under sized boiler ???

[Killy Bing]

Sizing on application.

every place differs

 

[SimonG]

Reminds me of the way the Gas Board (as it was pre privatisation), when they had the monopoly, used to advertise heating systems with "Guaranteed Temperatures". All they did was install oversized rads and an oversized boiler. No heatloss calculations - just a rule of thumb, which some online calculators still use: room volume in cu ft x 5 (if I remember correctly) gave you the BTU required. I remember picking up a radiator manufacturer's leaflet at B &Q in the late 80's which had this formula on it.

Thats an over simplified version of the finger in the air guess simplified version. Different multipliers based on the type of room. My old gaffer used to use that for his rad sizing. But those were the days when i was still at college and having to calculate heat losses using the r values of materials to get the u value. He was always near enough that it didnt matter.

 

[buckley plumb]

from the horses mouth - megaflow tech

The coil ratings in the Megaflo eco can vary from approx. 15 to 24 kW. In a standard domestic installation it is usually considered unnecessary to allow these high figures when calculating the boiler size. For the Megaflo we usually suggest adding 15,000 BTU = approx. 4.5KW to the boiler rating over & above the heating requirement.
(I have known for installers to allow just 3kW as this is usually the same rating as the back up immersion should it have to be used.)

For the maximum coil outputs of the Megaflo eco coil please refer to page 14 of the attached brochure.

 

[Roger Welton]

Imagine adding total cylinder loading to a boiler size if you fitted a 300l ACV bet part of 60kW IIRC:17:

 

[happyflyer]

It pretty obvious what is going to happen if you undersized a boiler, its going to take long to get everything hot from cold, the insulation on a Mega Flow is pretty good, I doubt if any MF on a domestic installation ever starts off from cold i.e 10 Deg C. Does anyone ever work it all out, I think not, you are doing what you always did, some will worked it out. It is a very complex subject that not everyone totally agrees with one another.

 

[Ermintrude]

I allow 4.186kJ/kg / K. Always calculate it from absolute zero. I.e -298c! Used this a few times in complex jobbies http://en.m.wikipedia.org/wiki/Dulong-Petit_law

 

[happyflyer]

I allow 4.186kJ/kg / K. Always calculate it from absolute zero. I.e -298c! Used this a few times in complex jobbies Dulong?Petit law - Wikipedia, the free encyclopedia

Jezz,

Does it get that cold up where you live -298 is that C or kelvin...looks like hell can freeze over after all....:32:

 

[GasmanxxxR1]

if you look at the domestic heating design guide as a general guide as many do in the industry you will find a table that they have recommended allowances for certain sized cylinders.

for example's

120l cylinder requires aprox 7.2kw to heat up in one hour (10-60) and suggests an allowance of 2.0kw when sizing your boiler.

225l cylinder requires aprox 13.0kw to heat up in one hour (10-60) and suggests an allowance of 3kw when sizing your boiler.

we all understand that modern cylinders have high recovery/high kw coils which can re heat the cylinders in 15 mins. But i would think that if your cylinder needs to recover in this time frame its prob undersized or not correct for use. I would say that if you wanted to be over cautious you may make an allowance for about half the cylinder rated kw in your calculations as suggested to shef by RM. But this could still be over the top really.

Spot on!

 

[happyflyer]

Spot on!

Me thinks too!

 

[Ermintrude]

Jezz,

Does it get that cold up where you live -298 is that C or kelvin...looks like hell can freeze over after all....:32:

0k = -298c point at which all molecules stop moving so no thermal energy. Theoretically coldest temp possible