31p worth of gas.

I don't have a bath, but assuming I used my oil boiler to fill a bath it would cost around 20p.

A six minute shower with my 8kW electric shower uses 800 Wh or around 22p, in theory, but it's a modulating shower so actually uses a bit less.

[wizzo227]

Our figures seem to be converging. 5.25 kWh of delivered heat to get from 10C to 100C is only a little bit above what I recall my little kettle using to get a measured litre of water plus the kettle housing plus loss to surroundings in six minutes and twenty seconds from a start temperature in September of 15C. More than 2kWh to get my bath sufficiently filled I agree with you on, and yes in September I had a miserly shallow hot bath.

I really don't like using pennies as a measure because some prices changed last week and will change again every three months, and I get gas at a different price to EoN. If we disagree over kWh, at least the argument is testable. I'm also fine with counting MegaJoules (MJ), where 3.6MJ=1.0kWh.

1 Joule = 1 Amp * 1 Volt * 1 second.

kWh seem more usual in energy billing. My best effort, only at lunchtime and only on one of the sunniest days of the year, has been to have a kettle bath using only rooftop solar power which did not cost anything and did not cause any carbon dioxide emissions.

[retrotecchie]

@wizzo227

Of course, if the only thing you are using the gas combi for is to heat your bath water, you have to add your standing charges on to the cost of that bath.

As I use electricity every day, my electric standing charges amortised over a day's consumption only adds a penny or two to the actual cost of my electric shower. If you only use gas to run a bath twice a week, you need to add around 3.5 days worth of standing charge on to the cost of each bath. With a standing charge around the 30p a day mark, that makes each bath cost about £1.35.

[meldrewreborn]

@retrotecchie

I think its only worth factoring the standing charge if one option is to eliminate it completely. I would regard the standing charge as a sunk cost, forgettable in these calculations. In my case getting a heat pump wouldn't mean replacing my other gas appliances so the standing charge is OK to be ignored. I fully agree that I should factor in the relative inefficiency of my old boiler, but even allowing for that getting a heat pump system would make no financial sense to me at the moment. In the future............

Perhaps an economic appraisal over a 30 to 40 year period would be required. But so many assumptions on the future price of fuels would effectively reduce it to academic guesswork.

Remember that the case for the introduction of smart meters was supported by such an appraisal. Reality has dealt its hand it frankly the outcomes predicted are rather worthless.

[retrotecchie]

@meldrewreborn

If you have gas cooking, or a gas fire, you can sort of forget the standing charge, or at least amortise it over your total gas consumption. But if the only use for gas is a combi to fire up the bath once or twice a week, it's very relevant and the biggest component of the cost.

Effective tariff rate might well be a better measure for determining cost of energy and I'm tempted to add an appropriate column to my spreadsheet.

ETR = (Total unit cost + standing charge)/total units. If you only use a couple of kWh a day, the effective tariff rate will be more per unit than if you used 10kWh per day. Got me thinking, that has...