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ASHP as preheater for conventional electric boiler

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(@dropbear)
Active Member Member
33 kWhs
Joined: 4 months ago
Posts: 3
Topic starter  

Hi everyone,

I'm reaching out to this knowledgeable community in hopes of some collective wisdom regarding my heating conundrum. My situation is a bit unique, and I'd greatly appreciate any insights or alternative perspectives you might have.

The Setup:

  • Large, 4-storey detached house with 6 bedrooms and 200 sq m of floor space.
  • Currently reliant on an electric boiler and radiators (no underfloor heating).
  • Existing 300L hot water cylinder.
  • Professional heat loss calculation confirms a 12kW heating requirement.

Fuel Options:

  • Solar: Unfortunately, solar isn't an option due to roof limitations and bay windows.
  • Wind: Similarly, wind power isn't feasible for my property.

The Shortlist:

This leaves me with three main contenders:

  1. Biomass Boiler: While efficient, the prospect of fuel storage and additional maintenance isn't overly appealing.
  2. Air Source Heat Pump (ASHP): Intriguing for its renewable credentials and potential cost savings, but a 12kW unit seems quite hefty.
  3. Fuel Cell: An interesting concept, but I'm unfamiliar with its viability and long-term costs in the UK.

My Initial Thought:

Given my existing radiators and desire to avoid major upheaval, I was considering exploring a hybrid approach:

  • Use a 12kW ASHP to preheat the water to 60-65°C in the existing cylinder.
  • Utilize my electric boiler for a secondary boost to reach the desired 80°C when necessary.

Questions for the Community:

  1. Is a 12kW ASHP truly necessary for my situation? Would a smaller unit with electric backup be a more economical solution?
  2. Has anyone tried a similar hybrid approach with an ASHP and existing electric boiler? What were your experiences and any unforeseen challenges encountered?
  3. Is a fuel cell worth exploring for my needs? I'm open to new technologies, but practical details and cost analysis would be very helpful.
  4. Are there any other options I haven't considered that might be a better fit for my scenario? I'm open to all suggestions and keen to learn from your collective expertise.

I'm committed to finding a sustainable and cost-effective heating solution for my home, and I believe your combined knowledge and experience can be invaluable in guiding me on this journey.

Thank you for your time and any insights you may offer!

 

DropBear 


   
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(@bontwoody)
Noble Member Contributor
2920 kWhs
Joined: 2 years ago
Posts: 416
 

Hi

12 kW isnt particularly large for a heat pump in a large property.

Can I ask why you are thinking of heating water to 80 C ?

I did try boosting water from a heat store with an instantaneous water heater when it started to be depleted. It worked but its flow rate wasnt as good as without so I switched it back.

 

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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(@dropbear)
Active Member Member
33 kWhs
Joined: 4 months ago
Posts: 3
Topic starter  

It probably doesn't need to be 80deg C. That was what the radiators were built for in our house before we bought it.... right before energy prices sky rocketed. Essentially I don't want a solution that involves ripping a whole system out (rads, cylinders etc). It works very well, other than the cosy. I Thought a solution like this might work by taking the bulk of the heavy lifting off the electric boiler. I'm open to alternatives though.


   
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(@derek-m)
Illustrious Member Moderator
13722 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4165
 

Posted by: @dropbear

It probably doesn't need to be 80deg C. That was what the radiators were built for in our house before we bought it.... right before energy prices sky rocketed. Essentially I don't want a solution that involves ripping a whole system out (rads, cylinders etc). It works very well, other than the cosy. I Thought a solution like this might work by taking the bulk of the heavy lifting off the electric boiler. I'm open to alternatives though.

I would suggest that you measure the size and type of radiators and check their specified output at a Delta T of 50C. From the total value it should then be possible to calculate the probable required water temperature at different outside air and indoor air temperatures.

 


   
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(@dropbear)
Active Member Member
33 kWhs
Joined: 4 months ago
Posts: 3
Topic starter  

I'll do that, but in the mean time, can I validate that this is actually a potentially viable solution? We haven't turned heating on for 4 years now, because it's just insane


   
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(@derek-m)
Illustrious Member Moderator
13722 kWhs
Veteran Expert
Joined: 3 years ago
Posts: 4165
 

Posted by: @dropbear

I'll do that, but in the mean time, can I validate that this is actually a potentially viable solution? We haven't turned heating on for 4 years now, because it's just insane

There are hybrid systems with normally a gas boiler along with a heat pump, so I would not see that it would be a problem. Please provide more information about your present system so that a more in-depth appraisal can be made.

 


   
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(@iancalderbank)
Noble Member Contributor
3640 kWhs
Joined: 2 years ago
Posts: 644
 

@dropbear there's an element of confusion in your write up. you talk about the 300L cylinder being preheated to 65 and then heat with something else to 80C, then you talk about the radiators being designed for 80C. those are not the same thing.

does your electric boiler run at a flow temperature of 80C when heating the house? that isn't the water that comes from your 300L cylinder.

The cylinder water is to supply  the hot water in the taps - and that definitely doesn't need to be at 80C.

your electric boiler must be on a separate circuit due to its power.  what is the e-boiler's power rating? what does it actually use when heating your house? Is there a way you could put a CT clamp meter on it? By running your house on the electric boiler and monitoring the consumption, you have a potentially really accurate way of measuring the heat loss of your house, and therefore how big an ASHP you actually need. That is the single most important question to answer when designing an ASHP system.

that said, 12kw doesn't seem an unreasonable first approximation for a 200m2 house.

It worth adding this (It took me a while to get my head round it - if you know this already I apologies, but if not this might help)

a 12kw heat pump doesn't use 12kw of electricity. It produces 12kw of heat  at maximum power - which is only needed on very cold - sub zero days. the usual design criteria for south of england is -2 or -3C. colder in the highlands of course, depends where you live . So at max heat output, the worst case with a very inefficient design it'll be using 6kw of electricity. In milder weather (approx 5-7c) with an efficient system design, it'll be using less than 1kw of electricity. This is where you save the run cost money.

you've said "Not had the heating on for 4 years due to insane" (I assume there's an implied "cost" on the end of that). Given that you are already familiar with the cost points of direct electric central heating, the transition from electric boiler cost to ASHP cost will be fairly easy estimate.  

so I know you've said you don't use it, but I would say: switch it on, keep the house at a steady comfort temp (don't switch it on and off), measure the outside temp, and measure what it uses at a given outside temp. you need to know that number.

If your radiators are genuinely only able to heat your house with 80C water in the circuit and you don't change any of them , you'll be able to run an ASHP at an efficiency (a.k.a COP) of no better than 2 ish. Meaning you won't get better than halving the electricity input (that you would currently would need with the e-boiler, if you ran it). This is not what we'd recommend.

If you do the job properly and change out the radiators so the house can run with a lower water temp, efficiency of high 3's to low 4's is do-able with radiators. Meaning electrical input 1/3 to 1/4 of the e-boiler's level. then you will not feel like you're being robbed when you run the heating.

the other serious option to add into this, if you have the capital, is to get a storage battery. This doesn't have to be combined with solar. buy cheap off peak electricity, keep it in your battery use the stored electricity to run the heating when the tariff is peak.

My octopus signup link https://share.octopus.energy/ebony-deer-230
210m2 house, Samsung 16kw Gen6 ASHP Self installed: Single circulation loop , PWM modulating pump.
My public ASHP stats: https://heatpumpmonitor.org/system/view?id=45
11.9kWp of PV
41kWh of Battery storage (3x Powerwall 2)
2x BEVs


   
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