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Dont understand this term in a heat loss calc

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JohnMo Advanced Member

JohnMo

Posted
Posted
  On 07/09/2024 at 06:16, ReedRichards said:

 

Surely it's just about the amount of warmed water you have per unit area of floor?  The closer you make the pipes then the more warmed water per unit floor area so the more heat it can give out for any given water temperature.  Since what you want is a fixed amount of heat for any given outside temperature then the closer the pipes the cooler you can make the water so the more economically you can run your heat pump (provided it's not also having to provide heated water for radiators elsewhere in the building).

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But once you get below about 15-20W/m², we are talking around 2 degree difference in flow temperature, when changing from 300 to 100 centres. 

SteamyTea Advanced Member

SteamyTea

Posted
Posted
  On 06/09/2024 at 21:18, SBMS said:

For those of us not in the know, and who are setting out on UFH design with an ASHP can you summarise the point regarding centres and flow temp

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That is a good question.

 

Like all good questions, there is not a simple answer as there are several interactions taking place at one time.

 

The simplest answer is that it is down to the power delivery (the W/m2).  That can be increased or decreased in two ways, pump more energy in (closer pipes) or raise the temperature (for the same size pipes).

In reality though, which is where it gets complicated, are the physical characteristics of the systems.

You cannot easily have pipes sitting next to each other as that would need a huge manifold for dozens of pipes to connect to.

You cannot have too high a temperature as you would not be able to stand on the floor, and the efficiency of the heat source may suffer.

You cannot change the weather, which is very dynamic, so you have to accept some compromise (an emergency fan heater).

The temperature difference between the flow and return (in and out of the time) needs to be within a sensible limit (you cannot extract all the energy supplied), this limits pipe diameter and pump size.

 

The more complicated answer is that as temperature rise and fall is not linear, even of it looks like it at small temperature changes.  As temperatures rise, so do loses, as it falls, the opposite happens until you get to parity (Laws of Thermodynamics explains it all).

 

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JohnMo Advanced Member

JohnMo

Posted
Posted (edited)
  On 07/09/2024 at 07:43, ReedRichards said:

Sorry, you've lost me there.  

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Have attached an UFH design chart, it shows the relationship between pipe centres, heat demand and mean flow temp (average between flow and return).

 

Draw a line from pipe centres through heat demand W/m² and you get mean flow temp required with a floor coverings with a Tog value of 0.1 (different floor covering materials will add or subtract actually flow temp required, depending on chosen materials).

 

Notes:

You actually have to extend the chart downwards below 20W/m².

ASHP have a min flow temp allowed it is generally 25 degs. ASHP generally have a dT of 5.

 

Using 20W/m2 as an example

300mm centres requires a mean flow of 27. While 100mm centres a mean of about 25.

 

The difference in CoP from the heat pump is in the region of 0.15 advantage to 100mm centres over 300mm centres at -7 outside.

 

 

Maxa SCREED-UFH-SYSTEM.jpg

Edited by JohnMo
Nick Laslett Regular Member

Nick Laslett

Posted
Posted (edited)
  On 07/09/2024 at 06:38, SBMS said:

Yes, it was more about the cycling that JohnMo mentioned that I was trying to understand. 

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I just watched this video which tries to explain the relationship between cycling, volume of water, ASHP power and modulation. I like the guy, but he doesn’t do the greatest job of explaining the topic. 
 

 

You can see that balancing all these factors does require some insight and number crunching. More UFH pipe increases the volume in the system. The modulation data for a lot of ASHP can be hard to find or difficult to compare. Rule of thumb appears to be 30%. Excessive cycling can be an issue if your ASHP is massively over powered for your heating needs. Sometimes they will spec a more powerful ASHP to speed up the UVC reheat time. My take away from when I first read up on this topic was that excessive cycling will drastically shorten the life span of your ASHP, but this might be misleading information. 

John Cantor has a good video about ASHP cycling. 
 

 

Edited by Nick Laslett
JohnMo Advanced Member

JohnMo

Posted
Posted
  On 07/09/2024 at 08:53, Nick Laslett said:

modulation data for a lot of ASHP can be hard to find or difficult to compare. Rule of thumb appears to be 30%.

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Mine is supposed to modulate down to about 3kW, but it never seems to drop below about 5.2kW, but is delivering a COP above 6 when running and runs a reasonable time. Most heat pumps have setting you change which alter cycling time and frequency.

MikeSharp01 Advanced Member

MikeSharp01

Posted
Posted
  On 07/09/2024 at 10:34, JohnMo said:

At that flow temp and outside temp max is about 6.2kW, so modulating down to 84%.

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That sounds like a very small modulation range - whats the model, you obviously thought it would do down to around 50%, I guess it would cycle less if it could get down to 3.1kW but maybe there are other trade offs when modulating down that far at such low temperatures & small deltas although I cannot think what they might be.

SteamyTea Advanced Member

SteamyTea

Posted
Posted
  On 07/09/2024 at 11:20, MikeSharp01 said:

I cannot think what they might be

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There was a statement from someone that at very low RPM/Phase Change pressures that the pump seals can fail. 

You also find that when the modulation i.e. pressure difference, is very small, the refrigerant condenses/evaporates in the wrong part of the unit.

 

There is a lot made of gas boilers modulating to very low outputs, but I wonder how efficient they really are at the extremes.

I can easily modulate the power output of my car by driving up a steep hill, in 6th gear, with my foot flat to the floor.  Low power, extremely poor fuel efficiency.

(on that note, yesterday I got 73 MPG over 550 miles, which is about 600 Wh/mile.  Only about double what an EV uses)

Beelbeebub Regular Member

Beelbeebub

Posted
Posted

One thing to note about modulation is, a HPs output range is dependent on outside air temp.

 

The spec migbt say output 3-10kw (so 30% modulation) but it might modulate from 3 to 7kw at -2C but 5-10kw at 12C

 

As you're more likely to bump into cycling when it is warmer outside, the minimum output at 12C and max at -2C (ie 5 and 7kw or 70% modulation) are the more relevant measures. 

 

 

JohnMo Advanced Member

JohnMo

Posted
Posted
  On 07/09/2024 at 11:20, MikeSharp01 said:

That sounds like a very small modulation range - whats the model, you obviously thought it would do down to around 50%, I guess it would cycle less if it could get down to 3.1kW but maybe there are other trade offs when modulating down that far at such low temperatures & small deltas although I cannot think what they might be.

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It's a Maxa i32V5, same as a Viessmann 100A. 

 

Just started with a target 5 degs higher than normal, to see what happens - was pulling quite a large dT at start up, so heat pump went to max power trying to get either the flow temp or dT to target. I then started moving the target temp back towards the more normal. Moved the target temp down in one degree increments, after a couple of steps the heat pump tone changes and it runs a lot quieter and the heat pump started modulating downwards. Gets to about 5.2 kW

 

During a normal run, you see the heat pump adding and subtracting power to maintain a 5 dT. I think the floor will happily take 5 to 6kW or more, even on 300mm UFH loop spacings. Think the ASHP has just found it's happy place, it can control dT and is running at point where heat given is equal to the absorption of the concrete floor, which is about 50 to 60 Tonnes of concrete to heat.

 

Cycles are very controlled, certainly doesn't have short runs and restarts every few hours. Thick screed floors are just strange respond differently to a radiator or thin screed floor. More like a storage heater.

PhilT Regular Member

PhilT

Posted
Posted
  On 07/09/2024 at 14:07, JohnMo said:

Cycles are very controlled, certainly doesn't have short runs and restarts every few hours.

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It's quite surprising that not all modern brands/models have this capability built in as standard.

JohnMo Advanced Member

JohnMo

Posted
Posted
  On 07/09/2024 at 17:30, PhilT said:

It's quite surprising that not all modern brands/models have this capability built in as standard.

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I think most do. You just have to try hard to find some settings or combination of settings to suit each home and heat pump 

PhilT Regular Member

PhilT

Posted
Posted
  On 07/09/2024 at 18:04, JohnMo said:

I think most do. You just have to try hard to find some settings or combination of settings to suit each home and heat pump 

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that's the whole point you shouldn't have to try at all!. It should be built in as standard and operate in the most efficient way automatically, never cycling and never operating at less than the most efficient power level

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