The temperatures you quote are the standard mcs minimum design temperatures.  The MCS and grant rules allow them to be raised but not reduced.  Try pursuading octopus to increase them.  Raising them at design time is unlikely to affect heat pump sizing but may well mean putting in slightly larger radiators.

In practice octopus are right you will be able to adjust, but it might affect running costs, particularly as octopus appear to design for a flow temp which is higher than many (45 is often picked for rads, octopus apparently design for 55 or 50 if you are insistent, which you should be).  Better if you get it right at design time if possible.

How did the hg rad sizing (physical size not watts) compare with octopus in the rooms in question?

Posted by: @jamespa

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The temperatures you quote are the standard mcs minimum design temperatures.  The MCS and grant rules allow them to be raised but not reduced.  Try pursuading octopus to increase them.  Raising them at design time is unlikely to affect heat pump sizing but may well mean putting in slightly larger radiators.

In practice octopus are right you will be able to adjust, but it might affect running costs, particularly as octopus appear to design for a flow temp which is higher than many (45 is often picked for rads, octopus apparently design for 55 or 50 if you are insistent, which you should be).  Better if you get it right at design time if possible.

How did the hg rad sizing (physical size not watts) compare with octopus in the rooms in question?

They've designed for 50c at -3.4.  If you could interpret something like "Centre 60DC120" and "Centre 45SC90" for me I can do a radiator comparison - I'm arranging a phone call to understand what that means so can respond later if you don't know.  In terms of heat loss calculations there is obviously some difference but not much and came out to 8.3kW and 8.14kW.  Octopus have quoted BTU figures in the radiator list whereas HG quoted W@dT50. 

I don't want to give any other impression other than I'm very happy with the service I've had from Octopus.  We just seem to be struggling on explaining/understanding this point about design temps - I'm even happy to spend a bit more for larger radiators if it means surviving winter 😀 

I suspect 60dc120 means 60cm high 120cm long double convector.  SC single convector.

Heat loss same so good, only issue seems to be rad size/flow temperature (same thing really as they are directly related).   Worth doing the comparison to see just how different they are, but octopus are, by repute, not willing to stretch beyond a certain point.  Remember smaller rads = higher flow temperature = higher running cost.

Posted by: @jamespa

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I suspect 60dc120 means 60cm high 120cm long double convector.  SC single convector.

Do you know how single/double convector relates to Type 21 or Type 22?  Does that relate to SC and DC respectively - sounds feasible?

Hi AndrewJ, 

There's several design principles which vary from designer to designer. 

The ones Octopus have designed to are generally considered the minimum comfortable temperatures for the rooms you have described. Eg a Gym might either be unheated or only heated to a max of 16 degrees, if that (imagine trying to bench in a 21 degree room!) and a bathroom might want 21 deg or even 22. 

If one is looking to keep the heat pump minimally sized, this might be the approach to take, for me personally unless the client has explicitly consented to lower than 21deg in a room, heat loss and emitter sizing is on the basis of every room (excluding obvious ones like gyms, stores, garages etc) going up to 21 deg. 

The emitter sizing is generally tailored to the room design temp, eg at a given outdoor temp say -2, the emitter at the design flow temp for that outdoor temp on weather comp will be sized in terms of output to reach that room design temp. 

We know the equation Energy = Mass  x Constant x (T2 -T1)

In this case to calculate heat loss simply we use 

rate of heat loss over time = U value x Area x (Temp room-outdoor temp)

Which is a slightly different version of this formula

For radiator sizing, which is essentially what your query is relating to, we must size the emitter accordingly to match the rate of heat loss over time (Q) which for now we will take as a combination of fabric and ventilation losses

This relationship, assuming no minute losses, is related by

Energy in must equal energy out to maintain room temp 

Energy supplied by a rad is simplistically defined by: 

Energy supplied by rad = coefficient of material x SURFACE AREA x (mean water flow temp - room temp)

What's the connection? The only variable we can change is the mean water flow temp if we want to maintain our given room temperature.

Remember that as we increase the dT across the heat pump (by increasing the average flow temperature across the rads, which links to our above relationship) we increase the electrical consumption of the unit.

Once a rad is fitted, realistically the only way to increase the output is to increase the flow temp since everything else can't be changed. Changing mass flow rate doesn't have that much of an impact so we can disregard this.

Pre-install, to increase the energy provided by the rad but maintain flow temp on the lower end (which we know is linked to HP efficiency intrinsically) we can increase the working surface area of the rad, its size. 

What this boils down to I believe, in your case, is that Octopus have seemingly designed to a (max?) 50deg flow temp which is higher that I would personally recommend to get the best efficiency out of your system. This may be to keep the HP size smaller or to reduce the emitter sizes. 
Depending on what unit you are having fitted, if you decide post installation you want warmer rooms you can increase the flow temp but if other rooms have emitters sized to 21 deg and you increase the whole system flow temp you will need to restrict the flow rate to those rooms to keep the temperatures under control. 

Overall you will reduce the system efficiency. 

I hope that makes sense. I try to keep the maths out of it but often it makes the most sense to people to see it laid out in this way. 

Best, 

Rob at Cirrus Energy

www.cirrusenergy.co.uk

Posted by: @andrewj

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Posted by: @jamespa

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I suspect 60dc120 means 60cm high 120cm long double convector.  SC single convector.

Do you know how single/double convector relates to Type 21 or Type 22?  Does that relate to SC and DC respectively - sounds feasible?

 

Not exactly.  Type 11 = 1 panel, set of fins, type 21 = 2 panels 1 set of fins, type 22 =  2 panels 2 sets of fins.  Unfortunately SC could be 11 or 21 and DC could be 21 or 22.  I just dont know the Octopus terminology. 

You may be able to deduce it from the variants on the list, or alternatively from the output.   Radiator output is proportional to deltaT to the power 1.3, and 1W = 3.41btu/hr so conversions could in principle be done, but it might be simpler to ask

@rob_cirrus_energy If I could perhaps summarise that to see if I understand correctly.  Let's say I have a house with 3 rooms.  If I were to calculate a room's heat loss and then specify a radiator size to give 21c at a flow temperature of 50c at -3c outside temperature then:

• if every room had a radiator sized to that spec, as the outside temperature increases, heat loss reduces and so does flow temperature and each room maintains its 21c temperature.  
• I don't need to do anything to any radiator to adjust its output and the whole house has the same temperature throughout
• If I wanted to cool the bedroom at night I could either (a) set a setback temperature and the whole house would cool; or, (b) put a TRV on the bedroom radiator and turn it down (or use a smart TRV to reduce and then raise in the morning)

Now let's say I have a house with 3 rooms with design temps of 16c, 18c and 21c and the radiators are sized to give that temperature at a flow temperature of 50c at -3c outside temperature then:

• Each room, regardless of the flow temperature/outside temperature, is at a different temperature relative to the design temp ratio.  As the outside temperature increases, the flow temperature decreases and the rooms maintain their design temperatures.
• If I want the room that is 16c to be held at 21c I could expect the other two rooms to be overheated (by 5c?) and the efficiency to fall through the floor.  I don't even know how I would achieve this under a weather compensation set up
• Assuming the bedroom is the one with the design temp of 18c then either (a) I do nothing because it is already at that temp; or (b) I have to reduce the flow rate back to something-or-other.

The first scenario I understand and it seems logical.  The second, with varying design temps, I don't understand and I don't see how I achieve temperature normalisation in an efficient manner.  I'm not so bothered in this about whether the flow temp is 50c or 40c I think that really relates to efficiency and cost of running.  What I'm trying to understand is how I achieve a comfortable temperature of 21c throughout the house so my wife isn't walking from a warm area into a cooler area.  To me, that must mean every room having the same design temp and appropriately sized radiator.  I hope I'm explaining this ok.

@jamespa  I have already started trying to do a radiator comparison so will wait until I speak to Octopus tomorrow or later in the week.  My fundamental question though is as above.

Posted by: @andrewj

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Notwithstanding the advice to sleep in cooler rooms at 18c

@andrewj Earlier with the gas boiler and Tado TRVs controlling the temperature, I used to think 18 is the right temperature. As otherwise the room used to get quite hot. But, with the heat pump the room temperature is steady, and our bedrooms are designed at 21 degrees. To be honest, I am actually liking this. There are no cold spots and it feels comfortable throughout the night. Maybe, a bit warmer for our dog, and I thought she will pant and keep us awake. But, even she is sleeping quite well.

@andrewj the trick is to get the radiator size roughly right and a bit too big, by design and then turn them permanently down with lock shield valve (not with a varying trv) to get the heat out that you want for the desired temperature.

There’s a guide to radiator balancing on this forum.

Octopus designed my home to 21 living areas ,19 bedrooms at 50c at -2 ish. In reality my flow temp is 38c at -3 and that keeps the whole house at 20.5 inc upstairs.

they recommended changing the radiators initially and I managed to convince them no to , and that wast needed.

the main lesson for me is tha: the heat loss studies not fact and likely a bit conservative . 350 days of the year the temp will be above the design temp and you will be able to get it above design temp (or lower flow temp).
At -5 outside my house is maybe 19.5-20 at 39c flow so on those days maybe a jumper inside but still plenty warm. I could bump flow but I take a jumper over my extra costs .

I wouldn’t sweat it too much . For instance you could balance the heat per room with trvs (although ideally you will keep as much on at low flow temp), or heat upstairs as you switch downstairs off the last hour before bed time, so you move the heat upstairs. 
I suspect most likely you will have a comfortable home though with a bit of configuring after the first few weeks 

@pie_eater that’s good to know at least.  Obviously every house is different and 13 radiators are being changed in mine. It’s a Finnish timber framed house so pretty well insulated and it only needs small single panel radiators to warm the house which aren’t good enough for a heat pump.  The MAIN issue I have is that if the rooms are different temps my wife will notice the transition from a, say, 21c room to a 19c room and complain.  It doesn’t matter if in reality 19c is warm, that transition will make it feel cold until she acclimatises. This is the situation we have at the moment and my assumption was that with weather compensation and each room radiatored to the same design temp, then the house would be pretty much the same temp everywhere.  I’ve told my wife that the house will be way more comfortable with a heat pump because it will be even-Steven’s throughout. This is perhaps where my understanding and reality depart company and what I’m trying to get to clarify here.