"water priority" and weather compensation

[fourseasons]

I've just had a new (viessmann 100)) system boiler installed as a replacement boiler to our central heating system which has 2 heating zones and an unvented DHW cyclinder all controlled by programmers/ programmable roomstats and cyclinder stat.

I was only told during the installation that, because of the weather compemsation control, the system could only be set up to provide water OR heating - i.e not both simultanesouly.

Can anyone tell me if this is an issue which arises with weather compensation control of other manufacturer's system (i.e not Combi) boilers (e.g Worcester bosch/ Baxi etc)?

Seems an obvious disadvantage to me - but Viessmann don't make it clear in their literature and neither did the installer - until too late.

 

[Boilerman2]

It is common for boilers with Direct Burner control Compensation to operate in Water Priority, given tht you have an unvented high recovery cylinder, this should only interupt the heating for no more than 15 mins after running a bath, this isbeacasuse to heat HW the boiler runs at 70 - 80'C, but while on heating the flow can drop down very low, so as to keep the boiler in condensing mode for as long as possible, at the end of the day, to maintain high efficiency the boiler & system has to be operated as the maker inteded

 

[mehran]

all WC systems which uses a tank for the dhw will heat the CH and dhw to different temperatures. it is the whole point of WC, if you try to do it the way you wanted you would get:

A: over heated rads or:
B: luke warm hot water...

Seems an obvious disadvantage to me - but Viessmann don't make it clear in their literature and neither did the installer - until too late

not really... unless you think the second your boiler stop heating your house the temperature will plummet or you tank will be cold the second the boiler stop heating that?

 

[D_Hailsham]

Have you noticed any problems e.g the house cooling down too much when the cylinder is being reheated?

The reason for having "hot water priority" when weather compensation is installed is simple: it means the hot water will heat up much faster. Imagine a situation where the weather compensation is running the boiler at a flow temperature of 50°C and you want to heat the hot water cylinder up to 60°C. The simple answer is: you can't, if you try to do both at the same time.

With HW priority, as soon as the cylinder stat asks the boiler to heat the water, the boiler automatically runs at full output, irrespective of what the weather compensation is doing, and the diverter valve also switches over so only the cylinder is heated. This means the water is heated up as quickly as possible.

As you can set separate times for CH and HW and, presumably, have a well insulated cylinder, try heating up the water during the night, when the CH is not on.

Most cylinder stats set to 60°C do not reheat the water until it has dropped to 50°C, and water at that temperature is still too hot for washing, so you may find that you don't need to reheat the water until well on in the day.

Experiment to find the best time to have the hot water reheated during the day.

 

[fourseasons]

It is common for boilers with Direct Burner control Compensation to operate in Water Priority, given tht you have an unvented high recovery cylinder, this should only interupt the heating for no more than 15 mins after running a bath, this isbeacasuse to heat HW the boiler runs at 70 - 80'C, but while on heating the flow can drop down very low, so as to keep the boiler in condensing mode for as long as possible, at the end of the day, to maintain high efficiency the boiler & system has to be operated as the maker inteded

Thanks

But unfortunately the theory doesn't always fit reality!!!

with several people running showers one after the other, and washing machines , car washing etc. the "15minute reheat" can be much much longer - and if the house is still not warmed you get cold.

I understand the theory fully - hot water for DHW and warm water to radiators - it's just that I thought weather comp control would be ignored when DHW "heat" was needed - and keep heating the rads too. As that facility has not been built in - out go ALL the savings from weather comp just to get hot DHW and a warm house (and its well insulated) at the same time.

Progress - I think not

 

[mehran]

i take it that your cylinder was not upgraded to a correctly sized high efficiency, fast recovery one then

as soon the boiler sees that hot water is being uses(via a stat or ntc) the boiler will fire to heat the cylinder as it is being used. 15 min is quite achievable.


a combi's are hot water priority but i don't hear cries of "the house is freezing as soon as i turn the tap on"


get a cylinders that meet your demands...

 

[fourseasons]

i take it that your cylinder was not upgraded to a correctly sized high efficiency, fast recovery one then

as soon the boiler sees that hot water is being uses(via a stat or ntc) the boiler will fire to heat the cylinder as it is being used. 15 min is quite achievable.


a combi's are hot water priority but i don't hear cries of "the house is freezing as soon as i turn the tap on"


get a cylinders that meet your demands...

that#s because with combis the heating's off for a short time.

and the cylinder was fine until the new boiler was fitted. so the problem is the boiler - or rather the way it has been installed

Anyway I was looking for constructive feedback on my idea to restore simultaneous DHW and heating - and keep the weather comp.

otherwise the weather comp is coming out.

 

[mehran]

simple, take the weather comp out

 

[D_Hailsham]

with several people running showers one after the other, and washing machines , car washing etc. the "15minute reheat" can be much much longer - and if the house is still not warmed you get cold.

There is no need for a hot supply to any washing machine or dishwasher; they should be cold fill only. If your machine has both, you can get a Y coupler so both connections receive cold water only. And do you really run the washing machine/dishwasher when everyone is showering? What about doing overnight washes? We have been doing them for over 30 years.

Do you really wash your car using hot water through a hose while everyone else is showering?

If your house is not up to temperature when everyone gets up and starts showering, that's not the fault of the system; it means the times are not set correctly. How are the heating times controlled? Do you have optimization?

So we are left with the problem of showers. Either your family spends too long in the shower or the HW cylinder is not large enough.

 

[htgeng]

So we are left with the problem of showers. Either your family spends too long in the shower or the HW cylinder is not large enough.

I agree with all that.. But the customer is always right I can see what the next complaint will be though. This thread does remind me to rig up the outside tap with hot water !

 

[fourseasons]

with several people running showers one after the other, and washing machines , car washing etc. the "15minute reheat" can be much much longer - and if the house is still not warmed you get cold.

There is no need for a hot supply to any washing machine or dishwasher; they should be cold fill only. If your machine has both, you can get a Y coupler so both connections receive cold water only. And do you really run the washing machine/dishwasher when everyone is showering? What about doing overnight washes? We have been doing them for over 30 years.

Do you really wash your car using hot water through a hose while everyone else is showering?

If your house is not up to temperature when everyone gets up and starts showering, that's not the fault of the system; it means the times are not set correctly. How are the heating times controlled? Do you have optimization?

So we are left with the problem of showers. Either your family spends too long in the shower or the HW cylinder is not large enough.

The old boiler and the tank preformed for 13 years - so the issue and the question is just water priority

Why do some users of this forum seek to presume and criticise the lifestyles and needs of others?

If I choose to hot fill washing machines and dishwashers - cheaper - faster and more energy efficient and wash the cars using mixed hot/cold water thro a hose that's my decision - not yours

As all I get is unfounded and prejudiced views rather than problem solving _ I'm off

 

[Gasman77]

Cya then

 

[Boilerman2]

Get a 2nd Cylinder, Store more Water for your demands I cannot understand your attitude 4seasons, you ask proffessional people for advice and the just start complaining when the answer is not the one you want. To get your WC working independantly, you will need to fit a 3 port mixing valve, controled by the WC, and an additional pump to circulate water around the heating cct, oh yeah and a Low Loss Header to separate the heating & hotwater primary ccts, it will cost£££££££££ seeya