ASHP - using a low loss header and modulation instead of a buffer

[J1mbo]

Output is not proportional (in a linear sense) to flow temperature btw.

Edited January 2, 2022 by J1mbo

[gravelld]

Thanks all. Looks like Kudox's W/m² for ∆50 works out around:

 

600x800  double convector = 1430W / 0.48 =~ 2979W/m²

600x1200 double convector = 2146W / 0.72 =~ 2981W/m²

 

Two sizes there to show the sizing appears to give roughly the same output per m². Obviously this changes with rad type, e.g. single, double convector. Right now I'm just trying to get some confidence in the model figures. I compared the Kudox output figures to Stelrad's and they come out about the same - about 3% difference. So some agreement there.

 

For ∆30:

 

600x1200 double convector = 1091W / 0.72 =~ 1515W/m²

 

Stelrad don't have a chart for ∆30 (instead they have a correction factor). Kudox's ratio is 0.508, and Stelrad's correction factor is 0.515, so more agreement.

 

Now, comparing these authoritative starting points with the original model.

 

The original model hardcoded 1480W/m² as the output for a double convector. So actually, I think after all the original model was roughly correct for ∆30.

 

If anyone could double check the above I'd be grateful. If the above is true, and we run a slightly higher flow temperature to meet ∆30, it means we have barely any rad upgrades to make.

 

 

[SteamyTea]

List of correction factors here.

https://www.buildingservicesindex.co.uk/entry/136540/AEL-Heating-Solutions-Ltd/How-to-calculate-the-delta-T-for-a-radiator/

[gravelld]

Yes,forgot to say i saw that. Either Stelrad copy and pasted or it's more confirmation ?

[SteamyTea]

4 minutes ago, gravelld said:

Yes,forgot to say i saw that. Either Stelrad copy and pasted or it's more confirmation ?

Work on half the output, won't be far out.

The maximum theoretical output only matters in extreme weather. 

[gravelld]

To make sure I understand the point you are trying to make:

 

... I thought this was about maximum theoretical output, because it's during the extreme weather you want to make sure you have enough output. i.e. if my rads were undersized, even if the HP was powerful enough to deliver the required Ws the emitters were not large enough, given the delta.

[Marvin]

 

6 minutes ago, gravelld said:

To make sure I understand the point you are trying to make:

 

... I thought this was about maximum theoretical output, because it's during the extreme weather you want to make sure you have enough output. i.e. if my rads were undersized, even if the HP was powerful enough to deliver the required Ws the emitters were not large enough, given the delta.

Correct  

[Marvin]

Remember the nearer the temperature of the water compared with the outside air, the more efficient an ASHP runs. 

 

So ASHP works on "low n slow"

 

So the more surface of emitter the lower the water temperature needs to be to meet the same temperature.

 

Hence the increase in radiator sizes if swapping form Boiler to ASHP. 

[Marvin]

What are you using as the most extreme cold weather? -5C outside??

[ReedRichards]

If you keep your rooms at 21 C then ∆30 might mean that the flow temperature is 53.5 C and the return is 48.5 C.  Your ASHP won't be running very efficiently at those sorts of temperatures so you want to minimise the amount of time for which it happens.  In your model, what is the outside temperature when you need the ∆30 radiator output?

 

Also, your RHI payments, if you could get them, will depend on the SCoP and that will go down the higher you make your maximum flow temperature for heating.  Setting up your ASHP to match your existing radiators will increase the running cost for the sake of a little bit of convenience. 

[gravelld]

1 hour ago, Marvin said:

What are you using as the most extreme cold weather? -5C outside??

 

@ReedRichards

Quote

In your model, what is the outside temperature when you need the ∆30 radiator output?

-4.2C

 

Yes, next stage is to understand the implications on RHI payments, and costs overall.

Edited January 3, 2022 by gravelld

[J1mbo]

RHI payments are simply based on the EPC annual demand and the ASHP approved SCOP for the quoted design flow temperature (each product has a table).

 

Aim for the lowest possible design flow temperature. Also bear in mind that in use, the number of daily compressor run hours will also affect the flow temperatures needed. If you only heat the house for 3 hours in the evening due to work patterns etc, go really big on radiator sizing. If you have it running all day or 24x7, the radiators can be much smaller relatively.

 

Especially with the cost of electricity now and it’s likely to rise significantly further, my advice FWIW would be to design for the lowest practical temperatures as it will reduce your running costs and increase your RHI payments at the same time. So just replace the radiators.

Edited January 4, 2022 by J1mbo

[gravelld]

Looking at the RHI figures then, going from a flow temperature of 55C to 40C will increase the grant by about £1.4k for the full seven years.

 

However to work out how many radiators need replacing, I still need the outputs for the radiators for ∆16.5 (or ∆21.5 for 45C). I can't see charts for this anywhere. Anyone know where I can get this info?

 

I'm just assuming there's a 5C difference in flow and return temperature, and the point for the delta is the mean of those two.

[ReedRichards]

Delta T (°C)	Correction factor
5	0.05
10	0.123
15	0.209
20	0.304
25	0.406
25.49	0.41666667
30	0.515
35	0.629
40	0.748
45	0.872
50	1
55	1.132
60	1.267
65	1.406
70	1.549
75	1.694

I

[ReedRichards]

To qualify for the RHI you need to use an MCS certified installer.  Part of the installation process is that they should evaluate the heat loss from every room in the house and how big a radiator it need.  You can make an input as to what design flow temperature you want but your installer should then do the radiator calculations for you.

[gravelld]

@ReedRichards SORRY IGNORE: that correction factor table is based on T50. I need one based on T30 if I'm going to use correction factors. [Just realised i can use the T50 correction factors by using the T50 output numbers from Stelrad at al]

 

All of this is post MCS calculations!

Edited January 5, 2022 by gravelld

[ReedRichards]

So what water temperature did the MCS calculations assume?  And why do you doubt them?

 

Was your original aim to see if you can use a higher water temperature and your existing radiators?  That's probably a false economy, as I hope you have now realised.

 

If you want a correction factor table based on T30, just divide all the numbers by 0.515.

[pdf27]

One thing to note here: thermostatic radiator valves are probably not helpful with a heat pump:

• As already noted, water volume is reduced when they close increasing short-cycling problems.
• Closing a valve reduces the available radiator area, meaning the water needs to be run hotter to heat the rest of the house.
• Unheated rooms will mean that the radiant temperature of the walls from those rooms is lower, so the rest of the house needs to be slightly warmer for comfort - again increasing the flow temperature.
• Ideally a heat pump would use weather-compensation to adjust the flow temperature to the minimum possible - TRVs will make this much harder.

Essentially they work by allowing you to run some rooms cooler, reducing total heat demand. However, this is not the same as reduced **energy** - heat pumps benefit very strongly from reduced flow temperatures, and it is probable that they'll be better off running cooler with the TRVs removed or fully open.

[SteamyTea]

46 minutes ago, ReedRichards said:

divide all the numbers by 0.515.

Not sure what "all the numbers" are. Assuming power then multiply by the correction factor.

[gravelld]

1 hour ago, ReedRichards said:

So what water temperature did the MCS calculations assume?  And why do you doubt them?

 

Was your original aim to see if you can use a higher water temperature and your existing radiators?  That's probably a false economy, as I hope you have now realised.

 

If you want a correction factor table based on T30, just divide all the numbers by 0.515.

 

I don't necessarily doubt them as such. But almost every installer I have spoken to is hand-wavy about the numbers, doesn't want to discuss system design, doesn't really understand things like higher performance building and runs a model that doesn't allow for inputting of all necessary data (such as air permeability or Uw of windows) to produce an accurate heat loss estimate (granted, this might partly be the MCS influence). So I do think the models need checking.

 

My original aim is to (1) keep us warm and (2) optimise cost and maintenance burden. I am yet to "realise" this, because it depends whether the cost of installing extra/larger radiators is covered by the increased RHI payments. I'll run some numbers tonight.

Edited January 5, 2022 by gravelld

[joth]

57 minutes ago, gravelld said:

But almost every installer I have spoken to is hand-wavy about the numbers, doesn't want to discuss system design, doesn't really understand things like higher performance building and runs a model that doesn't allow for inputting of all necessary data (such as air permeability or Uw of windows) to produce an accurate heat loss estimate (granted, this might partly be the MCS influence). So I do think the models need checking.

 

I know exactly what you mean here, but I found once I had signed up and paid a deposit my MCS installer was happy to share the spreadsheet with me, and I could tinker with the numbers to bring it in line with our EnerPHit spec. The MCS model does include room-by-room air change/hr and even thermal bridge allowances

 

 

Wish I'd known the the flow temperature was a key determining factor for RHI. We never run ours above 30ºC to protect the wood flooring, but the MCS cert has it as 35.

 

 

[SteamyTea]

13 minutes ago, joth said:

but I found once I had signed up and paid a deposit my MCS installer was happy to share the spreadsheet with me

You still got that spreadsheet and are you willing to share it?

[ReedRichards]

7 hours ago, SteamyTea said:

Not sure what "all the numbers" are. Assuming power then multiply by the correction factor.

I meant this:

 

Delta T (°C)	Correction factor	Correction factor relative to Delta T = 30°C
5	0.05	0.097
10	0.123	0.239
15	0.209	0.406
20	0.304	0.590
25	0.406	0.788
25.49	0.41666667	0.809
30	0.515	1.000
35	0.629	1.221
40	0.748	1.452
45	0.872	1.693
50	1	1.942
55	1.132	2.198
60	1.267	2.460
65	1.406	2.730
70	1.549	3.008
75	1.694	3.289

[gravelld]

BTW, what are the rules regarding RHI, MCS etc when sizing radiators? Must the installer prove the radiators are adequately sized on a per room basis? I know they have to prove heat loss and the suitability of the heat pump, just unsure about emitters.

 

For example, might it be ok to allow some rooms to be slightly undersized and other, next door rooms, oversized?

Edited January 5, 2022 by gravelld

[gravelld]

Flow temperature 45C (∆21.5)

I used @ReedRichards' chart above (thanks!) and interpolated (in my head) a correction factor of 0.63 from ∆30.

 

There are a number of rads that will need enlarging but it's not looking too bad atm. Some are way oversized, and some are in rooms with other heat sources, so I'm just going to wait on the response to my question above before I decide what to do.

 

For a flow of 40C upgrades look a bit more pervasive but will check that later.