Combi boiler - condensing - or not?

[Doctor Drivel]

Chav, you really are a dumbo. Here is the GasTec report:

[Tony Bryer]

Which says that during the summer season you can expect to cut your DHW energy usage by 12%. On a modern house DHW is responsible for about one third of energy use and most of this will be during the heating season (greater part of year and colder water coming in).

"Added to this figure should be the instantaneous saving of ~2.47% that this achieved whilst the boiler is operational in DHW mode. The total saving is thus projected to be ~(38+2.47) % ie fractionally in excess of 40%. This figure will only be valid if the boiler operates for sufficient hours to release the required amount of flue gas energy. If this amounts to 2.47%, then energy that the boiler must consume is:-

2.24/0.0247 = 91kWh

This is equivalent to about 3 hours operation per day for a ~30kW Combi, although this period should be reasonably uniformly distributed through the day. This operational period and such an operational profile is not unreasonable during the winter months."

For most people this assumption is totally unreasonable. If you have a

30kW combi it's almost certain that this is determined by DHW needs. For the typical house we use as a model (108m2 built to 2002 standards) the annual heating energy is 7325kWh - averaging about 30kW/day across the heating season. You can't recover what's not going out the flue.

[Doctor Drivel]

Chav, it is clear you are very lacking.

Payback could be 3 years. They aim to get it into mass produced combis as an integral part of the combi. Many makers are assessing it, and a housing associtions in Liverpool is testing out a number of them. Chav, tell them they are all wrong, as you know better.

See above.

Chav, you reall want it, but I never dance in Essex.

[Doctor Drivel]

It is a thermal store of condensate. In CH running the wasted heat heats the store of water to ~50-60C. As they said in most cases of small DHW draw-offs the vast majority of heat will come from this small store. So, in winter much of the wasted CH heat is reclaimed, and it is then used for DHW use.

There are clear gain. When makers are attempting to grap the extra 1 litre/min of the competition, this is a real quantum jump in efficiency.

With some clever piping and controls this could also be used to boost UFH as well.

[Tim Downie]

Why only 30%? Genuine question. If you run gases in opposite directions you always have a thermal gradient between incoming and outgoing gas which makes countercurrent heat exchangers potentially very effective.

Unless more than 30% is attainable.

Not if you can knock one up for £15. ;-) Bit of plastic drain pipe and a SS exhaust cutoff from the local Quikfit.

Tim

[Andy Hall]

OK, so even if it's 50 or 60%, it's still marginal.

Possibly, but unless you had a long length of it, you wouldn't get the surface area needed to achieve the transfer. Think about heat exchangers inside boilers and also plate heat exchangers for heating water. These have huge surface areas by comparison.

[Tony Bryer]

I assume that what is envisaged is an insulated doughnut containing combi pre-heat water with a length of ss flue passing though the middle so that the heat from the flue pre-heats the water. A key problem ISTM is that when the boiler stops you have this doughnut of warm water but if you don't make use of it pretty quickly it will lose heat back into the flue to the outside cold air. Likewise on one of those spring or autumn days that is nice and mild but cold at night this heat exchanger will turn into a pre-cooler.

[Ed Sirett]

Look over the BoilerCHoice FAQ it may help.

[John Rumm]

Chump, gullible could have been a word made for you...

Look at the hot water draw off pattern they used for testing. Lots of unrealistically small 1.8l bursts that could be satisfied from the stored water. For many instalations you will use 1.8l just running off the dead leg. 24l for a shower - great if you only want 2 mins in there

- but remember some folks might actually want to get clean. What about baths - even your mythical 100L bath will make a big dent in those figures. They also seem to be assuming that a 30kW combi will be running at that power output into the CH for three hours a day in order to have enough captured waste heat in the first place.

Not exactly a realistic scenario is it. Still realism was never your strong point was it.

[Andy Hall]

It's amazing what a manufacturer can achieve by paying for a test report.

[Doctor Drivel]

Chav, take it with a respected Gas test house in Holland if your Chavness thinks otherwise.

[Doctor Drivel]

Chav, you must read again. You are clearly not a very bright person.

[Doctor Drivel]

Matt, yep, an objective report. Amazing isn't it.

[John Rumm]

Yup, just define the criteria so that it is narrow enough, cough up the cash and you can get away with murder.... after all it worked on Hutton!

[Andy Hall]

Remind me to email you some other product reports and the number of one of my offshore accounts so that you can make the payment for your purchases.

Do you buy Lottery tickets at all?

[Andy Hall]

It certainly did. Literally.....

[Andy Hall]

Don't be silly.

You can give a commission to any test house or consulting firm and define the criteria in such a way as to produce the outcome that you want to have for marketing purposes.

They do need to tell any lies, but the results will not be objective either.

I suppose that you believe advertorials in magazines as well....

[Doctor Drivel]

Matt, have you any evidence of this firm skulduggery?

[Andy Hall]

OK. One more time.

Take a read through the report. Properly.

"Testwork"

"Zenex retained GaC to determine the energy savings possible from the Energy GasSaver."

This means that Zenex paid Gastec to do test work and produce a report. It was not that Gastec did this independently and out of the goodness of their hearts.

The basic pattern of use of the heating that was assumed is not reasonable, and neither is the volume of hot water use.

Much of the rest of the report is based on theoretical assumptions which the author points out should be verified experimentally.

Nobody is suggesting any impropriety in what was actually done. However, the measurements and assumptions are a long way from practical use and the further experiments recommended were not done as part of this work.

In other words, it's the minimum amount to suggest some promise in the product (but not under realistic conditions of use). The comprehensive work to actually establish useful data experimentally was either not carried out or not included in the report.

It doesn't have a lot of real value.

[Doctor Drivel]

Matt, so you haven't then.