@lucia 

There are too many delta ts in heating so it gets confusing 

As @derek-m says the delta t that matters for rad output is the delta t between the room temperature and average temperature of the radiator (which is the flow temperature minus half the design delta t across the radiator). 

So in a typical gas system where the flow temp is 80C,  the return 60C and the room temperature 20C, the relevant delta t is 70-20=50C.  This is the usual figure for which rad outputs are quoted.

In a typical heat pump system where the figures are 45,40,20 the relevant delta t is 42.5-20=22.5.

Btw rad output is proportional to delta raised to the power 1.3

If you would post a couple of example rad calcs we can see what's happening.

Posted by: @lucia

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@derek-m  So I can't just compare the delta T @ 30 for both rads? Oh. my. days... I think I need to go to college for this. Back to playing with Heat Punk.

Thank you for the calculator.  

 

It is easy to compare the estimated output of different radiators using the calculator I provided.

To estimate the thermal energy output of the type 11 at a DT of 30C, obtain the specified output at DT50 from the manufacturers data, and put this value in cell E3.

Set the value in cell C1 to the desired IAT (say 20C). Vary the LWT (cell J1) and RWT (cell J2) values until the value in cell A6 shows the desired radiator DT. Also pay attention to the DT value in cell K2.

The value in cell B6 is the estimated thermal energy output from that particular radiator under the set operating conditions.

For a type 11 radiator the output at DT30 would be in the region of 587W. A type 22 operating under the same conditions would be expected to output 1056W.

So if a type 11 is replaced with a type 22, the new radiator would be able to supply the same quantity of thermal energy at DT19 rather than DT30. This means that the LWT could be reduced from 52.5C to 41.5C and the RWT from 47.5C to 36.5C, which would allow the heat pump to operate with greater efficiency.

In an actual heating system the required minimum LWT and RWT will be dictated by the room containing the radiator with the lowest thermal energy output capacity to room heat loss. So if the room heat loss at a given IAT and OAT is say 500W and the room contains a type 11 radiator then the required LWT and RWT to maintain an IAT of 20C would be 49C and 44C respectively. By replacing the original radiator with a type 22, the minimum required LWT and RWT now becomes 39.4C and 34.4C respectively.

It is therefore more beneficial to assess the heat loss of each individual room and estimate the output capacity of the associated heat emitters to identify the 'weakest link in the chain'. It should then be possible to decide which heat emitters, if any, require to be replaced.

@derek-m Thinking about what @lucia says about quotes seemingly downgrading some replacement rads would it be wrong to upgrade rads not deemed necessary for upgrading in the installers plan? My quote suggests some old rads in the bedrooms do not need replacing as their output is ok, however as they are old I would like to replace them. Would it be wrong to replace them with ones that are capable of a higher output? (I think I understand your point that they may not actually be downgrading them)

@judith Will do as soon as I can. 😁

Posted by: @jancold

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@derek-m Thinking about what @lucia says about quotes seemingly downgrading some replacement rads would it be wrong to upgrade rads not deemed necessary for upgrading in the installers plan? My quote suggests some old rads in the bedrooms do not need replacing as their output is ok, however as they are old I would like to replace them. Would it be wrong to replace them with ones that are capable of a higher output? (I think I understand your point that they may not actually be downgrading them)

 

For optimum performance a heat pump should be operated at a low LWT that is consistent with reliable operation.

The thermal energy output of a heat emitter is dictated by its specified heating capacity (size) and the average water temperature. The required physical size and type of heat emitter therefore dictates the LWT required to supply the heat loss of a given room to maintain the desired room temperature.

I think that MCS states the required temperature for bedrooms to be 18C for heat loss calculation purposes, so a radiator with a lower specified heating capacity should be adequate to meet this. If a larger capacity radiator was installed then it may be necessary to lower the flow rate through the radiator by means of the lockshield valve, thereby reducing the quantity of thermal energy being supplied to the room.

In basic terms it is like having a 3 bar electric fire but only ever using 2 bars.

When designing a system it is preferable to estimate the heat loss of each individual room and then investigate the size and type of heat emitter that would be required to meet that heat loss at a given water temperature. The minimum acceptable water temperature is then set by the room with the greatest heat loss to heating capacity ratio.

@derek-m But if 🐙 design for 50º and you want to run the system at say 35º then more watts are needed no? Or am I still drowning in the Delta Ts?  

This is what I'm trying to figure out too. Because how it looks to me, the only way a 4kw hp has any chance of keeping my house warm without spending lots of $$ when it's properly cold is, according to the Daikin data, when the system is running at 35º-40º.

I don't think Daikin include defrost cycles in their data but if I go by my estimation of whole house heat loss of approx 5.6 (🐙 say 3.53 by designing for 'cavity wall insulation' @ 50º) I need a few bigger rads to run @ 35º to keep costs down? 

Posted by: @lucia

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@derek-m But if design for 50º and you want to run the system at say 35º then more watts are needed no? Or am I still drowning in the Delta Ts?

No.  You need exactly the same amount of watts which is determined by (in fact must match) the house heat loss.

What changes is that you need bigger radiators to emit the same number of watts.  This is because they are at a lower temperature and thus the temperature difference between the radiator and the house is smaller.

However, as you have noted, at the lower temperature you may be able to get away with a heat pump that has a lower rating, because most (not all) heat pumps emit more watts at a lower flow temperature.  

I think that what you are failing to appreciate is that the required LWT is dependent upon the actual heat loss and heating capacity of the heat emitters, and not the LWT at which you would like to operate the heat pump.

It may be useful to consider the relationship between IAT, OAT and heat loss. If we assume that your home has a calculated heat loss of 6kW at an IAT of 21C and an OAT of -3C. In basic terms when the IAT is 21C and the OAT is -3C then 6kW of thermal energy will leak from your home each hour. This is with a temperature difference of 24C between IAT and OAT, which means that for each 1C difference the heat loss will vary by 250W.

If OAT were to increase to 9C then the heat loss would be reduced from 6kW to 3kW. At an OAT of -7C the likely heat loss would be 7kW.

Assuming that the installed heat pump was actually capable of producing 6kW of thermal energy at an OAT of -3C, and that the total specified capacity of the heat emitters was 24kW, then the LWT would need to be 40.7C with a RWT of 35.7C. Increasing the capacity of the heat emitters could reduce the required LWT.

At an OAT of 9C, and hence a heat loss of 3kW, the LWT would need to be 33.6C with a RWT of 28.6C.

Unfortunately Daikin don't appear to publish detailed data tables as some other manufacturers do, so it is necessary to look at the various charts and estimate the probable heating capacity of the different Daikin models at the required LWT.

Looking at the charts for the Daikin 4kW model it would appear that it would not be able to provide 6kW of thermal energy at an OAT of -3C. The 6kW model may be able to meet the heating demand, though would be working at or near maximum output. The 8kW model would appear to have no problem meeting the heating demand down to -7C.

Obviously the larger capacity models may lead to more frequent cycling during milder weather conditions.

@derek-m Thank you. This is my dilemma. 

The BG original proposal designed for no cavity wall insulation @ 45º and came up with a heat loss of 5.6 and suggested a 6kw heat pump and several rad changes. This heat loss figure matched my rough estimate via the 'de Podesta gas consumption method' 

Octopus designed for 'cavity wall insulation' @ 50º - 4kw heat pump. 

I think the 8kw is too big for my house and is more expensive to run. The 6kw heat pump, even though it's a software limited 8, is probably a bit cheaper to run and will probably cover it.

But this is all theory on my part.  

It shouldn't be this tricky - no wonder they're all trying to push plug and play Cosy style kit. But Joe Public will pay for that with higher bills.  😏

Perhaps it might help if I add the following: If a house has a heat loss of 6kW at the design temperature (say, -2C), then to keep the house warm (at that outside temp) the ASHP needs to be capable of delivering 6kW of heat into the system AND the emitters need to be capable of emitting 6kW of heat to the house. The emitters cannot create more heat than the ASHP delivers, so they cannot compensate for an underpowered ASHP. But if they are too small then they can limit the rate at which that heat is transferred into the room, so not all of the heat the ASHP is capable of delivering can actually be used.

@lucia have 🐙 confirmed they definitely don’t want to fit a 6kW? I don’t know if you have Twitter but John_oes is the head of 🐙 Energy Services and I’ve always find him very helpful on there and he might take a second look at your case. 

You say their Cosy style offering will cause higher bills but higher than what? Higher than gas or just higher than an optimum ASHP system? Keeping costs just slightly lower than gas seems to be what they’re aiming for with the price point they offer. To get the big savings, the initial cost will be higher (as things stand with current installation costs/methods anyway). I’m with you in that I wish 🐙 would just change it to a 6kW, they have improved their offering since it first appeared, they now work with microbore and pass on leads to Heat Geek for example, so I hope they learn to offer a bit more flexibility. That said, the MCS is part of the problem but it looks like they’re changing too.

@dougmlancs I witnessed that the MCS are part of the problem; I wanted a higher temperature in the bathroom (~25-26 degrees C.) and was told that they (OE) could not go against MCS guidelines and therefore they could not fit a radiator of a size to suit my needs and wants. I looked into this and found that at a customers request, an installer could oblige by providing a higher temperature to those outlined in the MCS guidelines. I wrote to OE and pointed out the relevant paragraph in the MCS literature but by this time, OE had decided that my installation was ‘out of scope’. Regards, Toodles.