How do you calculate delta T for a heat loss calculation with UFH

[Adsibob]

The online heat loss calculator I'm using (https://starsapp.co.uk/basic-heat-loss-calculator/, recommended by @PeterW) requires the following input:

 

Specify the Delta-T (Δ-T) of your heating system. This is the difference between the Mean Water Temperature of your system and the ambient temperature of the room. If you’re not sure then leave it at 50°C in accordance with BS EN 442

 

 Given it's an app on a radiator site, isn't the assumed 50C only going to work for a radiator based system? What delta T figure shall I use for UFH? (My whole house is heated by water based UFH, apart from a couple of towel rads in the bathrooms, where we have UFH as well).

[Marvin]

Yes,  and a lower figure being between the temprature of the screed concrete around the pipes and the temprature of the water in the pipes. 

[Marvin]

UFH temp anywhere between 35 and 50 degrees C and floor temp 18C?

Say floor 20C. so Delta-T 15 to 30 

[Adsibob]

Hmmm, not sure how I’m going to do this. Ground floor is 68mm screed with 16mm pipes. First and second floor are 16mm pipes imbedded in XFLO extruded polystyrene boards, under a mixed variety of laminate, tile and engineered wood. Wouldn’t the screed and extruded polystyrene be the same temperature, or is the fact that they are on different floors relevant?

[Adsibob]

A bit of further research brought me to this page http://www.heatweb.com/techtips/Underfloor/underfloorheating.html which suggests floor temperature is much higher, around 27C. So if water is running at 42C delta T would be 15C. I'm way out of my depth though. Not really sure what i'm doing.

[Marvin]

The screed takes longer to heat up than the other floors. 

IMHO I would aim to keep screed floors at a more or less constant temperature. Others will tell you of their response time of their UFH.

 

The thermal reflective properties of the upper floor extruded boards will mean the heat will be felt quicker. The finish above the screed/ boards also affects the response result. This is in 2 ways: slower to heat up = slower to cool down. 

 

Yes the page you refer to lists the disadvantages as above.  

[Marvin]

11 minutes ago, Adsibob said:

I'm way out of my depth though. Not really sure what i'm doing

Water running through pipes in UFH is a lower temprature than for radiators so the surface of the pipe is cooler and produces less heat as the same amount of surface as a radiator. In order to achieve the same heat lots of pipe is run in the screed. Small radiator hot = lots of pipe cooler.

[Marvin]

I see your trying to find out the heat loss in a room.

 

I looked at the heat calculator your using. Your trying to find how much heat you need for your building! Use 50. Put in dimensions. The result will give you the heat required for the room watts or BTUs add up all the results of the rooms and there's your answer. The heat will be roughly the same using UFH or rads 

[Adsibob]

So I did the heat loss calculation with Delta T at 50C, putting in sizes of rooms, size and type of glazing, wall construction and approx* description of insulation and it came in at 23.7kw which is much lower than I was expecting., given this is a  3 bed 1930s semi converted into a 5 bed house. Heat requirement will be even less as it’s not factoring that I have installed 40mm EWI on all old external  walls. 
* there weren’t really enough options for my liking.

Edited October 16, 2021 by Adsibob

[SteamyTea]

Not really sure what you are trying to find out here.

Have you done a heat loss calculation on your place yet?

Or are you trying to do it with this radiator sizing we site?

[WWilts]

39 minutes ago, Adsibob said:

So I did the heat loss calculation with Delta T at 50C

Normally delta T is set at somewhere around 10 or 11 degr C
Not an expert, but I took it to mean the difference in temp between the water entering a circuit and the water leaving the circuit. ""
"Average temperature of the radiator [or circuit] minus the desired room temperature = Delta T"

LoopCAD does really good heat loss calculations, and you get a free trial for a month.

Edited October 16, 2021 by WWilts

[Adsibob]

13 minutes ago, SteamyTea said:

Not really sure what you are trying to find out here.

Have you done a heat loss calculation on your place yet?

Or are you trying to do it with this radiator sizing we site?

The latter. @PeterW recommended that site, and it’s pretty good, though won’t be as technical as LoopCAD I imagine. I’m only doing it as I need to buy a system boiler and don’t know what size to get. The range I’m getting though has a the highest modulation range on the market (1:17 ratio) so maybe oversizing by 6kw to be safe is the way to go. Not sure.

 Maybe I will try loopCAD. Does it take long to do on that? Do I need to have all my drawings in CAD to be able to do it?

[WWilts]

26 minutes ago, Adsibob said:

Do I need to have all my drawings in CAD

jpg accepted by LoopCAD.

I contacted a boiler manufacturer for sizing the boiler. They just took the floor area, assumed 70W/sqm for heating, added some W for hot water, and came up with a recommendation. Fairly rough and ready approach it seemed like.

[SteamyTea]

Never used LoopCad. Others on here have.

Maybe you should start a topic about it and get it turned I to a tutorial.

Could be useful to many people.

May need some help from the mods to keep it tidy.

 

I use Excel to calculate most things. By somewhere on here is a link to Jeremy Harris's heat loss spreadsheet.

 

Here it is  

 

[Adsibob]

2 hours ago, WWilts said:

jpg accepted by LoopCAD.

I contacted a boiler manufacturer for sizing the boiler. They just took the floor area, assumed 70W/sqm for heating, added some W for hot water, and came up with a recommendation. Fairly rough and ready approach it seemed like.

70W/m2 works out at only 14.2 kw for me. How much does one add to have enough to easily heat a 300L UVC? Just interested to compare how the 23.7kw figure which I got by using the online calculator compares to the 70W/m2 calculation.

[Marvin]

Calculating heating requirement:

There are so many variables to this conundrum IMHO even the most detailed analysis would only be able to give an approximate answer.

 

It reminds me of the question how many miles per gallon can my car do?

 

But, back to the building. Assuming you calculate exactly all the dimensions, and materials used in the building there are several other contributing factors for example:

What part of the country the property is in, what rotation it is ( facing north?) How high up, in a valley, close to other properties, shaded by trees, the standard of the construction, the maximum calculated internal heat required, the maximum external temprature assumed, how the occupants use the property, MVHR? solar gain? ( My home solar gain at some points in summer can produce more heat at those moments than required to heat the building at minus -8C in the depth of winter),

The equipment in the house, (I have now added the trickle power use of my home contents into the heating. This includes fridges, freezers, TVs,  Internet,) how many occupants, the list is endless...

 

But back to basics.

 

I think there is a list of significant items and the rest is a judgement call. I worked on the following and added 25% to the total because I did not want to be cold and I did not want the boiler (as it was when doing the calculation) to be running flat out in the dead of winter.

 

What I did was found out the thermal resistance of all the building elements.

Worked out how hot I wanted the building in winter.

Looked up the coldest recorded temprature where I live and made the assumed cold another 5 degrees lower.

 

These 3 give you the thermal envelope resistance of the building and the difference between the maximum cold outside and the warmth required inside. From these you can calculate the building heat requirement

 

The next thing was calculating the heat lost due to air replacement in the building. We have a MVHR unit.

I know the air flow and the assumed heat loss, so added this result to the heating requirement. 

 

Now we have a figure!

 

Unless the tool you are using to calculate the heat required uses info like your location it will only give you a CYA answer, so it will assume the worst situation and calculate using that. This is one of the reasons why you keep getting different answers.

 

Good luck.

 

Marvin.

 

 

[Marvin]

I think your original question mixes two questions:

 

How much energy is required to heat the building?

 

How much heat will the underfloor heating produce?

 

With reference to the latter the design of the pipe layout and the water temprature will fairly much dictate the result. Ignoring different water tempratures: the more pipe in the room the more heat produced, but as previously commented the floor construction and floor finishes are also other factors. Others here will know better how to deal with this.

M

[Adsibob]

Appreciate there is a fairly high degree of approximation. Will try Jeremy Harris’s spreadsheet next and see what figure that gives. How many kW extra do u need to add to one’s heating requirements for an unvented HWC? I read somewhere that the rule of thumb is to add 3kW, but that didn’tspecify what size cylinder that was for, which was odd. For 300L cylinder, how much should I add?

Edited October 17, 2021 by Adsibob

[SteamyTea]

15 minutes ago, Adsibob said:

Will try Jeremy Harris’s spreadsheet

I may not have linked to the latest versions.

15 minutes ago, Adsibob said:

How many kW extra do u need to add to one’s heating requirements for an unvented HWC? I read somewhere that the rule of thumb is to add 3kW,

It does rather, like most thing, depend on the size and insulation levels.

Have a look at the manufacturers blurb about heat loss, then double it.

[Adsibob]

4 minutes ago, SteamyTea said:

Have a look at the manufacturers blurb about heat loss, then double it.

2.32kwh/24hrs, so I need to add an extra 4.64kw. Thanks, very helpful.

[SteamyTea]

12 minutes ago, Adsibob said:

2.32kwh

 

12 minutes ago, Adsibob said:

4.64kw

kWh and kW

But think you meant kWh on the second one.

[WWilts]

On 17/10/2021 at 14:30, Adsibob said:

How many kW extra do u need to add to one’s heating requirements for an unvented HWC?

The boiler manufacturer I mentioned advised:
The cylinder would add an additional 8kW for the 250L cylinder and 9kW for a 300L cylinder.

[Adsibob]

1 hour ago, WWilts said:

The cylinder would add an additional 8kW for the 250L cylinder and 9kW for a 300L cylinder.

Interesting, there appears to be as many views on this as there are days of the week. What I find curious is that particularly in a house such as mine where apart from a couple of towel  heaters, there are no radiators, as it’s all UFH; and where the morning hot water requirements for three or four showers could be met by heating the 300L cylinder at 3:30am in the morning, when the UFH will be off, is it likely that the heating in the house will suffer if we give the cylinder priority over the UFH? UFH takes a while to cool down, so surely even in the depths of winter, and even in a house such as mine which only has half decent insulation (nothing close to passive), it can be switched off for an hour a couple of times a day to provide hot water priority to the cylinder. If that is the case, then I don’t see why I would need any extra for the hot water cylinder, but maybe I’m missing something.

Edited October 19, 2021 by Adsibob