Boiler Requirements for UFH

[richie9648]

Hi

Just wondered what the boiler requirements are for UFH ?  I am looking to heat 100m2 on the ground floor, 7 radiators upstairs, 3 bathrooms

 

Currently have a Worcester Bosch - https://www.boilerguide.co.uk/productinfo/worcester-bosch/greenstar-30si-compact-combi-gas-boiler  

and i think its 24Kw - enough for what I need?

 

[PeterW]

How many concurrent showers are you planning on running as a 24kW combi has a crap flow rate. 
 

Heating load depends on the heat losses of the house - new build ..?  Insulation levels ..??

[richie9648]

On a worst day I will have 2 showers running at the same time - usually when the kids get back and have to use both at same after a mucky day out. 
 

in terms of insulation I have none under my suspended floor but as part of the UFH I will be putting at least 100mm of kingspan in between the joists. 
 

Are you saying current boiler may not be enough?? 

[Olf]

If you currently use Worcester 30Si then it will surely cope with heat demand, even more so when you improve insulation

Your problem may be the UFH itself - the limit of floor surface temperature and thus heating water temperature as well as choice of surface may limit the heating power radiated by the floor.

[PeterW]

Your ground floor currently has radiators I expect ..? They are sized for a specific heat loss so first question would be do they work..? If they are sized correctly, and you have no problems then you could assume the boiler will be fine. 
 

If you have a  4x3m room with a heat requirement of say 1800W, which would have a radiator of 600x800 double convector. That is sized using a delta of 50°C between room and water temperature - so 70°C. 
 

The issue now is that you need to get the same 1800W from the floor, but you have to use much lower temperature water - 45°C max in screed, even lower under floating floors. You effectively need 150W/sqm to be radiated out of the floor at probably delta 15°C. Extrapolate that out to your whole floor, and you need 15kW through the UFH.
 

The boiler will want to kick 24kW out at full power but the problem will be the temperature as you need 70°C for the rads but only 40°C for the UFH. You need a buffer or low loss header as otherwise your boiler will keep cycling on and off as the UFH gets closer to its target temperature. 
 

And now factor in that you want probably 16kW into the water for the showers running and you start to run out of headroom with the boiler. 
 

I would suggest looking at your options on both UFH and DHW as I think you may start to struggle with that size boiler unless you considerably increase the insulation levels in the walls and the roof space as well as the floors. 

[richie9648]

Thanks for the input. Makes sense. 
 

What kind of output boiler would I need - someone recommended an unvented cylinder boiler which I thought was over the top !! 

[PeterW]

Unvented cylinder boiler ..?? Do they mean an invented cylinder and separate system boiler ..? Would make sense. 

[richie9648]

On 11/04/2021 at 22:46, PeterW said:

Unvented cylinder boiler ..?? Do they mean an invented cylinder and separate system boiler ..? Would make sense. 

I am pretty sure he said Unvented - like these - https://www.vaillant.co.uk/for-installers/products/domestic-boiler-cylinders-25601.html

 

I am struggling with the space and finding somewhere to put this will be a nightmare !!! Any other options?

[PeterW]

Ok so he’s talking about adding an unvented cylinder to the system which isn’t a bad idea. Mixing UFH and rads you will also need a buffer tank - probably in the region of 100 litres. 
 

Have you got attic space where these could go ..? Move the boiler too..?

[richie9648]

5 hours ago, PeterW said:

Ok so he’s talking about adding an unvented cylinder to the system which isn’t a bad idea. Mixing UFH and rads you will also need a buffer tank - probably in the region of 100 litres. 
 

Have you got attic space where these could go ..? Move the boiler too..?

Unfortunately no attic space  - any other alternatives?  Omnie and others say you should be fine with a combi boiler but not 100% sure - they mention that - we advise that the UFH will require 6.4KW (21900 BTU) based on a standard 1.0 TOG floor finish and a room temperature of 20°C with the estimated heat loss between 3.4kW and 3.8kW. 

That is just for the UFH - I dont even know what that means.  Any helpers??

 

Edited April 13, 2021 by richie9648

[Nickfromwales]

It means you need to get 6.4kW of energy into the UFH to maintain an internal ambient of 20^oC, in a nutshell. There are other dynamics like outside air temp but you can likely use that as a seasonal average heat input requirement to base a design on. Any gas boiler will do that on its head, but a lot will not go down as low as that on their minimum setting ( max modulation ) and still be happy, so hence the buffer tank needs to be introduced to stave off short cycling ( where the boiler produces much more heat than the heating needs at any one time even when the boiler is set to its lowest setting ).

It's down to how much w/m2 the UFH can emit, at a set max ( safe ) flow temp, vs the heat the room requires to stave off the heat loss of that room.

As far as hot water is concerned, you could fit a Vaillant 938 and that will run two 'just about ok' showers simultaneously but will be 100% reliant on cold mains pressure / flow whilst showering. If someone flushes a loo or the washing machine fills, you're Donald Ducked. Only an UVC or thermal store will give the top rates of flow to two showers. As you need a buffer for UFH, you may be better off with a thermal store ( a buffer heated by the boiler which has an instantaneous DHW coil inside it ) and go that way, but without room for a cylinder you may as well go for a heat-store ( high-flow ) combi like the Vaillant.