Estimating central heating costs

[gsplum]

Hi, we are about to install gas CH and would appreciate some help in estimating the running cost.

We have a gas water heater at present and so are looking at changing to a condensing combi boiler and adding radiators (under floor heating is not possible in our case)

I guess a typical timing configuration is 2hrs before we get up and 3hrs when we get back in the evening. I calculate heat loss at 20,9kw and propose a an initial additional startup load of 20% for 30mins (ie the time taken to raise the temperature of the pipes and radiators to steady state)

So maximum daily energy needs are (2+3 hrs) * (20,9 + 2,1 * 2kw) =108,8 kWh/ day. Does this seem reasonable?

The local climate is fairly mild and based on actual bottled gas convection heater use, we can estimate % of max heat loss which will occur in January:

% max Days
Oct 10% 31
Nov 50% 30
Dec 80% 31
Jan 100% 31
Feb 80% 28
Mar 10% 31

So, take the max daily energy requirements, multiply by % and number of days and I get 10785,5kWh total annual heating requirement. So far so good?

The condensing boilers we are looking at are rated as 98-109% efficient, so can we assume that on average at least 100% of the energy burnt is converted to heat transferred to the radiators? If so, does that make the price per kWh of gas the cost of annual energy requirements?

ie Annual cost = 10785,5kWh + 0,05 euros/ kWh = 585 euros + fixed costs + VAT?

thanks in advance for any input

 

[45yearsagasman]

The condensing boilers we are looking at are rated as 98-109% efficient, so can we assume that on average at least 100%

Tell us the make and model of these fabulous boilers. I want one.

 

[gsplum]

Ups. so is this where I find out I have missed something big. Here is a selection of condensing boilers that have performance claimed at 98-109%:

Ferroli Econcept Tech 25 C
Saunier Duval Isofast Condens F 35
Saunier Duval Isofast Condens F30
Vaillant ecoTEC PLUS VMW ES 346/3-5
Vaillant ecoTEC PLUS VMW ES 296/3-5
Vaillant ecoTEC VMW ES 236/3-5
Ferroli ECONCEPT TECH micro 25 C
Ferroli ECONCEPT TECH micro 35 C

Apparently they use the waste heat in the flue gases to pre-heat the cold water entering the boiler. So the water leaving the boiler appears to have gained more energy than should be the case of the gas burnt??

 

[big-all]

100% means no heat loss with the exaust being the same temperature as the air entering in other words impossible
110% means you are getting more energy out off the gas than it contains

perhaps what you have is a boiler with greater capacity than you require so the 110% means the boiler is larger than you need so has 10% spare capacity

another alternative is comparing to something else that is less efficient at converting the available heat

 

[gsplum]

OK not questioning the physics here, there is something about the way performance is being specified. For example, Vaillant in Europe specifies performance as 98-109% but the English website states an “efficiency of over 90%.”

So, it is wrong to assume 100% conversion from gas burnt to hot water above.

In order to get 10785,5kWh, I will need to burn 10785,5kWh / 0.90 = 11983,89kWh of gas. That is what we are saying, right? At 0,054 euros/kWh, the cost will be 650 euros and not 585.

Thanks- any other issue with the above estimate?

 

[gaswizzard]

Gross CV is what you should be using , 100% efficiency in real terms would be around 90% , the only accurate way of calculating gas usage would be to monitor at meter , just too many variables.

 

[muggles]

Of the boilers listed, the three Ferroli boilers, closely followed by the two Saunier Duvals, will use the least gas as they'll be the ones that spend the highest proportion of time waiting for an engineer to come round to make them work again.

 

[Agile]

OK not questioning the physics here, there is something about the way performance is being specified. For example, Vaillant in Europe specifies performance as 98-109% but the English website states an “efficiency of over 90%.”

It should be fairly obvious that you cannot get anything out that you have not put in !!!

Except in Europe!

But now look at the economy of Greece as the prime bad example in Europe. They expect to work for 20 years and then retire on a pension for 40 years!

Some things like boilers just dont work like that!

You put in gas and get about 90% of the available energy out as heat.

How big is your castello? 20 kW is a lot of energy !

Tonysito

 

[gsplum]

How big is your castello? 20 kW is a lot of energy !

Tonysito

My Englishman's castillo isnt that big, in fact that is actually one of the reasons I am in the forum now. One quote actually suggested I buy a 32kw boiler.. that seemed a lot for two showers.

 

[gsplum]

Gross CV is what you should be using , 100% efficiency in real terms would be around 90% , the only accurate way of calculating gas usage would be to monitor at meter , just too many variables.

thanks- the problem is we need to compare the costs of gas system with "biomass" (a boiler that uses wood pellets or olive stones..) to choose..

 

[Agile]

Dont expect a 32 kW boiler to run two proper showers at the same time!

If you want that you need a stored water cylinder!

A property with a heat loss of 20 kW would be very large. A well insulated English semi uses about 8-10 kW of heating.

Is yours a six + bedroom detached ?

In Spain many people get most of their hot water from a very simple solar panel and tank on the roof!

Tonysita

 

[swbjackson]

How big is your castello? 20 kW is a lot of energy !

Tonysito

My Englishman's castillo isnt that big, in fact that is actually one of the reasons I am in the forum now. One quote actually suggested I buy a 32kw boiler.. that seemed a lot for two showers.

Most combination boilers have an output to heating of 24Kw, the larger output is purely for hot water. As Tony said 32Kw will struggle to run two showers at the same time. I've got a 40Kw combi and it will just about cope with SWMBO running the hot tap in the kitchen whilst I stand under the shower.

If I was in spain I'd be looking at going solar for the hot water.

 

[gsplum]

Dont expect a 32 kW boiler to run two proper showers at the same time!

If you want that you need a stored water cylinder!

A property with a heat loss of 20 kW would be very large. A well insulated English semi uses about 8-10 kW of heating.

Is yours a six + bedroom detached ?

In Spain many people get most of their hot water from a very simple solar panel and tank on the roof!

Tonysita

hi tony, we have 180m2 with about 565m3 of air to heat. The heatloss calcs give between 16,4 and 20,9kw depending in the wall and roof material assumptions. The roof is shared and cannot be insulated easily. The other issue is marble floors..

The combi boiler we are considering has a micro accumulator and supplies max 12L/min at delta 35 deg.

A 300L solar tank will pre-heat the hot water, the boiler will topup as required (sun shines a lot here but there are periods of weak UV in winter ). Panels will be Vaillant- auroSTEP or Junkers (Bosch) F2/TS 300/FKB. Local mains is around 12deg, and I expect that to raise to a min of 25-30 deg via the solar storage- a reference site was at 45 deg yesterday.

So apart from efficiency error (90% not 100%) and potentially the max heat loss for the building, does the rest of the calc look reasonable?

 

[gsplum]

Most combination boilers have an output to heating of 24Kw, the larger output is purely for hot water. As Tony said 32Kw will struggle to run two showers at the same time. I've got a 40Kw combi and it will just about cope with SWMBO running the hot tap in the kitchen whilst I stand under the shower.

If I was in spain I'd be looking at going solar for the hot water.

which combi do you have?
The spec I haev here is for 24kw max for heating (80/60) and 30KW max for hot water.

 

[swbjackson]

I've got a Worcester 40CDi. It's now been superseded by the 42Cdi.