DIY Doctor

central heating

Postby april » Tue Mar 04, 2008 11:27 am

i have central heating in my house with 8ml pipework and a back boiler considering getting new boiler i have been advised that a combi condensing boiler would be the best choice and that i would also benefit from replacing the pipework to 15ml the more i have thought about it i dont really like the thought of loosing my storage of water and my tank i live in 4 bed house but only one bathroom 1 kitchen anyone out there that can advise pleASE
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Postby ericmark » Thu Mar 06, 2008 2:13 pm

My central heating has also played up after 25 years of service. When I phoned local heating engineer he gave me opposite advice and said combi boilers are only where there is no room to fit water storage and said how they could not be used with solar heating and if I could spare the room not to use combi in spite of my existing boilers giving instant water heating. He also pointed out my area is soft water and new storage tanks work on mains pressure. I also have mixture of 8, 10, 15, and 25mm pipes. Also Myson radiator in one room. So why did your expert advise on a combi as I must make same chose.

Postby Perry525 » Fri Mar 07, 2008 6:38 pm

The Americans have a saying. "If it ain't broke don't fix it."
If you have a system that works OK.
Why consider changing it?
Why should a combi boiler be the best choice for you?
You have the best system now!

Just think about it, you have an emergency store of water in the loft.
If the water is cut off for some reason, say a broken pipe up the road, you can still wash, flush the toilet, boil a kettle do all the things you do now, without thought or inconvenience.

Fit a Combi, no back up water store, can't flush the loo etc; problem you take a shower, someone turns on a tap elsewhere, you are standing there no water, cold, covered in soap, waiting for the water to come back.

Or worse! Some years ago I had just started a shower, when mud came through the shower head, covering my legs and feet. A broken main up the hill. Ended up wiping the mud off with kitchen paper towels.

Took three days before we turned off the garden tap, running all that mud out of the pipes from about a mile up the hill.

Size of pipes, if the 8mm pipe delivers the correct amount of heat to the radiator, then the man who designed your system got it right.

Why should someone come along and say different, when you know its right.

As an aside, if you want more heat from a radiator and its on full at the lock shield valve, then turning up the pump a click will push more heat through.

Final point. Combi boilers are sold on the basis that they are more energy efficient, that is sometimes true to a very small degree. There claim is based on the saving in energy made when the boiler first starts up by claiming back the heat from the exhaust gas. This situation only holds good until the home reaches its running temperature, once the returning water is no longer cold or cool - the effect is lost.

Think of the cost of your proposal, think of the minuscule amount of money you will save each year by changing your system and the time it will take to to recover your money, take into account the interest lost on your money in an ISA.

Its just not worth it.
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Postby ericmark » Sat Mar 08, 2008 12:11 pm

Water when it turns from liquid to gas (Steam) absorbs heat called the latent heat of evaporation and in round terms it takes 22 times (970.4 btu/lb or 2257 kj/kg) more heat to change water into steam as it does to raise its temperature from 0 to 100ºC this is a lot of energy.
The design of the compensating boiler takes advantage of this power and cools the gases to below approx 60 to 80ºC after which evaporation of water increases greatly. Some US models there is even include a heat pump to cool the gases even further. Most UK models just have two heat exchangers. The first (closest to flame) like any boiler heats the water to between 85 and 95ºC depending on if it is an open or closed system. The second pre-warms the water going into the first stage. To do this the return water must be cool around the 60ºC mark any cooler may cause problems with legionnaires disease. If the return water gets too hot then the boiler must compensate by reducing output, reducing pump speed or switching off.
The radiators have always had two controls one which the user is able to control (Either a tap or TRV) and the other which is pre-set by the installer. The latter, in the old days, was used to ensure a differential pressure so water went through all radiators, but now it is far more important and ensures the temperature drop across each radiator is about 15ºC when the user valves are full open.
Different makes and models of boiler use different methods of controlling water flow but one method is bypass valve and as each radiators TRV shuts it down more un-cooled water is returned to the boiler and the boiler reduces output to match.
I have one problem with all this. I have a Myson electrical cooled radiator which unlike all the radiators with TRVs instead of returning less hot water when the thermostat works returns more hot water as there is no control on flow rate of water only speed of fan. As a result the method of boiler control being pump speed, heat output, and pressure differential could make the difference between working well or being a complete flop.
Some boilers were very, hit and miss, and the condensating part only worked when it first switched on and as it heated up the condensating part all went by the wind, which took the water vapour away. Also sealed systems have higher water pressures which allow the water to get hotter, in same way as a cars radiator works, it also has limited damage if a leak should happen as it is not self filling and once the pressure is released it switches off minimising the damage that the water can cause. On the other hand with higher pressure more likely to leak and bleeding is a real pain. Also I do not want radiators hot enough to burn. Electrical regulations say a part which need not be touched for normal operation and is metallic has an upper limit of 80ºC non metallic 90ºC the Myson is OK but the other radiators unless convection covers are fitted would not comply and since the boiler is electrically controlled it comes under the regulations 423.1 Table 42.1.
It seems that every heating establishment has their own ideas. And each one gives a completely different answer to my request. Some saying I need a sealed combi boiler others sealed standard boiler with mains pressure heat exchanger with different reports as to which could at a latter date be used with solar heating or allow electric back-up if boiler failed. All seem to say getting another boiler to run 25 years with only two breakdowns to be very unlikely. The second only required cleaning, had I serviced it as recommended that would not have happened. At the moment, I have two boilers, the central heating boiler (open vented) is backed up with a gas fire, but hot water has no back-up. An electric back-up for hot water seems to need a water storage tank so I am leaning that way. It would also mean I could up-grade to a shower with side jets and turning on of taps in one place less likely to cause shower temperature change. I am looking for any corrections in my perception of the systems and any suggestions as to how to proceed. Or in fact if I should proceed at all. Main problem is hot water. Boiler pilot flame blows out with high winds, The temperature-voltaic unit that monitors the flame has needed changing on a regular basis about every three years, and once the gas valve stuck and boiled the water as tap was turned off and blew off a Yorkshire fitting with the pressure. Seems non-return valve is fitted in main and there is no pressure release valve in the boiler.
As to savings well I get views on old system 40% and new system 90% so should be half the cost but not sure I believe that?

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