Anti Cycling mode on hot water only

[Agile]

OP, do you work with computers or legal matters?

I have suggested 16 kW because the heat loss is probably between 12 kW and 20 kW depending on how well insulated your house is.

But those figures are based on an outside temperature of -1 C and for the summer we don't expect many nights when it goes below 8 C so for a summer setting 16 kW is bound to be totally adequate and probably for the winter too depending on the actual heat loss.

But for the time being the whole purpose is to set the boiler to avoid this cycling as far as possible with a seriously oversized appliance.

The Megaflow does need to be fitted with the correct controls as required by the maker and the G3 regulations. But that does not prevent the cylinder temperature being monitored by the Vaillant cylinder temperature sensor if used with their smart controls.

Tony

 

[hossacd]

Reading through the posts it seems you haven't tried the 70c flow temp yet ? Wouldn't have thought turning down the boiler will have much difference if you are still trying to heat the tank to 60c with a 62c flow temp.

I have just moved over to two cylinders and had to drop the native Vaillant controls for the cylinder but the Vaillant control centre allows a standard tank stat to be used rather the Vaillant VR10 sensor you lose the ability to set the tank temperature from the controls and monitor the hot water temp but it still allows the separate hot water/heating flow temp. What is the reason you can't you just use the standard stat on the Megaflow ? (question for the gas experts, just incase my system has been bodged by the installer although both mine are Vaillant cylinders)

 

[mrshr]

This week, I will try and get into the menus and have a play around with them. I need to do a bit of reading on it all first though.

But is my boiler actually oversized for the house?
Using the calc David937 suggested, 23 rads x 1.4kW comes out at 32.2kW

Vaillant do a 30kW and a 37kW, nothing in between.

I just tried Vaillant's own system finder survey (which I did way back when we selected the boiler)

http://www.vaillantsystemfinder.co.uk/Result.aspx

What happens in other people's houses with large boilers that only want to heat the cylinder? Do they get this any cycling thing too?

 

[Dan Robinson]

Yes they do.



Heatloss calculations can be fun. But nearly always overcooked.

My 4 bed detached house with extended ground floor only has a 15kw boiler.

 

[mrshr]

Wow.
My last house was a small 4 bed house and I'm sure that had a 24kW system boiler in it.

I doubt the guy that installed my 37kW boiler did any heat loss calcs at all.

Are we still saying mine's oversized? 23 rads in total and 250 litre cylinder.

 

[Dan Robinson]

In hot water only mode, definitely and I mentioned before that there is only so much mitigation you can do.

A design flaw with the ecoTec is with d.0 set to auto, an oversized boiler will struggle with short cycling.

Lowering the d.0 helps the boiler logic deal with the lack of demand better.


As for the genuine needs of your house, we have not the information needed.


There is a reason I call boiler over sizing "the spinal tap" method.

Bigger is seldom better.

 

[mrshr]

Just been looking at how to work out heat loss from a room and using U values etc.
Can't see that my installer did any of that.

I might well go round the house and work it out for each room, but probably not worth it now as it won't change the fact that there's a 37kW boiler on my wall!!!

Is there not a way to stick an additional load of some kind to the cylinder so that it has something else to help dissipate the heat? Not really sure what, just thinking out loud reallly. Maybe add a towel rail or something??

 

[Dan Robinson]

Wherever possible we do just that with towel rails.

But truly, don't lose sleep over it.

Knock the cylinder stat down a bit, knock d.0 down to 20. Or whatever.

If you get cold in the winter, bang d.0 up a little until you get the right balance.

It is something you will never truly get rid of unless you buy a different boiler or cylinder.

 

[The831Bunny]

Probably a 'balancing' valve on the cylinder return almost completely shut...

 

[Agile]

I normally fit the balancing valve on the flow! Why particularly the return 831 ?

Primarily for balancing the cylinder against the heating.

But it also helps to increase the delta T to improve the condensing efficiency of the boiler when heating HW only which with well timed HW should be the normal case.

Tony

 

[Dan Robinson]

Didn't you have that debate with steel inv the cc?

I always fitted them img the return, and have always found them thus.

 

[The831Bunny]

You are not going to lose the coil rating of 20+KW on a Megaflo with a restricted return. On a large cylinder imho it is always best to make sure it is charged and satisfied before the heating comes on. Not always possible with S Plan controls I know. Otherwise HW priority for me every time.

 

[Dan Robinson]

On unvented I seldom bother setting them. Just leave them full on. Mine certainly is. But I have a heat pump cylinder installed on a gas boiler with hot water priority.

 

[Steelmasons]

I normally fit the balancing valve on the flow! Why particularly the return 831 ?

Industry standard being throttling/balancing valve on RETURN , why do you feel installing on the flow pipe is better? , take into account aeration and noise amongst other issues.......

Radiator lock shield valves have always been fitted to return as are DRV's......can you explain your reasons as it will be interesting reading.

Oh sorry it was obviously a question you were putting to 831 due to you not being sure where the valve should be positioned.

 

[D_Hailsham]

It's a big 5 bed house so went for a biggest vaillant do.

But is my boiler actually oversized for the house?

Unfortunately you have fallen into the trap - as do so many others - of thinking that Big House = Big Boiler.

Using the calc David937 suggested, 23 rads x 1.4kW comes out at 32.2kW

You can't make the assumption that the average rad size is 1.4kW; in any case, how do you know the rads have been correctly sized? For example: I have 13 rads, ranging from 0.5kW to 2kw, which total 13kW; so average of 1kW. However, I only require about 8kW to heat my house. This is due to improvements in insulation since the house was built about 25 years ago.

Although the most accurate result will be obtained by using the specific U-values for your house, fairly accurate results can be obtained by using the Baxi Whole House Boiler Size Calculator. It uses data from the Building Research Establishment, so is unbiased - unlike so many other manufacturers' boiler size calculators.