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Gas Combi to ASHP - Making sense of survey results and proposal.

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(@irmartini)
Trusted Member Member
207 kWhs
Joined: 2 months ago
Posts: 20
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Hi Everyone,

I’m Ian and recently joined this forum as I’m in the process of switching from a gas combi to ASHP.

I did a little intro a few days ago but I’ve now got my heat lose survey and proposal from my chosen company and I’m trying to understand the results and what is being proposed. Although I understand how a ASHP works at a block diagram level I do not at the moment understand specifics.

Can I ask anyone willing to take a look at the below details and give me their honest opinions and advise please. I do realise this is a rather long post and that I don’t really know what I’m even asking!? As this is so new and different from anything I’ve every done before but thank you to anyone who takes the time to read and respond I really do appreciate it.

 

House details:
Detached property, 155m2, South East Shropshire near Bridgnorth
Good loft and cavity wall insulation, Double glazed and PV system.
Gas usage approx. 13,000kWh per yr.

**EDIT**

HEAT LOSS
Area M2 - 151.39
Power (W) - 7047
Energy HP (kWh) - 19281

*****

HEATING ANNUAL HEAT DEMAND
Demand                                                                      kWh/yr 19281
Heat supplied by HP, excluding auxiliary heaters             kWh/yr 19281
Seasonal Coefficient of performance SCOP                     SCOP 3.84
Electricity consumed by HP, excluding auxiliary heaters   kWh/yr 5021
Renewable heat supplied by HP                                     kWh/yr 14260

 

WATER HEATING ANNUAL HEAT DEMAND
Demand                                                                       kWh/yr 5619
Heat Supplied by HP, excluding immersion heater            kWh/yr 5619
SCOP of HP in Hot water mode SPF/SCOP 3.34
Electricity consumed by HP, excluding immersion heater   kWh/yr 1682
Renewable heat supplied by HP                                      kWh/yr 3937
Remaining heat, supplied by immersion heater                kWh/yr 156
Electricity consumed by HP, including immersion heater   kWh/yr 1838

 

PROPORTIONS, ENERGY CONSUMPTION, AND PERFORMANCE
Proportion of heating and water heating demand provided      % 100
by heat pump (excluding auxiliary/immersion heaters)
Capacity @ design conditions                                                kW 8
Renewable heat                                                                   kWh/yr 18196
Electricity consumed by HP                                                   kWh/yr 6703
(excluding auxiliary/immersion heaters)
Electricity consumed by immersion (supplied as part of HP)     kWh/yr 156
HP combined performance SCOP                                           SCOP 3.65
Star Rating / Flow Temperature                                            4 / 45°C
Outside air temperature                                                       -3.46°C

 

RUNNING COSTS (Based on Calculations)
Cost per unit of electricity for HP                                          p/kWh 25
Cost of electricity for HP                                                      £/yr 1715
(including auxiliary/immersion heaters)

 

The only thing that stood out from the above for me was the heating demand seemed rather high compared with my gas usage, but looking at the room by room heat loss calculations they are based on all rooms at 21deg except bedrooms at 18deg, where we have the living room thermostat set at 18 which we often up it to 19 on an evening and weekend and occasionally 20 but that only seems to happen when I’m not looking!

I’ve no idea about hot water demand? there are 4 of us in the house and 3 of us shower everyday and 1 teenage boy who might average 2 a week if we are lucky!

 

PROPOSAL
To supply and install an 8KW high temperature Samsung Air source heat pump with R290 refrigerant.
To connect heat pump to a RCBO size to be confirmed.
To Supply and fit communication cables between heat pump and plant room.
To run 2 X 28mm pipes from plant room to heat pump.
To supply and fit 250 litre Joule Cyclone unvented hot water cylinder
1 x 3 KW immersion heater for legionella purge installed as standard
Supply and install a 50 litre stainless steel volumiser or buffer tank
Connect heat pump to hot water tank and heating circuits.
To fill and flush the whole system and fill with 30% inhibited glycol.
To carry out all associated electrical work.
To commission system.
To provide handover pack which will include building regs certificates for the heat pump and the cylinder, an MCS certificate for the system and all warranty certificates.
Eco-Wright energy offer annual servicing and the opportunity for the owner to take out a service contract

 

I havn't copied everything into this post there is additional hot water demand calculations and the individual room and radiator calculations and i've trimmed the boave to try and keep it to the point.

 

COSTS
Radiators
Labour £600
New Rads £2,106.96

Extras
10m Rhau pipe £730

HeatPump £11,971.72

Sub Total £15,408.68

BUS Grant -£7500
Site Survey -£300

Total £7,608.68

 

As I said above any thoughts, recommendations and what the above actually means and how I interpret it would be appreciated.

 

Kind Regards

Ian

“Anything worth doing, is worth doing right.”


   
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(@ajn9000)
Eminent Member Member
109 kWhs
Joined: 2 months ago
Posts: 12
 

What did the heat loss survey say for your total heat loss [W] at design temperature?


   
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(@irmartini)
Trusted Member Member
207 kWhs
Joined: 2 months ago
Posts: 20
Topic starter  

@ajn9000 

I think this is it.

Area M2 - 151.39

Power (W) - 7047

Energy HP (kWh) - 19281

 

I've edited the original post to include this. 

Thanks

“Anything worth doing, is worth doing right.”


   
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(@derek-m)
Illustrious Member Moderator
13948 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4196
 

@irmartini

The heat loss calculation should detail the estimated amount of thermal energy required each hour to keep the rooms at the specified temperatures at the designated outside air temperature, normally in the region of -2C or -3C.

A suitable heat pump would need to have a maximum output capacity greater than the calculated heat loss.

This post was modified 2 months ago by Derek M

   
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(@irmartini)
Trusted Member Member
207 kWhs
Joined: 2 months ago
Posts: 20
Topic starter  

@derek-m 

Here are the last few lines of the heat loss sheet, there is no mention of time or temp.

However in the performance section of the report does state outside temp 3.46deg

image

The Total of 7047 and a suggested 8kWh heat pump seems to make sense to me unless I'm misunderstanding what its saying?

“Anything worth doing, is worth doing right.”


   
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(@derek-m)
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13948 kWhs
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Joined: 3 years ago
Posts: 4196
 

Posted by: @irmartini

@derek-m 

Here are the last few lines of the heat loss sheet, there is no mention of time or temp.

However in the performance section of the report does state outside temp 3.46deg

image

The Total of 7047 and a suggested 8kWh heat pump seems to make sense to me unless I'm misunderstanding what its saying?

It is difficult to assess from just a small section of the data, but the rated capacity of a heat pump is based upon the thermal energy output, not the electrical energy input.

I suspect that the column titled 'Power (W)' is the calculated hourly heat loss under the specified conditions, which at 7047 W would seem reasonable. The 'Energy HP (kWh) value of 19281 kWh, I believe is the estimated annual thermal energy required to keep your home warm.

 


   
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(@ajn9000)
Eminent Member Member
109 kWhs
Joined: 2 months ago
Posts: 12
 

Seems reasonable to me without going through the figures in detail.  Can't comment on £££.  The most important thing re design is to get your flow temperatures down - which will maximise your COP figures (efficiency) - so quizz your installer about how they are going to do that.  A good source of info : Renew the heating industry - HeatGeek.  Annual energy required is often over-stated in heat loss surveys so comparing that with what you already know about your home is useful.  That (your heat load) will only change if you make fabric improvements.  DHW demand can also be overstated - your figure looks a bit high.  For a 'typical' property space heating is 62% and DHW is 18% of total energy used. Energy Saving Trust provides info on DHW usage.  As Derek M said - at 8kW the HP looks like its sized OK for your 7kW load.  Hope that helps.        


   
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(@harriup)
Estimable Member Member
840 kWhs
Joined: 3 years ago
Posts: 83
 

I posted on another thread which may of some relevance -  https://renewableheatinghub.co.uk/forums/postid/30276 – I had a similar gas use figure and a similar heat loss figure which with the data from the ASHP use has proved to be about twice the real-world performance. Though your gas use is for a lower av temp than the heat pump is required to maintain, it is in the same ball-park.

That extra heat that the heat loss calc implies you need is what is driving the quote for radiators. Are they planning to replace about 10? £2700 is a substantial chunk. At least a target flow temperature of 45° at -3 is a more ambitious target than the usual 50°, and would give you scope to achieve better COPs in warmer temperatures. Is that a smallish gain for a larger cost now though if your real heat demand is lower than expected?

There seems to be shift away from using glycol to having anti-freeze valves to protect the outdoor unit. Glycol reduces the thermal efficiency of the heat transfer and makes the circulation more viscous therefore requiring more energy for the circulation pumps(s) – negative factors that affect it all the time it runs. I believe the Samsung units have their circulation pump in the outdoor unit (?) so it might be that freezing is perceived to be a greater risk for those components.

I would ask why a volumiser is required, and especially ask if a buffer tank is proposed. It used to be that a lot of installers put them in automatically as a backstop, even some manufacturers insisting that one be present. Buffers in particular can just cause a drop in working efficiency. But many systems don't need either at all, so I would ask the question – even if you can't follow the answer you should at least get a coherent relevant explanation.

Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop
210l Mitsubishi solar tank
Solar thermal
3.94kW of PV


   
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(@irmartini)
Trusted Member Member
207 kWhs
Joined: 2 months ago
Posts: 20
Topic starter  

Thank you for the all the replies I really appreciate it.

To sum up
I’m happy with the heat loss it seems to be as accurate as one can expect in my situation and the higher temps its based on gives some overhead in the system, also if the hot water is a little over spec’d for what we might need its more wiggle room in capacity which I’m sure my daughter will do her best to take advantage of as 15-20mins showers are not uncommon even with my protests!
From what I have read it seems to me that having an over spec’d unit can be as bad as an under spec’d unit but I don’t think going lower than a 8kWh unit is worth considering.

@harriup
Your on the money with the radiators that is to replace 10 and as our figures do seem to be very similar and you have a 8.5kWh unit it gives me more confidence that a 8kWh is suitable for my situation. Plus I'm thinking that if the radiators are more than required it would give the possibility of running a lower flow temp without changing the current proposal. I take your point about the move away from glycol but as i see it thats totaly dependant on if the Samsung unit is suitable to do that or not.

Questions for the forum if you don’t mind:
A. I understand lower flow temp increases efficiency and so leads to a higher COP, Is this just a given or are there scenarios where this might not be the case?
B. Is 35deg flow rate a sensible target considering my situation?
C. With regards to a volumizer/Buffer tank, can I ask what is the understanding of why it is better/more efficient without one? Are there pros & Cons or scenarios where it is required and others where it isn’t?

Questions I want to go back to the installers with once I have an understanding of the above:
1. What would the effect of specifying a lower flow temp of say 35deg have on the proposal.
2. Is the Volumizer/buffer tank actually required if so why?

“Anything worth doing, is worth doing right.”


   
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(@harriup)
Estimable Member Member
840 kWhs
Joined: 3 years ago
Posts: 83
 

The output of radiators running at 35° is around half that at 45° – so your radiators would have to be twice the size! And you can't really drop the flow temp much further as the heat output drops off so quickly,  so the unit would start cycling quite quickly. Ideally you want to use the ability of the pump to modulate its output as much as you can before asking it to run periodically.

The volumiser simply adds more water, therefore more energy, into the primary circuit which may be required to run a defrost cycle. Depends on the system, and the amount of water in the pipe runs – but not often needed I think.

Buffers can add inefficiency to a system because the return and flow can mix before the heat is delivered to the house; the flow to rads can draw in cooler water from the return and be less effective. They might be required for multi-zone, multi-circulation pump situations, but have to be done right.

Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop
210l Mitsubishi solar tank
Solar thermal
3.94kW of PV


   
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(@ajn9000)
Eminent Member Member
109 kWhs
Joined: 2 months ago
Posts: 12
 

Hi Ian

Re buffer tank: we have a Stiebel Eltron ASHP with 100 litre buffer tank.  SE are happy to work with installers to design a system with or without a buffer, but they prefer 'with' because they consider it to be difficult (not impossible) to fulfill all the criteria for a good design without a buffer tank. 

The criteria are:

- only one heating circuit which is always open

- to achieve a minimum flow rate

- to achieve minimum running times

- ASHP has access to sufficient energy for defrost cycles (it pulls this energy from the heating circuit) 

Heat pumps like a large volume of water - the buffer tank adds volume.

For our installation there were risks to all 4 of these criteria.  We have an old long thin stone house with a high-spec extension with lots of solar gain. We also don't like too much heat upstairs.  We therefore have 3 zones and the thermostats operate as timers and temperature limiters.  This configuration required a buffer tank.

The system is working very well.  Heat sCOP well above 4.00.  I have taken readings of the buffer tank flow and returns and they match quite well, ie, both return temperatures are very similar.

We have UFH down and rads up and both circuits operate at the UFH flow temperature (average 35 degC).  Heating curve = 0.6.

We tweeked the flow rate through the rads up a bit (2 degC delta T) - that takes a bit more energy to the rads.

Rads were sized for 45 degC - no problems with room temperatures even when we were down to -4 degC.   

Hope that helps and good luck!     


   
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(@mike-h)
Reputable Member Member
1197 kWhs
Joined: 3 years ago
Posts: 127
 

Hi Ian, as a Samsung ASHP owner, I have some concerns about what is being proposed. Regarding the buffer tank, I think it depends on the design of the tank. My buffer tank's outlet and return pipes were within a couple of inches of each other, so there was no chance that the flows would not mix. I was getting up to 4degC lower flow temperature to the radiators than was coming from the heat pump. This is equivalent to a 12% loss of efficiency and I noticed a big improvement when it was removed. Also, if it is outside your thermal envelope - say in a porch or garage, you will be wasting the heat loss from the tank. On the other hand, taller tanks with smaller circumferences may not produce as much mixing. See Protons for Breakfast's blog on having his Low Loss Header removed - it made no difference. https://protonsforbreakfast.wordpress.com/2023/10/10/a-heat-pump-experiment/

Posted by: @harriup

I believe the Samsung units have their circulation pump in the outdoor unit (?)

The Gen 6 Samsungs don't and I doubt if the Samsung HT does either. I would recommend an anti-freeze valve at the lowest part of the return as recommended by Ken Bone in another topic. No glycol!  I would be careful in putting too much credence on the SCOP figures - they can be like miles per gallon claims by some car manufacturers. Some ASHP manufacturers are more realistic than others. My 12kW Gen 6 Samsung quotes a SCOP of 3.68 at a flow temperature of 50degC, but gets nowhere near this unless it is 12 degC outside or more in which case you don't need that sort of flow temperature.

According to Samsung Tech support, the 8kW HT model can only modulate down to 4kW heat output. https://community.openenergymonitor.org/t/samsung-gen-6-8kw-efficiency/22132/97   Despite the thread title, SarahH was discussing her Gen 7 HT Samsung. Apparently the 8kW model uses the same compressor as the 14kW model, so this is probably the explanation - see https://community.openenergymonitor.org/t/samsung-gen-7-ehs-mono-quiet-r32-ashp-series-a-few-beginner-s-observations/25430 for more on this. The reason I point this out is that unless you have a property with a high heat loss, the inability to modulate below 4kW is associated with cycling when the outside temperature is above 7degC or so.

Sorry if this is a bit more negative than you were hoping for.

 

 


   
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