Quick question…I thought pipes to oversized radiators had to be bigger, but recent quote for 8.5 kw daikin air source said 15mm. Partner says it’s daikin so must be ok…am I missing something or getting the wrong end of the stick?!:)

Also refrigerated insulated pipes on outside of house…to the tank…bit surprised by this?
Thanks!

The amount of heat that a radiator can give out is a function of its size and the temperature difference between the temp. of the radiator and the surrounding air and the speed of that air. The closer the rad temp. is to the air the less heat will be transferred. (to the point where air and rad. temp. are the same then no heat is transferred) So in order to get a given quantity of heat out of a radiator the closer the rad. temp. to the air temp. the bigger the rad. needs to be. (If you increase the air flow over a rad. (fan assist) them more heat can be transferred to the air). Radiators are usually calculated with a 20 deg. temp. difference between input and output.

Water contains heat (energy) at a fixed rate of 4200 joules / kg regardless of the temp. So water with a start temp of 80 deg. falling to 60 deg. will loose 4200 x 20 = 84,000 joules transferred from rad. to air. However water going from 40 deg. to 20 deg. will have the same decrease in energy. but the ability of the rad. to transfer that energy (heat) to the air is reduced because of the reduced rad. to air temp. difference.

So bigger rads. are needed because of the reduced rad. to air temp. difference but because the energy contained in the water for a 20deg. temp difference is the same the same pipe size is sufficient. (Increasing the speed of the water is ignored here because a faster speed would give more energy available to the rad. but would not increase (by much) the ability of the rad. to use that energy).

Posted By: WillInAberdeenFor low temperature radiators, the deltaT between flow and return temperatures (eg 20⁰C that Peter refers to ) is usually reduced, eg to 5degC, otherwise the radiators would need to be very large. You then need more flow of CH water at that lower deltaT, to deliver the same heat capacity, so you need bigger pipes and circ pump. It's a trade off between spending money on bigger pipes versus spending money on bigger radiators,

Correct but if the pipe sizes are not running on the limit (and they usually aren't) it may be enough to increase the pump speed or fit a bigger pump. However more water speed increases the noise level from the pipes.

Where there is a drop down flow and return 10mm microbore manifold system e.g. in a bungalow, would it be beneficial to increase the flow by replacing the 10mm pipes with 15mm just where they are visible and therefore easily accessible? i.e. in the trunking coming vertically down the walls and then horizontally to the radiators.

<blockquote><cite>Posted By: Peter_in_Hungary</cite><blockquote><cite>Posted By: Jeff B</cite>Where there is a drop down flow and return 10mm microbore manifold system e.g. in a bungalow, would it be beneficial to increase the flow by replacing the 10mm pipes with 15mm just where they are visible and therefore easily accessible? i.e. in the trunking coming vertically down the walls and then horizontally to the radiators.</blockquote>
If the pipework was marginal so that a high speed was needed to get the desired quantity of heat to the radiator then going up to a 15mm pipe for the down pipes would probably negate some of the noise associated with high velocity water in pipes. It might also remove the need to turn up the speed on the pump quite so much or for a more powerful pump because whilst the restrictions of the 10mm pipe in the loft will still be there the 15mm pipe will have less resistance and it is the cumulative resistance that counts. Pipe resistance is a function of diameter, length, bends and fittings.

Resistance creates pressure drop so a high pressure is needed with higher resistance. To get a given quantity of water down a pipe a certain flow rate will be needed, increase the pipe dia. then a lower flow rate can be used. Calculations can be made to get the best fit but nobody (that I know of) does this for central heating.

A calculator for pressure drop can be found here
http://www.pressure-drop.com/Online-Calculator/
Have fun!</blockquote>

Thanks for that. My question was an academic one really as I have no intention of replacing any pipework at the moment. All the CH pipework is 10mm microbore and the majority of it is in the attic. It would be a big job to replace all the pipework up there starting back at the manifolds, but just replacing the vertical drops not quite so onerous. It might be worth it if this was on the critical path of any decision to install an ASHP in the future where having even just some of the 10mm replaced with 15mm could tip the balance in favour.

Thanks, that’s reassuring! I’m still surprised about the outside pipe though – as I understand it, it goes from the outside unit upstairs to the water tank… Is that usual?! I’m hoping that things have changed (it’s a daikin by the way) because last time I looked at Heat Pumps, it wasn’t a good idea to be heating your hot water with them but I’m assured that it’s fine now…?

Thanks,

Tania