Sizing rads and lower flow temps...?

[agreenleaf]

Hi all, I'm a long time lurker, first time poster....
I'm in the process of finally putting some plumbing back in my house after surviving with just a woodburner for some time!

I'm fairly confident the central heating layout would work but would welcome input from anyone who can see any issues with this setup...

A bit of background info if it helps:

- it's a 3 bedroom house, with a basement and a loft conversion so it's over four floors.
- I have a vaillant ecotec plus 832 combi boiler which is 32kw and has a 15.2l/min flow rate
- the lead pipe mains has been replaced and i've got 32mm mdpe piped all the way up from the road to the boiler cupboard.
- Mains pressure is 6.5/7 bar but i've got it set to 3.5 Bar with a PRV
- With wet UFH and a woodburner, I don't imagine the downstairs radiators will be turned up much but ultimately I need to design the system to be able to cope without the fire being on, for those odd cold snaps in spring/ autumn when fire isn't lit or UFH isn't in full swing.
- basement is infrequently used but a tickle of background heat would be welcome (it needs very little to stay warm as it's buried in the ground!)
- Planning for copper throughout and diagram shows 22mm, going down to 15mm
- I'd like the ground floor and basement radiators are on drops from the ceiling, so I don't have to bury plastic pipes below/ within the UFH area (it's already getting quite busy in there with various pipes for UFH, basement ventilation and soil pipes.)

I’ve performed some online heat loss calculators using this tool here: https://ukradiators.com/pages/btu-calculator, inputting window sizes, building fabric, adjacent rooms etc to get an idea of what’s required. The KW required for each radiator has been calculated using a Delta T of 40c

I’ve added a diagram to show the KW per radiator, total radiators are 5.81 kw, UFH is 2.2kw, given that it’s a 32kw boiler, this all seems in order?

I'm hopeful that given the external walls are internally insulated, new windows throughout and the loft conversion is stuffed to the gunnels with insulation, it should be a fairly warm home. It’s not going to be a totally accurate calculation using the online tools as they don't have options for outside walls that match the makeup of my house (18” thick stone wall, with 2” insulating plaster internally) so i’ve just done it based on a 9” uninsulated brick wall as that’s the closest option.

Funds are tight due to a young baby and having depleted all my savings so i'd like to tackle this myself so we can move in as a family ASAP. If any kind person out there has input/ comments on the system above then i'd be very grateful.

Many thanks
Josh

 

[Johntheo5]

You say
"The KW required for each radiator has been calculated using a Delta T of 40c"
do you mean that this is the (mean rad temperature) - (the required room temperature), if so, what are the flow and return temperatures and the required room temperature, the mean rad temperature is (flow temp + return temp)/2.

 

[agreenleaf]

You say
"The KW required for each radiator has been calculated using a Delta T of 40c"
do you mean that this is the (mean rad temperature) - (the required room temperature), if so, what are the flow and return temperatures and the required room temperature, the mean rad temperature is (flow temp + return temp)/2.

Hi John,

So that’s what the radiator calculator form quote. Which happens to be in line with my aims: 65c flow temp, 55c return, 20c room temperature. I’m just getting to grips with the delta stuff so forgive me if that isn’t right..

 

[Johntheo5]

Yes, that correct.
Rads are based on a T50 rating, like, 75C/65C/20C, a T50 rad running as a T40 will emit, (40/50)^1.3, 74.8% of its rated output so should be oversized by a factor of, 1/0.748, 1.34.
Mind you, its worded a bit strangely, its the kW for each room that is the important thing, then, if thats what you mean, the above applies.

 

[agreenleaf]

Yes, that correct.
Rads are based on a T50 rating, like, 75C/65C/20C, a T50 rad running as a T40 will emit, (40/50)^1.3, 74.8% of its rated output so should be oversized by a factor of, 1/0.748, 1.34.
Mind you, its worded a bit strangely, its the kW for each room that is the important thing, then, if thats what you mean, the above applies.

Right you are, I thought they'd calculated it at t40 but I ahve re-read what it spits out:

"Your room has the following heat output requirements:

Watts = 331
BTU = 1129
You will need to purchase radiator/s where the T40 (Δ40) is greater than the amount shown above." so i'll divide those values by 0.748 to get the true KW needed. Thank you for your sage advice!!

 

[MNW67]

Rads are based on a T50 rating, like, 75C/65C/20C

What puzzles me is that when calculating radiator outputs like in your example, they base it on a temperature drop between flow and return across each radiator of 10C. But modern boilers run on a dT of 20C.

 

[Mister Banks]

What puzzles me is that when calculating radiator outputs like in your example, they base it on a temperature drop between flow and return across each radiator of 10C. But modern boilers run on a dT of 20C.

The dT of 20c is only for a flow temp of 70c. The lower the flow temp, the lower the dT. My condensing boiler generally runs at a dT of 10c to 15c with a flow temp of approx. 60c.

 

[MNW67]

The dT of 20c is only for a flow temp of 70c. The lower the flow temp, the lower the dT. My condensing boiler generally runs at a dT of 10c to 15c with a flow temp of approx. 60c.

So, in the radiator output tables published by Stelrad etc. they use figures of 75C going into the radiator and 65C coming out of the radiator. This gives an average temperature of 70C inside the radiator. And they use a room temperature of 20C to calculate a T50 output for each radiator. But if you are running a modern boiler at 75C flow, then you are aiming for a 55C return temperature, rather than 65C. Which means to me that, in the real world, for modern boilers, all the output figures from the manufacturers are incorrect because they are using the wrong average temperature inside the radiator.

 

[Johntheo5]

If you require a (T50) rad to emit its rated (100%) output with a dT of 20C then, flowtemp/returntemp/dT/flow.LPM
must be 80C/60C/20C/0.717LPM (per kW).
At a dT of 10C these numbers are 75C/65C/10C/1.433LPM

IF you require 75C/55C/20C then these numbers become 75C/55C/20C/0.625LPM BUT the rad output will fall to 87.2% of its T50 rating and becomes a T45 rad, that's how TRVs work, they throttle the flow to reduce the rad output for the same flowtemp(s).

 

[agreenleaf]

If you require a (T50) rad to emit its rated (100%) output with a dT of 20C then, flowtemp/returntemp/dT/flow.LPM
must be 80C/60C/20C/0.717LPM (per kW).
At a dT of 10C these numbers are 75C/65C/10C/1.433LPM

IF you require 75C/55C/20C then these numbers become 75C/55C/20C/0.625LPM BUT the rad output will fall to 87.2% of its T50 rating and becomes a T45 rad, that's how TRVs work, they throttle the flow to reduce the rad output for the same flowtemp(s).

Thanks again John, after reading it several times, that makes sense now

 

[Johntheo5]

If you have the inclination or the time, this will pass away a few hours for you.

 

[agreenleaf]

If you have the inclination or the time, this will pass away a few hours for you.

hehe thanks, I had a play, i was surpised at just how much of an effect on kw output it has, when you adjust the flow temperatures! very low flow temps would need some very enormous rads!