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

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 JEH
(@jeh)
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@irmartini Hello Ian, I wonder if your house is really ready for an Heat Pump? Maybe you should spend the money on insulation, 13,000kWh is a lot. Our 162sq.m detached house used an average 7000kWh per year for heating over 2020-2023. But on top of that we are in all day long (working from home and using every room) with the thermostat set to 20-22°C and the boiler (at that time) was 40 years old. The way you're operating, with the house unoccupied and the heating off most of the day, is not going to work for a heat pump - you'll end up having to run it all day long or turn it on hours in advance if you want to keep the flow temps down. You could get an idea of the gas used for hot water by looking at peak summer usage when the space heating would be off. I'd suggest you stick with the gas boiler for now and try running that with the lowest flow temp it can do - save fossil fuel that way for now. Frankly, the BUS grant goes straight into the installers pockets and won't save you any money with the way the scheme works these days. Better to design a system that fits your needs not the one you have been given that is designed to tick boxes for MCS.


   
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(@irmartini)
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@jeh 

All that is left on my EPC is floor insulation. It predicts heating 15,397 for heating and 2,322 which is way above what we are currently using.

I have 300mm loft insulation, cavity wall insulation and modern double glazing I thought I was in a pretty good position!?

Here is my gas usage by month for the past few years is it really that high? I didn't think it was that bad?

 

Gas Usage

 

EDIT:

I should point out the 300mm loft insulation was only done a few months ago and the rather strict heating started last year maybe that goes some way to explaining a high usage?

 

“Anything worth doing, is worth doing right.”


   
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 JEH
(@jeh)
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Joined: 3 months ago
Posts: 5
 

I'll show my figures for 20-22 but we didn't use gas after May 2023. I suppose we're in different part of the country but looks like your numbers going in the right direction 👍 Our boiler was 40 years old and the pilot light was using as much gas as the hot water! As an experiment, we turned off the gas and switched to using the immersion in June 2023 and it cost about the same using off-peak Octopus Cosy. The old boiler had a high/low switch and we always used the low setting so our rads were never too hot to hold your hand on. If your boiler flow rate can go low, that would be the next thing to try. Our windows are all triple glazed but probably no better than your modern ones - u-Value 2W/m2°C. On the coldest winter days when it's usually a bright clear sky, we do get a lot of passive heating and while it might be freezing outside, our heating could be off all day. In the evening we'd lower Ikea pleated blinds which seem to help. To give you the whole picture, we do have MVHR - I wonder how much difference that makes? After Oct 2023 our heating has been with just two mini-split a/c units. I should admit in February we did finally modernise the gas boiler (11kW Vitodens 100W System) but we haven't used it for heating - only for hot water.

I'm hoping that helps, rather than confuses your choice. We have ended up with a kind of hybrid system that we're extremely happy with, It's versatile, has redundancy (2x minisplits and 1x boiler 1x immersion) and cooling—should that EVER be required! 

Gas Usage 20 23

 P.S Minisplit a/c heat pumps are cheap as chips, take half a day to install and you can keep the gas boiler just in case. In the first four months of this miserable year ours used 1013kWh for heating compared to your 6627kWh.


   
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(@harriup)
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@jeh 

in relation to your house I can see why you suggest 13000kWh is a lot, but it is around the average heat requirement for the UK, and the average dwelling is only around 100 sqm – so for a 155 sqm house it is a decent figure. The higher figures for heat needed in the original quote probably came from the EPC which is not dwelling specific just an application of those national averages to the sq meterage.

Once you have done the low hanging fruit of loft insulation etc, what is left? External wall insulation is expensive and of less benefit if you already have cavity walls done, lack of air tightness and poor thermal bridging would require some major building renovation to tackle, as would lack of insulation under a solid floor. They would improve performance yes, but take decades to get ROI, leaving you only with the warmth from a lower carbon footprint!

@irmartini

The heat loss survey as required when a heat pump is installed should define the appropriate size of radiator to match the heat loss of each room. The output of the radiators can be tweaked by balancing, which should be done anyway to compensate for the different flow rates through radiators at different distances along the loop.

The only TRV I try to use is in the room which gets a lot of solar gain, in order to prevent the heating system adding to it on the sunniest days.

 

Posted by: @irmartini

Here is the answer I got to this question:

With the HTQ it will be a buffer as this will prevent excessive cycling within the unit as well as give the heat pump a hydraulic interface and therefore will not restrict flow within the heat pump especially if some of the radiators have closed down due to the rooms reaching temperature.

A hydraulic interface does create a loop that the heat pump is in control of, whatever shenanigans go on in the secondary side, hence why manufacturers might be keen. But then does require a second pump to run.

I can see the logic of having a buffer that can take some heat from the pump and then allow it to used to heat the rads when the pump is off. But they also seem to cause inefficiencies. The algorithm used by the pump to set the flow temps and running time also has the potential to minimise cycling – the question is how good is the Samsung at it?

 

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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(@mike-h)
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Posted by: @irmartini

Here is the answer I got to this question:

With the HTQ it will be a buffer as this will prevent excessive cycling within the unit as well as give the heat pump a hydraulic interface and therefore will not restrict flow within the heat pump especially if some of the radiators have closed down due to the rooms reaching temperature.

 

I understand that if a number Radiators start shutting down via TRVs then a buffer would be helpful, but I don't plan to restrict the Radiators with TRVs rather I would be looking to lower flow temp.

I'm not convinced I need one in my situation?

Is it really worth having just in case?

My 50L buffer tank made cycling worse not better. At a LWT of 33C the radiators were only getting 30C due to mixing of the flow and returns in the buffer, so were not able to deliver the minimum heat that the ASHP could modulate down to. There are formulae for working out the system volume required to limit the cycle time, so a sufficiently large buffer tank may correct short cycling and a properly designed one may not allow mixing. However, if you remove most of your TRVs and have sufficient open system volume to exceed the minimum for that model ASHP, then I can see no advantage in having a buffer. You won't find many systems on heatpumpmonitor.org with buffer tanks.

How confident are you with your installer? The knowledge and skill of the installer is probably the single most important factor and many on this forum have wished they had chosen more wisely, myself included. Recommending a buffer tank for your install would be a little concerning in my opinion.

I am sure there are some happy Samsung owners, but it is of interest that apart from the 5kW Samsungs that Glyn Hudson self installed, Samsungs do not figure high up in the list on heatpumpmonitor.org. There is quite a preponderance of Vaillants and Viessmanns though.

Posted by: @irmartini

I wish i wasn't such a control freak and was just able to let the installer make the decisions and do what they think is right and stop worry about it.

Unless you have found a really top installer, that is the last thing you should do! You have plenty of time to do your research.


   
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(@irmartini)
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Many thanks to everyone who has helped with advice and info on this thread I really appreciate it.

Its been quite the learning curve and i still feel like I'm right at the bottom with a long way to go.

I have put my final points across to my installer with a request that we do not have a buffer but do have a volumiser (in the return of the heating circuit so isn't in use when in DHW mode only) and to have anti-freeze valves instead of glycol.

I'm waiting for there reply now.

There are a few other points I've requested like the full monitoring system which they don't seem to be aware of openenergymoniitor.org but understood what components would be required and said if I purchase the kit they would happily fit it.

 

 

Even if they agree to everything, I'm thinking about getting a 2nd full survey and quote even though it will mean another few hundred pounds for the survey I think it will be worth the  second opinion.

I've found a heat geek near to me that i'm going to contact.

https://www.nmbheatpumps.co.uk/index.html

 

 

“Anything worth doing, is worth doing right.”


   
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