Heat pump radiator sizing 50% larger is enough - fact of fiction

HI guys,I keep seeing and hearing designers and engineers saying that radiators should be 50% oversized for heat pump operation at 50deg C flow temp.Having looked at the manufacturer's data charts for different temperatures I can see this is not true.

Faral radiators would need to be over double the size to give the same output at a 50deg C flow against a flow of 75deg C. I have checked other data catalogues and they are all about the same. After all you cannot get more heat out than the surface area/water content/temperature and flow rate will produce.

I am sure many installations of air to water or water to water heat pumps will not work correctly if the 'mantra of 50% oversized is stuck to rigidly'. This will be true if the insulation values of the building are increased to suit the maximum output of the radiator at the new flow temperature.

In fact Faral quote an output per section of only 41% at a flow temp of 50deg C compared with a flow temp of 75deg C.

Is this why many systems are suffering during cold spells?

On ASHP and GSHP systems It would be best to avoid radiators altogether because of the required flow temperatures and stick to UFH where possible .
One of the main problems is the people installing these systems (primarily plumbers but that's for another thread) don't understand the fundamental operating characteristic differences between HP's and CH systems and see one as a direct replacement or alternative for the other.
Which more often than not leads to disastrous under sizing of HP systems ,Which it turn leads to astronomical running costs , COP's which are laughable and enduser's who are extremely p!ssed off.

Cheers

Stu

Hi Stu,

Just supplied an Altherma system to a guy, he sized his own rads to run at a flow temperature of 39deg C.

You should see the size of them! One is 28 sections long. It will definitely work, but I am glad i am not paying for the radiators!

Cheers

G

Hi G

That's a big rad alright .:eek:
Well if he's happy to pay for it so be it.
Just make sure you get your's too .;)

Cheers

Stu

Been paid already, been waiting since March for the plumber and owner to get their finger out:cool:. I hope to commission before the end of the decade:D

Dude, it doesn't matter what the heat source, a bigger rad will run at a lower temp. I know that a lot of UK houses when retrofitted with central heating used, to our logic over here, really small piping......10mm or something (we would retrofit with 25mm+), to do the retro cheaply and easily. You needed 75C to get any heat out.

The thing to remember is that at 75C, most of the heat will be convective heat which is not as efficient as radiant heat which occurs at a much lower temp. Rule of thumb is that the percentage of radiant heat (of the total heat emitted) goes up as you drop the surface temp and the the amount of convective (or less efficient) heat goes down as you drop the temp (as a percentage of total heat).

The point here is that it is not a simple linear relationship with temp and real heat output. Floor heat seems to do really well with very low temps and very low flow rates.

Good thing you are not in the opposite situation.....Just my 2 cents worth....

That's a good point Refrigologist. Like all rules of thumb it's only a starting point. Check data sheets carefully, particularly where 'equal or approved' or 'value engineering' type terminology is being used!

Also check the room temperature that's being referenced when selecting rads. I've seen outputs quoted at 18'c which drop off significantly when you extrapolate out to 21 or more.

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Originally Posted by MikeHolm

Dude, it doesn't matter what the heat source, a bigger rad will run at a lower temp. I know that a lot of UK houses when retrofitted with central heating used, to our logic over here, really small piping......10mm or something (we would retrofit with 25mm+), to do the retro cheaply and easily. You needed 75C to get any heat out.

The thing to remember is that at 75C, most of the heat will be convective heat which is not as efficient as radiant heat which occurs at a much lower temp. Rule of thumb is that the percentage of radiant heat (of the total heat emitted) goes up as you drop the surface temp and the the amount of convective (or less efficient) heat goes down as you drop the temp (as a percentage of total heat).

The point here is that it is not a simple linear relationship with temp and real heat output. Floor heat seems to do really well with very low temps and very low flow rates.

Good thing you are not in the opposite situation.....Just my 2 cents worth....

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I take your point, but Faral do quote lower and higher temperature figures in increments of 1deg C. Their catalogue quotes true output of radiator, so whether it is radiant heat or convective surely the outputs quoted by the manufacturer are the ones that should be taken note of. If we were to select a radiator based on a flow temperature of 75deg C with a 50% oversize and then operate at only 50deg C it would remain undersized regardless of radiant heat increase.

At the end of the day if the radiant heat output goes up but the customer requires 22deg C room temp and you can only achieve an air temp of 18 to 20 you will still be in trouble, no matter that it is allegedly possible to keep rooms at a lower temperature because radiant heat is more comfortable.

Try telling my wife that she should be feeling warmer due to the increase in radiant heat and the lower temperature shouldn't be a problem:D

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Originally Posted by Refrigerologist

I take your point, but Faral do quote lower and higher temperature figures in increments of 1deg C. Their catalogue quotes true output of radiator, so whether it is radiant heat or convective surely the outputs quoted by the manufacturer are the ones that should be taken note of. If we were to select a radiator based on a flow temperature of 75deg C with a 50% oversize and then operate at only 50deg C it would remain undersized regardless of radiant heat increase.

At the end of the day if the radiant heat output goes up but the customer requires 22deg C room temp and you can only achieve an air temp of 18 to 20 you will still be in trouble, no matter that it is allegedly possible to keep rooms at a lower temperature because radiant heat is more comfortable.

Try telling my wife that she should be feeling warmer due to the increase in radiant heat and the lower temperature shouldn't be a problem:D

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Haha, yes it is a real test to be able to convince someone that they will be comfortable with an air temp of 18 when they are used to seeing 22 on the t-stat but the radiant component is a hard one to measure.

We have always had t-stats with numbers on them here so when I can get a Danfoss or some other European t-stat with just a red and blue arrow, I am really happy. Some people are just not happy unless they see 22 on the dial even if we rig it to read false.;) (which we have done on occasion)

The 50% sizing rule, comes from 2 directions,
1; normal boiler is sized to run only a few hours a 10hrs( well it was was when I lived in the UK) so this run time must total the losses for 24 hours. With heat pumps they run 24 hrs (lets say 20hrs) so by interpretation the rads are now 2 * bigger than normal, plus they need to be further down rated because of the lower temps another 50%, so in fact your rads are 3 times bigger than a standard.
if you have flow temps higher than 40C, then heat pumps are not really the way to go.

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Originally Posted by mad fridgie

The 50% sizing rule, comes from 2 directions,
1; normal boiler is sized to run only a few hours a 10hrs( well it was was when I lived in the UK) so this run time must total the losses for 24 hours. With heat pumps they run 24 hrs (lets say 20hrs) so by interpretation the rads are now 2 * bigger than normal, plus they need to be further down rated because of the lower temps another 50%, so in fact your rads are 3 times bigger than a standard.
if you have flow temps higher than 40C, then heat pumps are not really the way to go.

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Mmmm. Food for thought, but if the heat loss at peak load is 2.0kW in a room and your '50% oversized' rad only produces 70% of your required output then your room will get colder due to the losses outweighing the input. Most boiler based systems coped with off times because they were so oversized in the first place. This enabled fast response times to cope with the heat losses and off times. Plus as everyone seems to agree most rads were oversized anyway for a boiler system. Heat pumps do not, or should not have that luxury of being so oversized.

How many people have pulled out a boiler with a capacity of 18 to 24 kW and replaced it with a correctly sized heat pump of half the capacity with correctly sized heat emitters? Plenty that I know of and they are working correctly. A friend of mine built his house in 1958, he extended it to double the size and is still using the same boiler. Now that is oversizing. And no he hasn't insulated it to part L just some loft lagging and double glazing.:D

Having looked at the data I am convinced it is not a good starting point unless the building insulation is being radically upgraded. But to pull out a boiler, install a low temperature heat pump and expect the original radiators to provide the correct amount of heat because they could have been 50% oversized is a dangerous game to play. And as we all know, those guys that tie their horses up outside every job will most likely use the 'rule of thumb' and may even take it as the gospel truth.

Maybe my wording was incorrect, my statement comes covers where the 50% rule of thumb comes from.
It comes from increased run time and lower temperatures.
When it comes to peak demand, you must also introduce thermal mass into the equation. You maybe short in heat production at peak load, but the reduction in actual temperature is dampened by the mass of the structure.
As much of the issue is determining the your design equalibrium point, a boiler will basically always produce its designed heat output, but a heat pump heat output reduces with lower ambients (lower SST and defrost) and increased water temps (increased SCT), so I see on many occassion that the designer has used the nominal heat pump output for peak demand when peak demand is at considerable detremental reduced heat pump output.

So if your peak demand is 10Kw at -5C then your heat pump output should 10kw at -5C, at the designed water flow rate temp. The nominal heat pump rating may well be 20Kw. ( unless you are relying on thermal mass "deep embedded underfloor" where loads can be diversified

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Originally Posted by mad fridgie

Maybe my wording was incorrect, my statement comes covers where the 50% rule of thumb comes from.
It comes from increased run time and lower temperatures.
When it comes to peak demand, you must also introduce thermal mass into the equation. You maybe short in heat production at peak load, but the reduction in actual temperature is dampened by the mass of the structure.
As much of the issue is determining the your design equalibrium point, a boiler will basically always produce its designed heat output, but a heat pump heat output reduces with lower ambients (lower SST and defrost) and increased water temps (increased SCT), so I see on many occassion that the designer has used the nominal heat pump output for peak demand when peak demand is at considerable detremental reduced heat pump output.

So if your peak demand is 10Kw at -5C then your heat pump output should 10kw at -5C, at the designed water flow rate temp. The nominal heat pump rating may well be 20Kw. ( unless you are relying on thermal mass "deep embedded underfloor" where loads can be diversified

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I understand all that you say. But if there is a prolonged cold spell and the heat emitters are undersized the thermal mass of the building will lose heat that wont be replaced and so the rooms get colder than they should be.

Heat loss is not lessened by having continuous operation if the output does not meet actual requirements. It would be the same if there were not enough pipe embedded in the floor. If the emitter is too small you can run the heat pump for as long as you like and unless you can raise the flow temperature it will make no difference. Tough if it is a wooden floor as I understand that the flow temperature should not exceed 40 deg C and the upper surface temperature should not exceed 27deg C.

It all comes back to the fact that if the heat emitter is not capable of providing enough capacity at peak low temperature the house will not get up to temperature.

Totally agree about sizing of the heat pump though, it needs to be looked at from the point of equilibrium, but that has always been the case even in straight air to air heat pumps. Of course there will come a point where the selection of a large heat pump is not cost effective, then it will require the use of a back up heat source. In the case of the Daikin Altherma a built in back up heater element. So a pretty standard choice these days as the manufacturers seem to have it sorted in the packages offered.

Heat loss is continuous, how ever over a 24hr period it does vary. With most boiler systems, were sized to cover the total loss of the structure over this 24 hour period with a relatively short run time say 10hrs, this includes the heat emitters. for example loss 10Kw so daily loss is 240Kwhr, boiler would and system would be sized at 24kw. this would give a daily thermal energy balance. So the rads are sized at 24kw at design flow. So if a heat pump was to run 24 hrs (not that I am recommending this, but solely as an example), the rads need to be down rated to meet the 10Kw load, this is achieved by dropping the flow temp. Even this reduced temp is still to high to achieve good COPs from a heat pump and low ambient. You need to further increase the surface area of the rads (more rads to be added)
Fundamental refrigeration principles determine that if you want good COPs then you must have low flow temperatures.
Using an element for auxiliary heating, maybe OK as a last resort, for the few hours a year when ambient drop below normal design criteria, any more than this is just bad design and cop-out by designers.

It is also about tempering the expectations of the end user. Another example I installed a combi underfloor (ground floor) and rad system (first floor) "supply flow temps 32C", house temps were perfect, but the client complained that his rads where not hot and he could not dry his clothes, in the same way his old boiler system did.
He then complained about his winter power bill, He was using 48kwhr per day for his heating. This is 350M2 house with windows covering 70% of the external walls. (and not quality triple glazed like in Europe)
I turned of my machine and gave him a 2 KW fan heater to heat his home (same power use) Within 2 days he was pleading with me to turn his system back on.
At the end of the day it was found that he had been on the net, read all the positives of heat pumps and never considered the negatives of the system. (he was a smart arse)
Heat pumps are the be all and end all, they are an excellent solution when applied right and when clients understand the limitations of such systems.
To me the best system for a water heating heat pump, is a deep embedded system with low temp, high flow rates, for background, with an overlay in living areas of surface mount piping (for rapid response) and a combination of PCM materials to cover the variants in load profiles.

Mad, have you ever found a customer willing to pay for double the tubing in the floor( deep and surface) as well as PCM walls? I suspect it would be an expensive system.

BTW, the price you pay for standard panel rads like Stelrad or Myson is half what we pay here per watt output. I bought a container of them once for an apartment building cause the price was so much better over there. Pretty cheap to put a few more rads in to get better performance from the HP.

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Originally Posted by MikeHolm

Mad, have you ever found a customer willing to pay for double the tubing in the floor( deep and surface) as well as PCM walls? I suspect it would be an expensive system.

BTW, the price you pay for standard panel rads like Stelrad or Myson is half what we pay here per watt output. I bought a container of them once for an apartment building cause the price was so much better over there. Pretty cheap to put a few more rads in to get better performance from the HP.

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Two words, Wall Heating. If you can't use UFH, use wall heating and if you can't use either, leave the job to an oil boiler, woodchip boiler etc. BTW Mike, are woodchip and log burners big in Canada? They're huge in Central Europe.

There are a lot of wood furnaces in rural areas but not internal wood boilers. There have been some spectacular house fires because people didn't take care of their chimneys so the insurance industry discourages them.

Our wood boilers are mostly the open type that sit some distance from the house and owners typically shove 100kg of green wood in them which pollutes like crazy. Some towns have banned them but they are pretty popular. Otherwise the only proper wood "boilers" come from Europe.

I installed a few Viessmann units 20 years ago and they were quite nice. They brought them in as a market test and I think I installed most of them...hehe...

Because of our stupid pressure vessel laws, all boiler water jackets must be welded and inspected by highly qualified and certified welders and inspectors and where you could get away with 1mm steel for example, we would need 3mm. None the less, there are a few manufacturers like Kunzl and Baxi who have spent the bucks to get their units certified. They would be mostly log burners anyway. Although we are a BIG supplier of pellets to Europe, most of our pellet heaters are space heaters, not boilers for the above reasons.

BTW, I have done wall heating on occasion but not usually necessary in the houses I work on.

Mad, you make a very good point about users expectations.

A home buyer will be used to 24kW boiler and Rads at 80/70 and will expect their 'new' system to be 'better'. That might mean to them: turn on the heating at 6am on a snowy winter morn and toasty and warm by 6.15. If you have a 10kW ASHP, that might mean leaving it on all night.

Funny you should mention drying the washing, I have had that complaint myself!