Vaillant 12kW question

[Grey Sage]

I have an installer who is fixated on an Ecodan 11.2kW with a flow temperature of 50oC and a SCOP of 3.3 (declared) but I like the look of a 12kW aroTHERM. My heat load calculations, done by a heat engineer that I trust, has suggested that 12 radiators need to be replaced to get the heat delivery of 12.6KW. My question is this: Would a 12kW aroTHERM deliver the heat at a higher flow temperature, and still have a SCOP to match the Ecodan, thereby allowing the existing radiators to be used?

 

This would be an cost advantage for the aroTHERM but I need someone with more knowledge than me to show the calculations.

 

TIA

Copy of Radiator Schedule (004).xlsx

[J1mbo]

You can find the Arotherm Plus SCOP values for all flow temperatures up to 55°C on the MCS directory here: Product directory - MCS (mcscertified.com)

 

Search by manufacturer, 'Vaillant Group UK Ltd', then the 12kW is on the second page. The rating is 3.63 at 55°C (and 3.92 at 50°C). The increased SCOP will directly affect the RHI payments vs the Ecodan (i.e. they will be higher with the Vaillant).

 

Personally I wouldn't skimp on the radiators. You want the flow temperature design as low as possible because this increases the declared SCOP for the product and in turn this increases your RHI payments, as well as of course reducing the electricity consumption in practice. I would aim for a flow temperature design of 45°C if you can or even lower. Radiators like Faral Tropical95 can really help.

[Grey Sage]

18 hours ago, J1mbo said:

You can find the Arotherm Plus SCOP values for all flow temperatures up to 55°C on the MCS directory here: Product directory - MCS (mcscertified.com)

 

Search by manufacturer, 'Vaillant Group UK Ltd', then the 12kW is on the second page. The rating is 3.63 at 55°C (and 3.92 at 50°C). The increased SCOP will directly affect the RHI payments vs the Ecodan (i.e. they will be higher with the Vaillant).

 

Personally I wouldn't skimp on the radiators. You want the flow temperature design as low as possible because this increases the declared SCOP for the product and in turn this increases your RHI payments, as well as of course reducing the electricity consumption in practice. I would aim for a flow temperature design of 45°C if you can or even lower. Radiators like Faral Tropical95 can really help.

Thanks for that J1mbo. Can you see the excel file that I tried to upload with my question? The heat engineer used the existing radiator outputs based on  40oC  compared to his calculated heat demand figures, implying that each radiator provides an average of less than half of the required heat. I don't know what temperature they operate at. I do know that they supply sufficient heat.  Then he has specified a 35oC temperature and worked backwards to bigger radiators.

 

My question is can the Vaillant reliably heat at 55oC (and a SCOP of 3.63 - better than Ecodan) those existing radiators to deliver the calculated heat requirements?

[J1mbo]

Yes it will give 55 degrees all day long but it will cost you much more to run like that and it won’t achieve 3.6 in practice. Really, spend the money on the radiators. And also be sure to specify Vaillant’s own controls, like Ambisense.

[Grey Sage]

9 hours ago, J1mbo said:

Yes it will give 55 degrees all day long but it will cost you much more to run like that and it won’t achieve 3.6 in practice. Really, spend the money on the radiators. And also be sure to specify Vaillant’s own controls, like Ambisense.

Thanks again J1mbo. I'll take that on board. Do you have any experience with "eco-radiators? I think that they are supposed to flow through one panel first at the design temperature and the through the second panel. Sounds like a contravention of the laws of physics to me but the claim to be able to deliver more heat for a given flow temperature.

[J1mbo]

I don't. But, radiator output is a function of surface area and surface temperature. Looking back at your spreadsheet, your requirement is going to be quite challenging. No heat pump will work with what you have and qualify for RHI.

 

Radiators are usually rated at DT50. This means, the average surface temperature of them being 50°C higher than the ambient air temperature. i.e. at flow of 75°C and a return of 65°C at 20°C room temperature. To get decent running costs the heat pump probably wants to be run at 45°C flow and 40°C return (or maybe 50/45). Therefore the target DT from a radiator perspective is just 22.5°C. Hence, the surface area needed is much, much greater and the water speed needed also double (since the radiator is being specified to give up 5 instead of 10 degrees or water temperature).

 

Looking in particular at the lounge. The heat demand is 2.8kW. Some of the most effective radiators are aluminium fin style such as Faral Tropical 95 (datasheet is available from here). The 672mm high FT9-672 provides about 50W per section at dT=22°C. That would mean a total of 56 sections needed for this room. At dT=30° (52.5 flow / 47.5 return) it's 76W, requiring 37 sections.

 

The piping may also need some thought. To deliver 12kW with 5° across flow and return the heat pump will need to deliver 2,100 litres per hour or so, meaning 28mm flow and return there as usual. But the way the radiators are linked should be considered. The lounge will need 500 litres per hour so if the radiators are on the end of a run serving others, this might be a problem. The plan calls for 2 radiators but it might be worth thinking about more if the room design allows for that and have that room served from 22mm pipe going back to the 28mm.

 

There also won't be anything like enough water in the 12 panel rads specified, so a larger buffer tank will be needed - reckon on total water volume of 240 litres being needed in the system to avoid short-cycling mid-season.

 

If there is anything you can do to reduce the thermal load, such as maybe updating glazing or adding wall insulation, it would be a good idea to do so.

[J1mbo]

The other issue is that the heat demand is listed as 12.3kW. Assuming you want the heat pump to deliver DHW, there is no power left to do so with a 12kW ASHP. Maybe it would be worth looking at a 16kW ASHP or use 2x 8kW-ish outdoor units, which would also give some capability to survive breakdowns.

Edited November 21, 2021 by J1mbo