Radiator sizing with ASHP?
Discussion
Moving from oil boiler to ASHP in a few weeks, as the existing system is on its last legs.
A couple of the radiators will be marginal at the lower temps of a heat pump.
The one in the kitchen diner is a double panel vertical Milano Alpha, 1780h x 340w.This style is available in various widths, so it would be relatively easy to just get a wider version.
The specs show heat output at T30, T50 and T65.
What T number should I work off to match or exceed the heat output of the existing setup, which is running at around 65 degrees at the inlet?
From what I can make out, ASHP works best between 35 and 45 degrees. Is that the case for best economy?
If so, for comparison, should I go halfway between the T30 and T50 values?
It's a Mitsubishi Ecodan 5kw unit going in.
A couple of the radiators will be marginal at the lower temps of a heat pump.
The one in the kitchen diner is a double panel vertical Milano Alpha, 1780h x 340w.This style is available in various widths, so it would be relatively easy to just get a wider version.
The specs show heat output at T30, T50 and T65.
What T number should I work off to match or exceed the heat output of the existing setup, which is running at around 65 degrees at the inlet?
From what I can make out, ASHP works best between 35 and 45 degrees. Is that the case for best economy?
If so, for comparison, should I go halfway between the T30 and T50 values?
It's a Mitsubishi Ecodan 5kw unit going in.
thats not how you do it
how may (heating) watts do you need to heat the room your talking about?
then match that number as close as you can to the t30 number as the difference between the delta t's isnt linear
but all this should be specified by the installer of the heatpump as obviously they have fully done a heatloss survey
how may (heating) watts do you need to heat the room your talking about?
then match that number as close as you can to the t30 number as the difference between the delta t's isnt linear
but all this should be specified by the installer of the heatpump as obviously they have fully done a heatloss survey
Yes, they did do the survey, and pointed out that 2 of the radiators were likely to be too small.
The one in the lounge doesn't matter too much, as there's a woodburner in there.
The one in the kitchen/diner can be pretty borderline on really cold days with the oil boiler, so that one I'll get replaced.
I'm unclear about what T number I should be working from though to get the watts I need.
The existing radiator is 1559 watts at T65, 974watts at T50, and 498watts at T30
The biggest replacement is 2598 watts at T65, 1832 watts at T50, and 928 watts at T30
The one in the lounge doesn't matter too much, as there's a woodburner in there.
The one in the kitchen/diner can be pretty borderline on really cold days with the oil boiler, so that one I'll get replaced.
I'm unclear about what T number I should be working from though to get the watts I need.
The existing radiator is 1559 watts at T65, 974watts at T50, and 498watts at T30
The biggest replacement is 2598 watts at T65, 1832 watts at T50, and 928 watts at T30
Just had this done as part of an ASHP install. As others have said the installer should specify this as part of the heat loss survey etc.
The only conversation I had about radiators was what size they were and where they were sited in the room. The specification was all down to the installer.
The only conversation I had about radiators was what size they were and where they were sited in the room. The specification was all down to the installer.
clockworks said:
Yes, they did do the survey, and pointed out that 2 of the radiators were likely to be too small.
The one in the lounge doesn't matter too much, as there's a woodburner in there.
The one in the kitchen/diner can be pretty borderline on really cold days with the oil boiler, so that one I'll get replaced.
I'm unclear about what T number I should be working from though to get the watts I need.
The existing radiator is 1559 watts at T65, 974 watts at T50,
The biggest replacement is 928 watts at T30
So if your currently running at T68 temps which would be common for an oil fired system, even the biggest is going to be a good bit smaller and you might need two of the new ones! (or three of the old size)The one in the lounge doesn't matter too much, as there's a woodburner in there.
The one in the kitchen/diner can be pretty borderline on really cold days with the oil boiler, so that one I'll get replaced.
I'm unclear about what T number I should be working from though to get the watts I need.
The existing radiator is 1559 watts at T65, 974 watts at T50,
The biggest replacement is 928 watts at T30
I would also certainly get the one in the lounge swapped over, you dont want to have to light the wood burner every day for 3months of the year, and can always turn the heating down
Within reason, you cant go too big with a heat pump, and you would probably best off changing a good number of the rads for a 50-100% bigger one.
Unfortunately I can't find the heat loss calcs and original quote. It was over 4 months ago, I put it "somewhere safe"....
I've done a couple of calcs online, comes back at around 1100 watts. This fits with the heat geeks "cheat sheet" number for the property type and room size, and the ASHP size specced by the installer.
When I said that the existing radiator was borderline, that's more about the time it takes to warm up in the morning. As I understand it, I wouldn't ever shut an ASHP off like I do with the oil boiler. The kitchen/diner heating is currently off from 18:30 until 06:00.
At T30, the biggest same design radiator will be undersized when it's really cold.
At 40 degrees, it would be fine (I think it would output around 1350 to 1400 watts at 40 degrees).
So, does an ASHP really only run at 30 degrees? Or, is 40 degrees OK?
Also, bear in mind that I'm in West Cornwall, and the outside temp rarely drops down to frost on the car levels - maybe 3 or 4 nights per year.
I've done a couple of calcs online, comes back at around 1100 watts. This fits with the heat geeks "cheat sheet" number for the property type and room size, and the ASHP size specced by the installer.
When I said that the existing radiator was borderline, that's more about the time it takes to warm up in the morning. As I understand it, I wouldn't ever shut an ASHP off like I do with the oil boiler. The kitchen/diner heating is currently off from 18:30 until 06:00.
At T30, the biggest same design radiator will be undersized when it's really cold.
At 40 degrees, it would be fine (I think it would output around 1350 to 1400 watts at 40 degrees).
So, does an ASHP really only run at 30 degrees? Or, is 40 degrees OK?
Also, bear in mind that I'm in West Cornwall, and the outside temp rarely drops down to frost on the car levels - maybe 3 or 4 nights per year.
clockworks said:
So, does an ASHP really only run at 30 degrees? Or, is 40 degrees OK?
Also, bear in mind that I'm in West Cornwall, and the outside temp rarely drops down to frost on the car levels - maybe 3 or 4 nights per year.
The flow temp depends on age of property, how well it is insulated, how big the rads are and how big the pipes are. Also, bear in mind that I'm in West Cornwall, and the outside temp rarely drops down to frost on the car levels - maybe 3 or 4 nights per year.
To give you an idea I live in a period property….system is designed around 40 deg flow temp. We’ve got big rads, big pipes though recently installed double glazing, well insulated loft etc.
There is no definitive answer.
I suspect as things bed down we’ll end up with a lower flow temp (system has only just been installed)
Cheib said:
The flow temp depends on age of property, how well it is insulated, how big the rads are and how big the pipes are.
To give you an idea I live in a period property….system is designed around 40 deg flow temp. We’ve got big rads, big pipes though recently installed double glazing, well insulated loft etc.
There is no definitive answer.
I suspect as things bed down we’ll end up with a lower flow temp (system has only just been installed)
What I've been reading just now on the Mitsubishi website suggests that the ideal flow temp is 35 to 45 degrees, so 40 degrees seems like a good starting point.To give you an idea I live in a period property….system is designed around 40 deg flow temp. We’ve got big rads, big pipes though recently installed double glazing, well insulated loft etc.
There is no definitive answer.
I suspect as things bed down we’ll end up with a lower flow temp (system has only just been installed)
At 40 degrees flow the bigger radiator that I'm thinking of using should be outputting around 1350 watts, which should be fine.
I'm probably overthinking this. The plumbing manager and electrical manager are coming to do the final pre-installation survey next Friday, so I'll discuss any upgrades with them.
clockworks said:
Cheib said:
The flow temp depends on age of property, how well it is insulated, how big the rads are and how big the pipes are.
To give you an idea I live in a period property….system is designed around 40 deg flow temp. We’ve got big rads, big pipes though recently installed double glazing, well insulated loft etc.
There is no definitive answer.
I suspect as things bed down we’ll end up with a lower flow temp (system has only just been installed)
What I've been reading just now on the Mitsubishi website suggests that the ideal flow temp is 35 to 45 degrees, so 40 degrees seems like a good starting point.To give you an idea I live in a period property….system is designed around 40 deg flow temp. We’ve got big rads, big pipes though recently installed double glazing, well insulated loft etc.
There is no definitive answer.
I suspect as things bed down we’ll end up with a lower flow temp (system has only just been installed)
At 40 degrees flow the bigger radiator that I'm thinking of using should be outputting around 1350 watts, which should be fine.
I'm probably overthinking this. The plumbing manager and electrical manager are coming to do the final pre-installation survey next Friday, so I'll discuss any upgrades with them.
Possibly we have some confusion about 'delta T'.
There are two 'delta T's.
The delta between the (mean) temp of the radiator and the room determines the heat output.
The delta between flow and return water temperatures, multiplied by the flow rate determines the heat input.
In the simple steady state case, flow rate is adjusted to get the heat required,
The mean temp of the rad will be somewhere between flow and return temp, so when a rad is throttled, 40 flow might give a 24 return, so a mean temp of about 32.
The delta between rad and room is then about 12 and the delta between flow and return is about 16.
Max power, unthrottled, the flow return delta is small and the rad/room delta is big.
Don't forget that at the boiler/heat pump end, it's more efficient to have a low return temp.
Which boils down to bigger rad is more efficient.
A very big rad with a low flow temp is more efficient still, but you need to shift a lot of water.
When it comes to your annual energy spend, average efficiency on average days matters.
But the system has to perform in 'Blizzard from Russia' conditions, then the finer points of efficient may be less important than 'effective'.
There are subtleties, the more you think about it, when each rad in the house is throttled differently and contributes more or less water at a differnt temp to the retrun stream to the heatpump or boiler. I'm not sure balancing the rad sizes is the ideal answer, but it's the easiest one to understand IMHO.
There are two 'delta T's.
The delta between the (mean) temp of the radiator and the room determines the heat output.
The delta between flow and return water temperatures, multiplied by the flow rate determines the heat input.
In the simple steady state case, flow rate is adjusted to get the heat required,
The mean temp of the rad will be somewhere between flow and return temp, so when a rad is throttled, 40 flow might give a 24 return, so a mean temp of about 32.
The delta between rad and room is then about 12 and the delta between flow and return is about 16.
Max power, unthrottled, the flow return delta is small and the rad/room delta is big.
Don't forget that at the boiler/heat pump end, it's more efficient to have a low return temp.
Which boils down to bigger rad is more efficient.
A very big rad with a low flow temp is more efficient still, but you need to shift a lot of water.
When it comes to your annual energy spend, average efficiency on average days matters.
But the system has to perform in 'Blizzard from Russia' conditions, then the finer points of efficient may be less important than 'effective'.
There are subtleties, the more you think about it, when each rad in the house is throttled differently and contributes more or less water at a differnt temp to the retrun stream to the heatpump or boiler. I'm not sure balancing the rad sizes is the ideal answer, but it's the easiest one to understand IMHO.
I got the ASHP size wrong in my first post. It's actually 6kw.
Floor area is 103 square metres if I exclude the unheated integral garage and conservatory.
6kw seems to be the correct size for the floor area in a reasonably well-insulated property.
The "T" numbers that I'm talking about are those quoted by the radiator retailer. I assume that they mean the flow temperature at the radiator inlet, not the delta across the rad, or the delta between rad and room temps.
I was initially concerned that 6kw seemed a bit low, as my oil boiler is 15kw. I've done some quick maths on annual consumption.
We have oil delivered once a year. In the 14 years that I've lived here, it's been between 750 and 900 litres.
900 litres of oil gives 9300kwh.
Boiler efficiency of 90% means 8400kwh into the heating system.
Heating is on for 8 months a year, with 4 of those months being cold.
I'm guessing the coldest month (with limited solar gain) might use 20% of the oil, so 1680kwh, or 56kwh per day.
56kwh would mean running the ASHP for less than 10 hours a day.
Since (I think) ASHP is supposed to keep ticking over 24/7 for maximum efficiency, it seems like a 6kw unit is good enough for the worst case days - as long as I'm not trying to heat from ambient.
The woodburner is currently just used as an emergency backup during power cuts, and as a feature at Christmas. Over the 15 months since the lounge was remodelled, it has been lit 3 times.
Floor area is 103 square metres if I exclude the unheated integral garage and conservatory.
6kw seems to be the correct size for the floor area in a reasonably well-insulated property.
The "T" numbers that I'm talking about are those quoted by the radiator retailer. I assume that they mean the flow temperature at the radiator inlet, not the delta across the rad, or the delta between rad and room temps.
I was initially concerned that 6kw seemed a bit low, as my oil boiler is 15kw. I've done some quick maths on annual consumption.
We have oil delivered once a year. In the 14 years that I've lived here, it's been between 750 and 900 litres.
900 litres of oil gives 9300kwh.
Boiler efficiency of 90% means 8400kwh into the heating system.
Heating is on for 8 months a year, with 4 of those months being cold.
I'm guessing the coldest month (with limited solar gain) might use 20% of the oil, so 1680kwh, or 56kwh per day.
56kwh would mean running the ASHP for less than 10 hours a day.
Since (I think) ASHP is supposed to keep ticking over 24/7 for maximum efficiency, it seems like a 6kw unit is good enough for the worst case days - as long as I'm not trying to heat from ambient.
The woodburner is currently just used as an emergency backup during power cuts, and as a feature at Christmas. Over the 15 months since the lounge was remodelled, it has been lit 3 times.
The oil boiler is not 90% efficient more like 70% at best especially at the temps your running !
6kw is possibly correct my place is about 50% bigger than yours I think and the other day at about -2.5 I was putting in a bit less than 5 to heat the entire house to 20-21 ISH with a boiler outputting water at about 40c
6kw is possibly correct my place is about 50% bigger than yours I think and the other day at about -2.5 I was putting in a bit less than 5 to heat the entire house to 20-21 ISH with a boiler outputting water at about 40c
Trustmeimadoctor said:
The oil boiler is not 90% efficient more like 70% at best especially at the temps your running !
6kw is possibly correct my place is about 50% bigger than yours I think and the other day at about -2.5 I was putting in a bit less than 5 to heat the entire house to 20-21 ISH with a boiler outputting water at about 40c
Is that "a bit less than 5" kwh of electricity going into the heatpump, or 5kw of heat coming out of it?6kw is possibly correct my place is about 50% bigger than yours I think and the other day at about -2.5 I was putting in a bit less than 5 to heat the entire house to 20-21 ISH with a boiler outputting water at about 40c
The unit they've specced for my house is rated at 6kw, with a consumption of 1.88kw.
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