All power is given to the DHW heat exchanger. Some combi's derate the power when in CH mode to prevent excessive boiler cycling, which still gives a hell of a lot more power than what the average house needs. This is easily overridden. The better combi's have built-in anti-cycle control anyhow.
All power is given to the DHW heat exchanger. Some combi's derate the power when in CH mode to prevent excessive boiler cycling, which still gives a hell of a lot more power than what the average house needs. This is easily overridden. The better combi's have built-in anti-cycle control anyhow.
All power is given to the DHW heat exchanger. Some combi's derate the power when in CH mode to prevent excessive boiler cycling, which still gives a hell of a lot more power than what the average house needs. This is easily overridden. The better combi's have built-in anti-cycle control anyhow
The flow rates from combi for _showers_ are pretty reasonable most of the issues are to do with control and stability of temperature.
Most combis have just one gas/water heat exchanger. For once I'll support IMM on this claim. Because the boiler is rated typically at 24 or 28 kW this is usually far in excess of the space heating requirements so then boiler is cabable of heating the radiators up 'zippo' or 'real fast'. In fact it might do so too well and not give the wall thermostat or TRV's time to respond until after the rads have been filled with very HW whcih subsequently overheats the house - things settle down and the boilers invariably have the ability to moderate the gas rate and have other gadgets/features in the control systems to operate more efficiently.
HTH
I you don't support me on al my claims you little of this business. :)
"Ed Sirett" wrote | >> > > For the | >> > > majority of British homes combi's are | >> > > a panacea. They also heat the house | >> > > up "real fast". | >> > Please explain "real fast". Why would they be any faster | >> > than a conventional boiler? | >> Most are 85,000 to 100,000 btu/h, | > Is this true for the CH exchanger? I thought this was lower | > than the hot water exchanger? | Most combis have just one gas/water heat exchanger. | For once I'll support IMM on this claim. | Because the boiler is rated typically at 24 or 28 kW this is usually far | in excess of the space heating requirements so then boiler is cabable of | heating the radiators up 'zippo' or 'real fast'.
I may be missing the point here but ...
Surely the heat which can be dumped into the rooms is dependent on the size of the radiators? and having a bigger boiler isn't going to change anything unless the radiators are increased. If the radiators stay the same (ie undersized compared to the boiler) then the return temp of the radiator water will be higher and the boiler will shut down or go bang.
Owain
Where I disagree with IMM is that being a combi makes any difference. Any modern system boiler does this too. In fact, they are normally exactly the same as the combi version of the boiler, but with the secondary plate exchanger, flow switch and diverter valve removed.
Christian.
Are they? Some are, mainly system boilers, many are not, just set output rates. Heating boilers (no pump or pressure vessel) tend to be this way. Some installers fit set output rate boilers as they perceive modulated boilers to be troublesome (mainly it is because they don't understand them)
see:
Yeah, but from cold, an oversized boiler can get them hot in no time at all. After the radiators are hot, it makes no difference (assuming it can modulate down to be equivalent to the fixed rate and thus avoid short cycling).
Christian.
"Tony Bryer" wrote | Owain wrote: | > Surely the heat which can be dumped into the rooms is | > dependent on the size of the radiators? and having a | > bigger boiler isn't going to change anything unless | > the radiators are increased | You're thinking about the steady state situation. It takes | energy to get the temperature of all that water and steel | up to working temperature, and the bigger the boiler the | faster this happens. A BMW 316 and a BMW 328 will both do | 70 on a motorway but one gets there a lot faster from a | standing start.
But that makes little difference to the overall journey time between London and Edinburgh?
I can understand that the rads themselves will get hotter quicker, but that happens quite quickly in most cases anyway. That's not the same as heating the house up "real fast". My rads get hot within a few minutes. I have a boiler which could heat a 3-bed house but the flat takes an age to heat up from cold, because the radiators are undersized (and wired in microbore). They are the limiting factor, not the boiler.
I still don't understand therefore how 'combis are a panacea. They also heat the house up "real fast"'
Owain
The case in point is that _from stone cold_ the combi boiler is likely be able to heat the radiators up more quickly.
Of course if you compare a 24kW system boiler (eg. Vaillant T/max 624e) with the combi model (824e) then there will be no difference at all. However if you were to have a stored hot water system you might realistically install 15kW system boiler (eg. Vaillant 615e). In this case there might be a difference in the warm up times. However we are really picking nits here as we might be talking about a few minutes difference.
I replaced a 40,000 BTU/r Neataheat with a 27kW combi and the warm up was real zippo in comparison - a few minutes all the rads were hot all over. Then the combi modules down. When you come after having the heating off for a week or so in winter then you appreciate a zippo warm up.
A few simple explanations have been given, after that well....
Contact Geoff - he can sell you a new one....... .andy
To email, substitute .nospam with .gl
"Ed Sirett" wrote | > Surely the heat which can be dumped into the rooms is | > dependent on the size of the radiators? and having a | > bigger boiler isn't going to change anything unless | > the radiators are increased. | The case in point is that _from stone cold_ the combi boiler is likely be | able to heat the radiators up more quickly.
I can see that there could be an improvement with a combi, because if you return to a cold house and put both CH and HW on, the boiler output is divided between the two, or the HW gets priority, for the first half hour or so to heat the cylinder of cold, whereas with a combi all the heat would go to the CH ... even if you immediately started running HW through a combi (which would AIUI mean absolutely no heating) that situation shouldn't last long, after which all the heat is available for the CH. That's irrespective of the boiler power. But surely the heat to the rooms over time is dependent on the radiators/piping?
Owain
I was only supporting this claim in the quite common case of where a medium sized conventional boiler has been replaced with a combi boiler.
Heating the radiators is one thing heating the house happens later, but clearly the radiators have to heat up first so if that happens quicker then the eventual process is quicker.
Of course, there are plenty of cases where this might not be true, such as the one you have outlined.
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