epsilonGreedy

Radiator power output, relative to what?

Recommended Posts

Radiator manufacturers quote the "power" of different size radiators in BTU's and Watts. Does the industry use a defined setting at which the power rating is achieved e.g. room temp of 20 degrees and water circulating at say 60 degrees?

 

I ask because I am trying to calculate how many wet radiators I will need in addition to UFH and I am reassured to read that a middle size double panel radiator is rated at 1.5 Kw.

Share this post


Link to post
Share on other sites

it should list a "deltaT" usually 50 or 60 i.e the flow temperature is 50 or 60 degrees c above room temp, what type of heating system do you have? i presume you have something with a low flow temperature because you mention UFH, it is possible to use a correction factor to calculate the output at your selected flow temperature

Share this post


Link to post
Share on other sites

We have UFH throughout the ground floor 

I was amazed how small the radiators for upstairs were calculated 

All single panelled

We moved in in December and still not used the first floor heating

The UFH is great

But think the exstra insulation and airtightness plays a big part 

Share this post


Link to post
Share on other sites
Posted (edited)

+1 to delta T but beware it's not always linear.

 

As I understand it some rad are specified for "70/55" meaning an inlet temp of 70 and an outlet of 55 with an average of just over 60. So the delta T to a 21C room is just over 40C. However have heard some rads are specified at 90/70.

 

This article explains that doubling or halving the delta T doesn't double or half the power output of a radiator. It's because radiators work by both conduction and radiation. Radiation is proportional to T^4 I think...

 

https://www.castrads.com/frequently-asked-questions/buying-cast-iron-radiators/how-does-delta-t-affect-heat-output/

 

 

 

Edited by Temp

Share this post


Link to post
Share on other sites
8 hours ago, epsilonGreedy said:

Radiator manufacturers quote the "power" of different size radiators in BTU's and Watts. Does the industry use a defined setting at which the power rating is achieved e.g. room temp of 20 degrees and water circulating at say 60 degrees?

 

I ask because I am trying to calculate how many wet radiators I will need in addition to UFH and I am reassured to read that a middle size double panel radiator is rated at 1.5 Kw.

 

How much heating does your house require?

 

A single 1.5 kW output radiator would be enough to heat our whole 130m² house in very cold weather.

 

Like others, we've found that we definitely don't need heating upstairs, if anything we could do with a bit more cooling up there.

  • Like 1

Share this post


Link to post
Share on other sites

Our last house had a low temperature radiator / heat pump system, flow temp of 33C at 0C ambient. Worked perfectly well. The radiators were sized between 3 and 4 times larger than they would have been in a conventional CH system.  Dividing the stated output by that multiple would therefore give you a rough idea of what you will get output wise at UFH flow temps.

  • Like 1

Share this post


Link to post
Share on other sites

Appears different manufacturers use different flow/return/room temperatures when they spec their output powers.... 

 

Stelrad 70/55/20 

AEL 80/60/20

  • Like 1

Share this post


Link to post
Share on other sites
4 minutes ago, Temp said:

Appears different manufacturers use different flow/return/room temperatures when they spec their output powers.... 

 

Stelrad 70/55/20 

AEL 80/60/20

good luck with getting that temp drop across a stelrad tsd unit

 

 got very anal about this when i had  std heating in house 

spent many hours with 2 thermo couples on  the in+out to rads and played about with pump speeds +boiler temps + adjusting flow valves to balance whole system 

best i could ever get was 12c drop across the rads 
for upstairs in modern house --maybe electrical heating mats are the way for the small time you will use them ?

  • Like 1

Share this post


Link to post
Share on other sites
8 minutes ago, scottishjohn said:

good luck with getting that temp drop across a stelrad tsd unit

 

 got very anal about this when i had  std heating in house 

spent many hours with 2 thermo couples on  the in+out to rads and played about with pump speeds +boiler temps + adjusting flow valves to balance whole system 

best i could ever get was 12c drop across the rads 
 

 

Interesting... while looking at official radiator power ratings I had concluded just 3 mid sized radiators and no UFH would be enough to heat a 1600 sq ft house built to just 2013 thermal regs in all but the coldest snaps. This intuitively feels wrong. Given your best CH tuning endeavors could only achieve 60% of the rated input/output temperature drop maybe I should assume a similar real world performance factor.

Share this post


Link to post
Share on other sites
Posted (edited)
1 hour ago, JSHarris said:

How much heating does your house require?

 

 

I have not yet worked this out for my house design, so far I am using the House Builder's Bible model house 160 sqm which is a no frills rectangle. My house is 139 sqm in an L-shape and for the moment I am working with heating/thermal losses at the 2013 regs standard.

 

The HBB model house sheds 6 Kw of heat at a 20 degree delta and 3Kw in Passive house form. Once incidental gains are included the space heating demands are 4.7kw and 1.7kw respectively (bit of guesswork here because the table showing incidental gains is an annual KwH/Cost table.).

 

Looking at the elements of heat losses for the HBB model house built to 2013 regs I hope to improve in a few areas:

  1. If I can half the 2013/Passive performance gap for air leakage and ventilation that would equate to a 20% improvement.
  2. Thermal bridging accounts for another 39% of the remaining heat loss but he does not break this figure down so I am not sure what improvement I can make in this aspect of my brick & block build.
  3. The model house also has fairly pedestrian U values of 0.18/0.22/0.18 for attic/walls/floor.

So all in all I hope for a 30% improvement on the 2013 standard = 3.3 Kw space heating demand at a 20 degree delta.

 

Edit: Maybe not a 30% saving because the HBB 2013 house does not have UFH and using the @JSHarrisspreadsheet my UFH floor at 25 degrees with 150mm of insulation will loose 5.8% of its heat downwards.

 

1 hour ago, JSHarris said:

Like others, we've found that we definitely don't need heating upstairs, if anything we could do with a bit more cooling up there.

 

 

My hunch is a small towel radiator in each bathroom upstairs and a radiator in the master bedroom in my less than passiv house which will be 2 to 3 times more thermally leaky than yours.

 

 

Edited by epsilonGreedy
Forgot UFH losses.

Share this post


Link to post
Share on other sites

Why not model your actual house?

 

The simple heat loss spreadsheet I've uploaded here for others to use seems to be reasonably accurate as far as defining the heating requirement, even though it doesn't include incidental heat gain.  Others here have used it and reported back that it seems to be reasonably OK, and lots of people have now used it and reported back on how it's performed.

 

In case you've missed it, here it is: Heat loss calculator - Master.xls

 

We have heated towel rails in the bathrooms, run on a time switch circuit so they are only on for an hour or so morning and evening.  They do a reasonably good job, in that the towels are always warm and dry, and the bathrooms are never chilly, but it would be nice if the floor was warmed slightly in the bathrooms, just for comfort in bare feet.

Share this post


Link to post
Share on other sites
28 minutes ago, JSHarris said:

We have heated towel rails in the bathrooms, run on a time switch circuit so they are only on for an hour or so morning and evening.  They do a reasonably good job, in that the towels are always warm and dry, and the bathrooms are never chilly, but it would be nice if the floor was warmed slightly in the bathrooms, just for comfort in bare feet.

 

Done exactly this on one recently with UFH mat and rads on a timed circuit and contactor from the time clocks. 

Share this post


Link to post
Share on other sites
1 hour ago, epsilonGreedy said:

 

Interesting... while looking at official radiator power ratings I had concluded just 3 mid sized radiators and no UFH would be enough to heat a 1600 sq ft house built to just 2013 thermal regs in all but the coldest snaps. This intuitively feels wrong. Given your best CH tuning endeavors could only achieve 60% of the rated input/output temperature drop maybe I should assume a similar real world performance factor.

temp drop across the rad in real world will depend on how high the ambient room temp is.

sure if room was very cold you might get much bigger temp drop .

 

Share this post


Link to post
Share on other sites
46 minutes ago, JSHarris said:

Why not model your actual house?

 

 

I had a quick look last year and was daunted by its size, having looked at it this morning I now realize the mandatory data input in white spreadsheet cells is much less.

 

55 minutes ago, JSHarris said:

even though it doesn't include incidental heat gain.  Others here have used it and reported back that it seems to be reasonably OK, and lots of people have now used it and reported back on how it's performed.

 

 

Re. the spreadsheet's overall scope am I correct in thinking that thermal bridges are not part of the calculation?

Share this post


Link to post
Share on other sites
24 minutes ago, scottishjohn said:

temp drop across the rad in real world will depend on how high the ambient room temp is.

sure if room was very cold you might get much bigger temp drop .

 

 

True but am I correct in thinking the quoted manufacturer radiance power should have been measured at an ambient 20 degree?

 

So looking at the following you tuned your way to 80% of the Stelrad rated power and 60% of the AEL.

 

2 hours ago, Temp said:

Stelrad 70/55/20 

AEL 80/60/20

 

Share this post


Link to post
Share on other sites

I tried to keep the data input down to as little as was needed to get a reasonable estimate, without being overly tedious to fill in.  It is just a tool for estimating; I originally put it together to do quick "what if?" checks to see the relative impact of changing things, to allow more accurate trade offs between cost and performance to be done, back when I was designing the house.

 

It doesn't account for thermal bridging, for the simple (and selfish!) reason that I knew from the outset that our house was going to have thermal bridges designed out, no matter what build method we chose. 

 

For a more detailed model, that allows for incidental heat gain and thermal bridging, then SAP is sort of OK (not great for a very low energy house, though, IMHO), and PHPP is very good, but also very complex.

Share this post


Link to post
Share on other sites
Posted (edited)
29 minutes ago, JSHarris said:

It doesn't account for thermal bridging, for the simple (and selfish!) reason that I knew from the outset that our house was going to have thermal bridges designed out, no matter what build method we chose. 

 

 

Until this week I was not aware that thermal bridging was such a major factor. The HBB model house calcs estimate that thermal bridging accounts for 30% of heat loss in both the 2013 and Passiv standard versions of the model house.

 

Starting with a basic 2013 design the HBB shows the relative heat losses are:

  • 32% Thermal Bridging
  • 27% Air leaks & ventilation.
  • 16% Doors and windows
  • 14% Walls
  • 5% Roof
  • 5% Floor

 

Edited by epsilonGreedy

Share this post


Link to post
Share on other sites

If you look at SAP, then you can either put in the assessed value for psi (by calculation of the actual thermal bridges or from advanced details) or you can accept the default value.  SAP uses a  way of determining the total impact of thermal bridging, by assigning it a value called Y.  The Y value has the advantage that it can be compared with the U value for fabric elements, but that's pretty much the only thing it has going for it (IMHO, I'm not a fan of it).

 

The 32% figure you've quoted is a bit extreme, as I believe it most probably comes from the default value in SAP.  In reality, any sensibly designed new build will have significantly lower heat loss from thermal bridging, for the simple reason that it can be challenging to get the FEE down without moving away from the default psi values and either using those from the advanced details (bit of a fiddle, IMHO) or doing a proper analysis based on the designed structure (far and away the best method, and not hard to do).

 

FWIW, designing out all thermal bridges, including geometric bridges at angled external wall/roof/floor corners, isn't hard to do.  There are standard details for reducing thermal bridging to negligible levels available, anyway, so it's a bit of a mystery as to why they are neither more widely used nor more widely understood, especially as thermal bridging can present a slight condensation risk, especially in a very well insulated house, by creating cold spots.

  • Like 1

Share this post


Link to post
Share on other sites
47 minutes ago, epsilonGreedy said:

 

True but am I correct in thinking the quoted manufacturer radiance power should have been measured at an ambient 20 degree?

 

So looking at the following you tuned your way to 80% of the Stelrad rated power and 60% of the AEL.

 

 

its not a matter of "tuning my way"

i was looking for the biggest temp drop across the rad  

if you get that then you are getting the best from the rad .

maybe they have improved them over the years 

Share this post


Link to post
Share on other sites
5 minutes ago, scottishjohn said:

its not a matter of "tuning my way"

 

 

Not sure what else to call the process, you claimed you "got very anal about this" and entered into an iterative adjustment process that few new houses would benefit from.

Share this post


Link to post
Share on other sites
Just now, epsilonGreedy said:

 

Not sure what else to call the process, you claimed you "got very anal about this" and entered into an iterative adjustment process that few new houses would benefit from.

ALL houses need to be balanced   to get  the best from the system   ,especially with modern boilers so return temp of water is correct  and your plumber should do this on comisioning , not just wind the valves full open, feel the rad and say "thats working" -

and why it was anal was because i was attempting to get the temp drop the rad makers said   and I could not  even after many attempts  to balance upstairs with down stairs

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now