overlay UFH or Rads for 1980's house

[SuperV8]

Hi,

Have been reading many topic's regarding UFH and to basically not to bother unless you don't have a very well insulated slab.

My project:

1980 small detached house - concrete slab floor, filled cavity walls, recent double glazing, 300mm loft insulation.

I have done a heat loss survey using heat engineer - which comes in around 4200w heat loss on a -3.5degC day. lower than I was thinking, mainly helped I think as it's not a large house at 76m2 total floor area.

I am fitting a new kitchen and removing a stud wall so now (if ever) would be a good time to fit an overlay UFH system - the type you can tile directly onto. They are 20mm think panels - I only have about space for 45mm total floor increase available due to front door opening in. Can't go to the length of digging up floors (doing this around my small family)

Current system is gas system boiler/rads and hotwater heat store. Looking at going to a heatpump in the near future.

 

I have gone through the UFH loss excel sheet shared elsewhere on here which looks like this:

using 20mm XPS/aluminium UFH panels at 0.035W/m.k

My heat loss survey said my downstairs rooms would required 2200w on -3.5deg day.

 

So not great - loosing 900w into the floor!

Is this 900w per hour so I can calculate by the ASHP COP and kW/h electricity cost to work out cost per hour?

But with only 27 deg floor temp and delta T of 5 that gives a flow temp of 28.5? so a heatpump should be very efficient at that low temp.

 

I am ultimately bound by running costs.

I know fitting large rads would loose less heat into the floor - but how do I calculate the actual running costs UFH vs rads?

Even though I loose more heat through the floor - if this heat is provided more efficiently it could still work out cheaper?

 

Any thoughts/help greatly appreciated.

 

Thanks,

Tom.

 

[JohnMo]

The temp in the spreadsheet will be the temperature your feet feel, not really much to do with flow temp. The flow temp is a product of upward, downwards and sideways flow. Look at the attached charts and have a play, even at 100mm centres you will be over 40 mean flow. (That assumes hard tile floor not carpets) You can design radiators for a lower flow and then way less downwards heat loss.

 

If you really want to spend money on your heating - install radiators and post £5 per day to me that will make both of us happy.

 

Yes 900W per hour at a cop of about 2.5, would be £2.50 per day just on downwards heat loss. Plus the difference between flow temp of 45 and 35 is a cop of 0.6, hence me asking for £5 a day. 

 

Put the insulation down, but not the pipes, put nice radiators designed for a DT 15, so max flow temp is closer to 35 instead of 45 plus.

[SuperV8]

Thank you for the detailed reply.

Right - i'll scrap the UFH idea and concentrate on rads.

 

Looking at rads, they seem to be listed in watts at DT50 - I've found a conversion factor to divide by 0.21 to get to DT15 output.

I have 900 and 1140watts heatloss in my kitchen and living room respectively - which would mean rads rated at DT50 of around 4300w and 5500w which seem huge!

 

Unless i'm missing something - (highly likely!) is the radiator DT different to system DT? 

[SuperV8]

I think I was confusing DT! and deg!

 

from here:

https://www.clyderadiators.co.uk/delta-t-conversion

Quote

 

What is Delta T?

Delta T, or ∆T, specifically relates to the difference in temperature between the water circulating in the central heating system and that of the ambient or room temperature. If the ambient or room temperature is 20ºC and the mean water temperature inside the radiators is 70ºC, the Delta T or ∆T value is calculated as 70 ºC - 20 ºC = 50ºC.

Delta T 50° is the UK standard for all domestic gas boilers and is set to allow professionals, end-users and consumers alike, to make a fair and reasonable comparison of hydronic radiator and towel rail outputs from various suppliers and across a mix of product types.

 

 

so using the 35deg recommendation  

 

- the conversion factor should be 0.63!

Which makes my heatloss:

900watts = 1430

1140watts = 1810

 

So I need to look for 1430 and 1810ish DT50 rads - which seems much more reasonable.

 

 

[crooksey]

Why not just replace the front door and raise the floor height further, is that an option? Have you got enoguh height in the doorways to add 140mm (this would be 70mm PIR, screed and finished flooring)?

[TonyT]

https://trystanlea.org.uk/heatpumpradiators
 

some interesting reading from someone who works in monitoring, self installed a heat pump too.

 

 

[SuperV8]

56 minutes ago, crooksey said:

Why not just replace the front door and raise the floor height further, is that an option? Have you got enoguh height in the doorways to add 140mm (this would be 70mm PIR, screed and finished flooring)?

I installed lovely new composite front and rear doors 2 years ago - so won't be changing these anytime soon.

 

I did 'briefly' think about digging up our floors - but this is simply not feasible whilst living in the house with my young family. I am up for a challenge but this would be a step too far!

 

[SuperV8]

50 minutes ago, TonyT said:

https://trystanlea.org.uk/heatpumpradiators
 

some interesting reading from someone who works in monitoring, self installed a heat pump too.

 

 

Thanks, i'll have a read.

[crooksey]

1 hour ago, SuperV8 said:

I installed lovely new composite front and rear doors 2 years ago - so won't be changing these anytime soon.

 

I did 'briefly' think about digging up our floors - but this is simply not feasible whilst living in the house with my young family. I am up for a challenge but this would be a step too far!

 

 

You are clearly very concered on heat loss, if the front door is the only point holding you back on going for UFH, its worth considering.

 

If you are going for radiators, you could consider a H5 panel with a wood burner boiler, this will take excess heat out of the wood burner and use it to "top up" the output from your ASHP. I considered this for a long time (even purchased the panel) but then decided to just put new PIR, UFH and screed down. Pleased I did it now as can get away with 35C flow temps from the heat pump even at -3 outside and maintain 25 degrees inside.