Radiator Sizing with heat pumps???

"In My Opinion"
firstly I believe that common water radiators are not primarily devices that radiate heat, that are natural convention heating devices, more energy is transmitted to the heated space via convection than radiation!
If we look a standard radiator, with use with a boiler, the water is circulated lets say 60C (I know in many cases it is higher), the air temp is 20C, the rad has a total area of 1 squ meter and is rated at 4kw.
It would "SEEM" to make sense that the rad could be rated at 100w per C per m2 (100*40*1 = 4000w in the above case)
So if we had a heat pump that produced 40C water and we needed 4kw, it would SEEM to make sense that we would need double the surface area.
4000/100/20=2M2.
But where is the mistake?
Is the heat transfer coefficient going to remain constant. "NO"
Why not?
The water velocity is the same, the conductivity of the metal is the same, the ambient is the same!
What changes is the velocity and mass of the air that passes over the external of the rad.
The hotter the water, the more air is drawn over the rad, so more mass is heated and improving the heat transfer at the point of contact between the metal and the air (breaking boundry layers), so by reducing the water temp, you reduce the amount air, the temperature of air and reduce the effective heat transfer of the rad to the air.
So what would be the practical effect, we could say that the air mass flow is reduced by 50% and the flowing air temp has been reduced by 50%, The 4kw rad would now become a 1kw rad at 40C, the system can no longer reject the heat produced by the heat pump at 40C, either the heat pump will turn off, or the water must become warmer for the system to reach equalibrium, which would reduce the capacity of the heat pump whilst increasing the power draw of the heat pump (reducing the system COP)

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Originally Posted by mad fridgie

"In My Opinion"
firstly I believe that common water radiators are not primarily devices that radiate heat, that are natural convention heating devices, more energy is transmitted to the heated space via convection than radiation!
If we look a standard radiator, with use with a boiler, the water is circulated lets say 60C (I know in many cases it is higher), the air temp is 20C, the rad has a total area of 1 squ meter and is rated at 4kw.
It would "SEEM" to make sense that the rad could be rated at 100w per C per m2 (100*40*1 = 4000w in the above case)
So if we had a heat pump that produced 40C water and we needed 4kw, it would SEEM to make sense that we would need double the surface area.
4000/100/20=2M2.
But where is the mistake?
Is the heat transfer coefficient going to remain constant. "NO"
Why not?
The water velocity is the same, the conductivity of the metal is the same, the ambient is the same!
What changes is the velocity and mass of the air that passes over the external of the rad.
The hotter the water, the more air is drawn over the rad, so more mass is heated and improving the heat transfer at the point of contact between the metal and the air (breaking boundry layers), so by reducing the water temp, you reduce the amount air, the temperature of air and reduce the effective heat transfer of the rad to the air.

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Thats it in a nutshell MF. A radiator at 40C won't give off much more heat than yourself if you were walking round in the nip. You should only consider rads where the "rads" are fan coil hydronic convectors.

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Originally Posted by mad fridgie

The hotter the water, the more air is drawn over the rad, so more mass is heated and improving the heat transfer at the point of contact between the metal and the air (breaking boundry layers), so by reducing the water temp, you reduce the amount air, the temperature of air and reduce the effective heat transfer of the rad to the air.

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I have attached a power point page from my level 3 lecture on system balance principles. This page was on the very topic you raise.

If the height of the radiator is 1m then the difference in pressure of the column of air at the face of the radiator can be approximated for illustrative purposes thus...

The cooler room air across that meter height applies a downward pressure mgh where m is in fact the density. The warming air next to the radiator exerts a lesser downward pressure mgh because the warmer air is less dense. In fact force is mass x acceleration (mg) and per area the mg of the warm air is less than the mg of the cooler room air. The warmer air will accelerate upwards by the difference in the forces divided by the weight of the warm air, so (acceleration) a = Fdiff/m.

Pressure difference or pressure pushing the lighter air next to the radiator upward = dP = density of radiator air x gravity x height x(Trad - Troom)/Troom

For illustration - If you half the pressure difference between the two columns of air, the room air and the air against the radiators, you will accelerate the warmer air up at 75% of it's original speed.

If we reduce the air temperature against the rad from 70°C to 50°C (arbitrary numbers) then we have reduced the pressure from 2 Pascals to 1.23 Pascals which is about 60% which means the air flow will be about 77%. Then 77% of 60% is 46% - that is 77% of the original air velocity x 60% of the original TD gives us about 46% of our original radiator capacity.

Something like that - all very crude - but for illustration of the causal forces in play perhaps adequate :)

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Originally Posted by Bigfreeze

Thats it in a nutshell MF. A radiator at 40C won't give off much more heat than yourself if you were walking round in the nip. You should only consider rads where the "rads" are fan coil hydronic convectors.

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In NZ when I have installed a few rads on a heat pump system (two storey houses underfloor heating on ground and rads upstairs), i increase rads by 400%, in the the UK being colder and to balance better with the reduction in capacity at the lower ambients, i would increase by 600-800%.

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Originally Posted by mad fridgie

In NZ when I have installed a few rads on a heat pump system (two storey houses underfloor heating on ground and rads upstairs), i increase rads by 400%, in the the UK being colder and to balance better with the reduction in capacity at the lower ambients, i would increase by 600-800%.

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Increasing size by 100% is sufficient :)

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Originally Posted by DTLarca

Increasing size by 100% is sufficient :)

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Firstly I do not believe that 50C is a good temp for heat pumps. Look at the differnce in performance of a compressor at 2 different condensing pressures say 10C apart (40SCT and 50SCT as an example, lets say SST is -5C we talking about heaters in winter SH and SC remain constant)
I do like you presentation, but as stated it is basic, and it would seem to presume that the co-efficient of the rad remains constant.
The biggest single issue is that when it becomes colder, your house need more heat, but when it becomes colder, your heat pump produces less heat.
So when sizing your rads, you then have to allow for colder water enetering temperatures, which futher derates the rads. It is a question of where do you choose your equalibrium point, for the house/system.
A system design it is not a question of understanding the steady state of a system (this is where pure theory works well) but understanding the process variables, that deviate from the steady state design.
In most case this where excellent system fall over.

Example Copeland R410a
40SCT HOR(heating) 12.1Kw power draw 3Kw (COP 4.03)
50SCT HOR 11.5kw power draw 3.9Kw (COP 2.95)
Lets say a heating season of 100days and a daily heating load of 180Kwhrs
40SCT= Run hours 1488, power drawn = 4464kwhrs
50SCT= Run Hours 1565, power drawn = 6104kwhrs
Difference= 1640.
Adds up and you would be more cosy (with big rads)

Pictures would allow the uninitiated members members from warmer climes, to contribute.

Natural convection (laminar, turbulent) versus forced convection, on the air-side - that makes a huge difference to the external heat-transfer coefficient... :)

@ Marc,

You may want to refer to later texts for the laminar/turbulent equations for natural convection flow for air over vertical surfaces. A good one is "Convection heat transfer", Adrian Bejan.

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Originally Posted by desA

@ Marc,

You may want to refer to later texts for the laminar/turbulent equations for natural convection flow for air over vertical surfaces. A good one is "Convection heat transfer", Adrian Bejan.

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Des, that would be something to do one day when I have retired - as they say :)

Not really - perhaps next year after I have completed a great list of many other things.

I wanted to give only a basic reply to the subject of radiator performance adjustment coefficients.

I wanted to show that first, yes, we have the reduced TD but also that Mad Fridgie has reason to be confident that the reduced air flow rate is the next major contributing factor as he had taken care to point out himself.

If you have 60% of the pressure difference on account of reduced air density difference then you will have (0.6)^1/2 = 77% of the air velocity and therefore somewhere along the same proportions a reduction air volume - and capacity is also related to air volume as in Q = m C dt.

I wanted to show that the guesstimates I used kind of agree with the index of 1.4 used by radiator manufacturers.

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Originally Posted by DTLarca

Des, that would be something to do one day when I have retired - as they say :)

Not really - perhaps next year after I have completed a great list of many other things.

I wanted to give only a basic reply to the subject of radiator performance adjustment coefficients.

I wanted to show that first, yes, we have the reduced TD but also that Mad Fridgie has reason to be confident that the reduced air flow rate is the next major contributing factor as he had taken care to point out himself.

If you have 60% of the pressure difference on account of reduced air density difference then you will have (0.6)^1/2 = 77% of the air velocity and therefore somewhere along the same proportions a reduction air volume - and capacity is also related to air volume as in Q = m C dt.

I wanted to show that the guesstimates I used kind of agree with the index of 1.4 used by radiator manufacturers.

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I do not have very low temp data from rad manufactures, only seen data from 80C to 60C, I maybe wrong on my sizing (i have not gone into in so much detail) if i remember rightly, it was not a straight line reduction.
Thanks for the input.

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Originally Posted by DTLarca

Des, that would be something to do one day when I have retired - as they say :)

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It's not overly difficult. Should be light bedtime reading. :)

From producers catalog take a capacity for a 80/70or whatever is given. Than tho formula looks like this:
q=qn*(dT/F)^m
q-capacity for given conditions(temperature regime and room tempreature)
dT=((Tsupply+Treturn)/2)-Troom

F = ((Tsupply+Treturn)/2)-Troom but for a heat pump regime.
m is coefficient which is mainly for radiators 1.39. This exponent should be given by radiator producer.
The flow for pipe dimensioning is than installed capacity and DT if there is no thermostatic valves. If thermostatic valves are installed than the flow shall be calculated according to the heat loss

I have been to many dwellings where there has been a change from gas to GSHP as an exercise funded by government for local authorities. They didn't change the rads or any of the original central heating system. fitting a GSHP which delivers up to 65C out put temp. ANd, quite surprisingly they are very adequate in performance. The occupiers are happy a with the lower physical outputs from the rads, just that they have learned to use the system more conservatively by having the system on for longer and sooner.

OK guys, time to throw a monkey wrench into the works.

I put a lot of old style cast iron rads in houses and i prefer them to the new style "stelrad, Myson or equiv" and in the 200+ houses I have done this in, I can run the boiler at a lower temp given the equivelant sq m of radiation of both types of rads.

We have 3 types of heat transfer to deal with, conduction, convection and radiation but no one here is talking about the gradual increase in radiant energy output (which is a more efficient type of transfer than convection) over convective as the heat emitter temp drops. Absolutely, given something with the limited surface area of a panel rad we will have to rely on the less efficient convection, for which we need higher temps just to get the convection started (unless you power it), so I think this needs to be added to the mix.

Don't know the math but I do have the experience (working on the math though).

BTW, I have a target maximum supply temp of 50C for any heating system I do with boiler and that is at -20 outdoor.

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Originally Posted by MikeHolm

OK guys, time to throw a monkey wrench into the works.

I put a lot of old style cast iron rads in houses and i prefer them to the new style "stelrad, Myson or equiv" and in the 200+ houses I have done this in, I can run the boiler at a lower temp given the equivelant sq m of radiation of both types of rads.

We have 3 types of heat transfer to deal with, conduction, convection and radiation but no one here is talking about the gradual increase in radiant energy output (which is a more efficient type of transfer than convection) over convective as the heat emitter temp drops. Absolutely, given something with the limited surface area of a panel rad we will have to rely on the less efficient convection, for which we need higher temps just to get the convection started (unless you power it), so I think this needs to be added to the mix.

Don't know the math but I do have the experience (working on the math though).

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Have seen this in action too. Old cast iron rads seem to outperform standard rads.

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Originally Posted by nevgee

I have been to many dwellings where there has been a change from gas to GSHP as an exercise funded by government for local authorities. They didn't change the rads or any of the original central heating system. fitting a GSHP which delivers up to 65C out put temp. ANd, quite surprisingly they are very adequate in performance. The occupiers are happy a with the lower physical outputs from the rads, just that they have learned to use the system more conservatively by having the system on for longer and sooner.

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This simply can't be cheaper than oil. At those temps and if I'm generous the unit will have a COP of 2. So for every kilowatt of energy produced it would cost the user 9cent electricity (i'll use irish figures). A litre of oil costs 73cent. If you have a boiler burning at 90% efficiency, that would mean it was costing you 8.1 per kw. So if 20,000kw was produced over the year the costs would stack up as follows
Heatpump €1800 not including costs for defrosts
Oil €1629
Gas would be even better

I haven't included night rate but I'm being quite generous at a COP of 2.
I fail to understand how anyone would be happy with a heatpump at those levels. Unless it was going to cut my bills by 60-80% i wouldn't even consider it

I love those cast iron rads, hate moving them though. So many people rip them out to put in "scorched air" heating. Drives me nuts.

DON'T ANY OF YOU SMART EUROPEANS EVER GET MIXED UP IN USING DUCTED AIR FOR HEAT DISTRIBUTION. LOWEST COMMON DENOMINATOR HEAT, REGARDLESS OF THE SOURCE.

Sorry i had to yell that one out.

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Originally Posted by nevgee

I have been to many dwellings where there has been a change from gas to GSHP as an exercise funded by government for local authorities. They didn't change the rads or any of the original central heating system. fitting a GSHP which delivers up to 65C out put temp. ANd, quite surprisingly they are very adequate in performance. The occupiers are happy a with the lower physical outputs from the rads, just that they have learned to use the system more conservatively by having the system on for longer and sooner.

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What has been found, is that plumbers over size rads anyway, plus take into account that a boiler system, is designed to run 8-10 hours a day (to delievery enough energy lost over 24hours per day) a ground source system will say run 20 hours day, so effectively you have halved the rad capacity. If we also accept the GSHP, tend to have a more constant energy source compared to that of ASHP. A 24 hour energy equalibrium is reached.
It would seem that on new installs the time factor has already been taken into considderation, then they are matching the rad to the output of the heat pump at max temp outlet conditions.
(retrofit info came from Worchester Bosch, when i was over in the UK)

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Originally Posted by Bigfreeze

............... if I'm generous the unit will have a COP of 2. ............

I haven't included night rate but I'm being quite generous at a COP of 2........................

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I not sure how you calculated a COP of 2, however, these are the manufacturer's published figures :-


SOURCE WATER/BRINE ON 0ºC*

OUTPUT TO WATER AT 55ºC# kW 2.80 3.60 8.10 12.00
ELECTRICAL INPUT kW 0.76 1.1 2.6 3.9
COP 3.6 3.27 3.1 3.08

OUTPUT TO WATER AT 35ºC# kW 3.40 4.80 8.30 12.50
ELECTRICAL INPUT kW 0.75 1.06 2.01 2.9
COP 4.5
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The project was wholy government funded and carried out by a local authority into their properties. In that respect there was no question of the capital cost implication.

I was was just pointing out that regardless of the cost, these systems can work satisfactory at reduced output temps. :D

Wasn't having a go at your statement nevgee, just I know from experience that there is no payback in a system working to these conditions.
The figures you printed there are not completely right as I fail to see what a water/brine temp of 0 has to do with an air to water heatpump.
The next figure is different from the COP of 2.6 I found on the Daikin website but thats at an air temp of 7C, if you work to the european standard of 2C that would be closer to a COP of 2.
To me, it doesn't make sense to fork out 8-10k for a heating system that will cost as much if not more than oil to run, cost far more to fix if something went wrong and actually creates more CO2 than burning oil because of the inefficiencies in the way electricity is produced.
Running a heatpump at high temp makes no sense to me at all and I would never sell one to a customer. If you want to "go green" on a high temp system then install a wood pellet boiler or a micro chp. Heatpumps are not suited to the application

I was refering to ground source heat pumps.

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Originally Posted by Bigfreeze

Wasn't having a go at your statement nevgee, just I know from experience that there is no payback in a system working to these conditions.

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No offense taken.

I was refering to Calorex ground source heat pumps. Granted they have to drill a 70m borehole which in its self isn't cheap. I really don't think the economics are sound but as I said, its a gov funded scheme, so I guess instal cost becomes irrelavent!

Putting that to one side, I've not heard any grumbles from the people who have to live with the new kit. generally they seemed happy and were pleased with the running costs compaired to their previous fuel. Now I don't know if they were on gas, quite possibly they were all electric, with old style fan blowers. So in that case there would be a significant saving.

Really though I guess if the numbers are crunched correctly then it's "horses for courses" some will be ok others maybe not.

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Originally Posted by nevgee

I was refering to ground source heat pumps.

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True, ground source would be better but then you're getting into even more expense. And you're also shortening the life of the equipment by about 50% by constantly running at 55C.
I get where you're coming from and there are slight benefits to using a GSHP in this arrangement but personally I don't think you should ever look for more than 40c on heating mode with a heatpump. The payback would be close to 15years in that scenario.

Again, I'm not arguing against your point that they work and I know cost didn't come into it in the installations you saw due to government grants. I'm just looking at it from a consumers point of view, who would be stumping up cash.

Regarding the cast radiators....

The heat output of those radiators is a bit better also because of cast structure....But the old systems are mainly over sized and when the outdoor sensor is not working.....they heat quite well actually...