Which size for most efficient Boiler?

[Dan Robinson]

Pdf is invalid according to my phone.

 

[Darems]

Pdf is invalid according to my phone.

.XLSX is not allowed according to the Forum.

Have uploaded a work-in-progress file here http://www.filedropper.com/heatlosscalcs

 

[Agile]

The one I referenced to above wanted an SB40 fitted. I tried to get him to have an Eco RF24 but he refused point blank - he agreed to an RF30.... He thought (naturally enough I suppose) that the numbers referred to the actual kW's

For those who don't know, the RF30 gives out more central heating power! But that's irrelevant if the property cannot use it.

But the minimum power output is far lower on the RF24.

Just shows how customers always "think" they are right.

The main problem with boilers is that if oversized they will cycle excessively when not much heat is required. When it is 14 C outside and the house is up to temperature the heat demand might only be 2 kW and to deliver this the boiler needs to modulate on/off which is inefficient and provides more wear on some boiler parts.

Tony

 

[Agile]

If I raise external to -3C and only allow 10% for heat up I end up with a heat loss of 13KW (I've made a few more adjustments too (like no external walls in the loft)

Thanks

I am also in London and would never expect to size a system for -3 C outside. Nor do I remember any recent cold weather with that low temperature during heating times.

Even if the temperature was below that, the built in allowance for heat up would provide enough heat to cope. But it would in theory take longer to heat up.

In practice, during most normal timings that would not be a problem because heating is usually turned on about 5 pm when the outside temperature is higher. The temperature usually falls during the evening and is often at the lowest about 3-4 am when most are tucked up in bed.

Even if the system does not have enough power to cater for a cold outside temperature it is not the end of the world, it just does not quite reach the expected 21 C.

In the lounge here, the heat produced from the TV, audio and computers increase the actual room temperature by 3.5 C !

Tony

 

[Dan Robinson]

For those who don't know, both the RF24 and the RF30 give out the same central heating power!

You what?

 

[Steelmasons]

For those who don't know, both the RF24 and the RF30 give out the same central heating power!

You obviously don't know either.

 

[Agile]

Thanks Dan. I have edited to correct my previous post.

Must have been thinking of the Viessman!

 

[D_Hailsham]

Pdf is invalid according to my phone.

Worked alright on my Windows 10 Laptop using Reader DC Version 2015.

Darems

I see you have kept the 70C default mean temperature, i.e 75C Flow, 65C Return. This is OK when heating the cylinder as the flow needs to be above 70C to heat the cylinder water to 60C. But it won't allow the boiler to run in condensing mode when heating the rads. For this the return temperature needs to be below 55C. Vaillant boilers recommend a maximum 20C differential, e.g 75C/55C.

This will not affect the heat loss, but it will affect the size of rads required as rad output depends on flow, return and room temperature. I don't know if the Myson calculator takes this into account.

To get the maximum efficiency from your system you should have weather compensation controls. These automatically adjust the flow temperature of the water according to the outside temperature, which increases/decreases the radiator output as required.

According to the Megaflo link you provided the heat loss is 1.2 kWh, which works out at 4.32MJ. This means a temperature drop of approx 6C over a 24 hour period if the cylinder is not reheated.

I have just noticed that the heat loss summary (page 1, top left) does not include the 10% allowance. Did you realize this? If we include the 10% heat up, the total requirement would be 15.9kW, which suggests an 18kW boiler. This would give a cylinder heat-up time of 30 minutes, recovery time of 22 min and 7 mins for 10C rise. The 18kW boiler has the added advantage of modulating down to 4.2kW (5.7kW for the 24kW) this means the boiler would still be modulating until the outside temperature rose to 15C (13C for the 24kW). The choice is yours.

 

[Darems]

To get the maximum efficiency from your system you should have weather compensation controls.

I would dearly like to include Vaillant's new vSmart controller, unfortunately they have advised it is incompatible with multi-zone setup (UFH).

Thanks for the rest of your feedback, very useful and interesting to know.

Tough decision for a novice: stick resolutely to what the numbers tell me, and get the 18KW, or play safer and get the 24KW

 

[D_Hailsham]

I would dearly like to include Vaillant's new vSmart controller, unfortunately they have advised it is incompatible with multi-zone setup (UFH).

The vSmart is not true weather compensator. All it does is obtain predicted temperature information for your area (how large?) from the internet, which may not be correct for your precise location.

You can have weather compensation for two zones if you use the VRC470, or VRC470F programmer/ weather compensator, with a VR61 wiring centre, VR81 thermostat (zone 2) and VRC693 external temperature sensor. You can find details in the Ecotec Technical Brochure, pages 48 & 49.

 

[Darems]

Thanks for all the comments and advice to all.

I've requested our Engineer to fit the Vaillant Ecoplus 624, but now stumped about the cylinder: I thought it would be a MegaFlo 170DD direct since we no longer have a header tank, not the 170i (indirect)...

 

[Dan Robinson]

Sod Megaflow - Joule are the way and the light.

 

[Darems]

Sod Megaflow - Joule are the way and the light.

Haven't heard of them... Will look them up

Am I right about the direct vs. Indirect though?

 

[Dan Robinson]

No - you want Indirect, not Direct.

 

[Darems]

Thanks for the quick reply...

I seem to have been lead astray by another site explaining the difference (not sure on etiquette about posting links to other sites)
It claims "Both unvented and vented cylinders can be either what is called 'direct' or 'indirect'" it also expands on each in terms of "Water Delivery" and "Water Heating"
The misleading bit was that it was not immediately obvious that Direct Water Delivery = Unvented, and Indirect Water Delivery = vented because in the same section it bundles together Direct Water Heating = Elements, and Indirect Water Heating = coil.
Thankfully I've just re-examined the Unvented MegaFlo spec and it does indeed make the distinction between Indirect Coil rating and Direct Element rating.

Funny how simple things get muddled up when your immersed in learning and over-analysing everything.