# Any point in over-speccing BTU vs. what is required?

#### S1llyB1lly

Hello guys!

I'm self refurbing a ground floor flat in a mid-terrace property (built around 1880!). The renovation is extensive and goes back to bricks and joists.

After destroying it, I've began to build it back up. I've taken accurate measurements of each room, inputted these into 3 different online BTU calculators and am repeatedly coming to a figure of 30,000 BTU, or so.

The boiler is a Potterton Combi, rated at 28kW DHW / 26kW CH Pnc (condensing)...whatever that means. By my maths, 28kW = 95,000 BTU

I have 2 questions:

(1) Is there any harm / negative effect in over speccing and get radiators equivalent to, say, 40,000 BTU?

(2) I don't know whether my system is delta 50 or delta 60 as all the pipework has been ripped out, so I don't know what the inlet / outlet water temperatures would have been...what should I assume, as this has an effect on BTU values

I'm new to this but doing my own, my understanding is that radiators cant be too big, its purely cosmetics, you can always turn them down. It also allows a lower boiler temperature which should mean its running in condensing mode more of the time. I'm calculating based on a difference of 35c.

Heating outputs have been specified in kW since 1980.

For an older property a simple rule of thumb is that 100 W /m2 will be about adequate.

You cannot over spec outputs as the TRV will turn the rad down if need be.

Tony

Thank you, Tony. Is there a maximum output that a quality TRV can handle or, again, doesn't matter?

Different TRVs have different internal orifice sizes but most on a condensing system can easily handle the largest domestic rads of about 5 kW.

Few boilers will be stable with a differential of over about 25 C.

Tony

Potterton have made many combi boiler models. Which one do you have? (If you can find the data plate, give the GC number.)

A combi boiler will always produce much more heat than is required for warming the house. This is because, when supplying hot water to a tap, it has to heat cold water instantly from 10-20C up to, may be 60C.

The correct delta will depend on which boiler you have. You can then install the appropriate rads and pipes.

Potterton have made many combi boiler models. Which one do you have? (If you can find the data plate, give the GC number.)

Mine is the Potterton Combi 28 HE Plus A boiler, installed in 2012 (not by me).

The Drayton TRV4 can handle up to a maximum of 5kW (10C delta) and 10kW (20C delta). But 3kW or 6kW is preferable as it will allow the TRV to control the room temperature more accurately.

I stumbled across this...

http://www.eceuk.com/assets/Documents/KS08_How_to_Design_a_Heating_System.pdf

And there is an absolute heap of variants that affect how much power is used to heat a home (u-values, materials, location, heat emission blah blah blah).

Realistically, when you chaps are quoting a client for a new system, say boiler and rads, who will use an online / Mears calculator and who will take the time to plug all the possible variants into a spreadsheet or do a full and very detailed piece of analysis?

It would be a very silly person who spent much time doing calculations on individual rads for a quotation when the customer is likely to just use the cheapest.

Most might use a whole house calculation for the boiler and many would use their experience to make a judgement.

Tony

OP, since you are going to such lengths with the building, perhaps you might want to calculate heat losses for each room. Oversizing the radiator and running the boiler at a lower temperature will improve efficiency

If there is a thermostat, good. If not, run in a cable for a thermostat

Here is the manual
Thanks.

Although not specifically stated, the flow/return delta can be calculated by reference to the "Pump - Available Head" graph on page 10. It is 20C.

Assuming Flow/Return temperatures of 70C/50C and a room temperature of 21C, a radiator will produce only 70% of the BS EN 442 quoted output. This mean that, if you want 1000W output, you will have to install a nominal 1400W radiator.

Assuming Flow/Return temperatures of 70C/50C and a room temperature of 21C, a radiator will produce only 70% of the BS EN 442 quoted output. This mean that, if you want 1000W output, you will have to install a nominal 1400W radiator.

If you have a room temperature of 21C, why do you want the radiators to give up any heat?! Hot enough!

Nozzle

Here is the manual
Thanks.

Although not specifically stated, the flow/return delta can be calculated by reference to the "Pump - Available Head" graph on page 10. It is 20C.

Assuming Flow/Return temperatures of 70C/50C and a room temperature of 21C, a radiator will produce only 70% of the BS EN 442 quoted output. This mean that, if you want 1000W output, you will have to install a nominal 1400W radiator.

That is very useful. Does that mean that I should be adding another 30% or so due to loss to whatever the individual room BTU / kw figures are?

#### DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.