new boiler change or keep hot water tank

jenny2009

What would be the savings in hot water costs, not CH, if a hot water system was changed from a hot water storage tank to instantaneous hot water via a combi boiler?

I get increases costs as below.

At present hot water for 1 bath and 2 power showers and washing dishes every day uses approx 21 kWh of gas, 3 kWh of which are used by the pilot light, costs at present 65 pence per day, £237 per year.

If the system was replaced with a combi boiler and the hot water tank removed, a new shower would have to be fitted, either electric or using the combi boiler, whereas if the boiler was replaced by a like for like albeit more efficient nothing would need to be changed.

So 10.5 kWh electric shower would cost roughly £200 or more to fit and 2 10 min showers would cost £280 approx per year based on 11p per kWh of electricity.


If the combl boiler was rated at 35 kWh (no idea if this is correct} then 2 10 min showers would cost £128.

have no idea how much it would cost to heat the water for a bath by gas but would say 40 pence per bath so that is £146 per year.

so gas costs £274 per year plus the cost of a new shower

So to change from a hot water storage tank would cost over and above the boiler costs

£200 for an electric shower and increased costs of £420 as against £237 at present for hot water.


Or £200 for a shower fed from the combi boiler plus £274 per year in gas as against £237 at present for hot water.

So could someone explain to me why the plumber is so insistant that a combi boiler is cheaper to run than the present system?

Jen

Cardew

Jenny,

When calculating shower costs the only concern is the true cost per kWh of the gas or electricity to provide the hot water.

Electricity is approx 10p/kWh, gas is approx 3.5p/kWh. For gas you have to consider the efficiency of the gas boiler which at an average of 75%(they range from 60% to 90+%) makes the effective price of gas approx 4.5p/kWh

So it will never be cheaper to use an electric shower rather than heating water by gas and getting that water from a hot water tank or combi.

The savings of a Combi against a conventional Hot water tank system are much more difficult to calculate.

The main consideration is the heat loss from the HW tank. A well lagged tank will lose between 2kWh and 3kWh a day when filled with water at 65C(that is the BS to which they are tested)

As your tank is unlikely to to be full 24/7 with water at 65C, 2kWh a day would IMO be at the high end of losses. So some 730kWh per year perhaps £30-£35 a year.

However the heat lost from the tank isn't really lost as it warms the fabric of the house - which is why they are often in airing cupboards. You have in effect a small radiator giving out heat.

Another consideration is the length of the HW pipe runs to/from the boiler/tank/taps/showers.

So whilst a combi will be cheaper than a boiler/HW tank the difference is really not that high.

jenny2009

As your tank is unlikely to to be full 24/7 with water at 65C, 2kWh a day would IMO be at the high end of losses. So some 730kWh per year perhaps £30-£35 a year

I haven't a clue about that, I just know that on average over the past 3 years, when I started taking notice, I have used on average 21 kWh of gas per day for 2 10 min power showers and a bath, irrespective of whether my well insulated with a foam cover then a fibreglass blanket, hot water tank loses 2 kWh per day through a mixture of conduction, radiation or convection.

The exam question is,

would I use more or less than 21 kWh of gas, on average per day, if I replaced my hot water tank and so called inefficient boiler with a pilot light with a super combi boiler to have 2, 10 min showers and a bath daily?

Jen.

C_Mababejive

It is my view that you should buy a high efficiency system boiler and keep your storage tank. Make sure you tank and all its pipework/drawoffs is thoroughly insulated. In this way,you will have a plentiful supply of hot water whenever you need it and will not be wasting money with a combi. The stored system is better for most family homes IMHO.

jenny2009

It is amazing how putting things down in writing helps to make understanding things like this so simple.

I spend £237 per year for hot water.

If I replace what I have with a new system to save 20% on costs I will save about £47 in the first year so if I spend £470 it will take 10 years to recoup the cost, £1000 will take 20 years to recoup the cost, even if the cost of gas doubles it will still take about 15 years to recoup the cost.

If my usage of gas only decreases by the cost of the pilot light, £35 per year, we are looking at a longer term yet.

And the biggest laugh of the lot is my hot water system does not need replaced, there is nothing wrong with it, it is just supposedly inefficient because it has a pilot light and should be replaced to save the planet.


Yes that's right, spend thousands of pounds to save pennies to save the planet. I'd rather spend it on holidays.

Jen.

thills

There is common sense, and then there is government thinking. I wonder how the CO2 impact of building a boiler, transporting it, installing it etc really stacks up against the CO2 savings? Same like scrapping a perfectly good car. CO2 is the modern demon, one cant deny its any good for the planet, but it is a minnow compared to methane, which is 25x more greenhouse, which comes from the population of 9? billion eating etc. We have schemes producing bioethanol that use 1.5kw "dirty" energy to make 1 kw "clean" energy.
If it first makes financial sense to "us" do it, and it's better with CO2, great.