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Is ASHP Going to Work for us - terraced retrofit?


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Good evening,

 

I could really do with some of the expert help on here to point me in the right direction.

 

Currently renovating a 1960 terraced house. Current plan is to install solar (5.46kp) and an Aereco dMEV system. Have been looking to add an ASHP but keep going round in circles whether its a complete waste of money in our circumstances. Currently running on a gas combi-boiler that's around 15years old, but doesn't seem to have any running issues. Main reason for installing ASHP would be to hopefully make use of the solar electric generated, and hopefully live as off grid as possible.

 

We have had a heat loss survey completed as required by the ASHP companies we've contacted. which shows a Coefficient Of Performance of 3.68 and a peak heat load of 8.92kW and a recommended 12kW ASHP. The terraced property is approx 115m2.

 

Are we completely wasting our time looking at ASHP as a viable alternative presently? I've read a few posts suggesting money would be better spent reducing heat loss further and to hold off on ASHP. We can definitely add more loft insulation and insulate the suspended ground floor. Are there any alternatives to ASHP which we could look at to make use of the solar generated power to heat our water or rooms and would suit our situation better? 

 

Thanks.

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Max out on airtightness and insulation before considering any change to your heating system.

 

To illustrate a point the cheapest heat pump is about £3k, this would buy you 2.5 meters depth of insulation for your attic. Silly I know, but you get the point I hope. 

 

Are you planning on renewing the entire plumbing system anyway? 

 

To throw another spanner in the works Aereco do an AWN ECO+ DCV unit that has an integrated heat pump. No idea of the costs or efficiency. 

 

 

 

 

 

 

 

 

 

 

 

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It also depends on why you are considering a HP 

If it is to save money Then stick with gas Even in a newly built well insulated home A HP will be more expensive to run than gas 

HPs are better for the environment Or will be eventually when the electricity they run off isn’t produced using Fossil fuels 

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Whichever heating system is installed, if you reduce the house's thermal losses, then you have a system that is cheaper to run.

The reason for this is that you save on two points:

You can run the system less time i.e. only when the outside air temperature is below 6°C.

You need to put less energy in i.e. 6 kW as opposed to 9 kW

 

dMEV is an admission of failure, get the airtightness sorted out properly and you can fit MVHR, does the same and scavenges some waste energy.

 

Have you checked with your DNO that you can connect 5.46 kWp of PV.

That amount of PV is capable of supplying most of your DHW, though during the winter it will only help a little bit.

 

 

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Thanks for the comments. As I see it then.

 

  • Park the ASHP idea for the moment
  • Increase insulation / air tightness. Likely to be more guidance required around this.
  • Regarding the DNO, I'd assumed the solar installer would be checking compliancy, should I request evidence before proceeding?
  • Disappointed to hear about the Aereco potentially being a poor option. Aereco was suggested as an option by the Energy Trust which shared with me a document indicating  the same performance as an 80% heat recovery system. And although the complete install won't be cheap at nearly 5k to supply and fit, I understand it is still cheaper than an MHVR system. I have looked into an MHVR system from BPC, but even their approved installers don't won't to install it, citing quality issues. 
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2 hours ago, Richini said:
  • Regarding the DNO, I'd assumed the solar installer would be checking compliancy, should I request evidence before proceeding?

Just check that the installer is fitting a G98 compliant inverter that will limit output to the grid at 16amp. Thats the limit the DNO sets for fitting systems without prior approval. Certainly with solaredge you can connect 5.4kw of panels to a G98 limited inverter

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Posted (edited)

Reference condition based dMEV you referred to.  I am not in agreement with the comments made with respect MVHR.  I have MVHR as our airtightness is less than 3, so had no option.

 

But dMEV, operating based in air quality is a great way to go as ventilation is only done when needed and on a room by room basis, MVHR is all or nothing.  I have attached a report on such a system which make interesting reading.Atamate_SDAR+Paper+2019+(1).pdf

Edited by JohnMo
Missed word
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If you need a new CH system then you should definitely consider an ashp. At 28/8 unit prices you’d need a COP of 3.5 to break even. Size your rads right and that achievable.

 

And you’ll get decent mains pressure hot water too….

 

Agree with everyone else, insulate and seal up the gaps.

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Thanks for all the comments, advice taken on board.

 

Would there be a case for installing something like an iboost & Megaflo Eco Solar PV ready unvented cylinder for hot water only? Just looking to maximise what we can get out of the solar system, but appreciate there are limits to what we can expect to generate, particularly in winter. 

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I have a 1960s end of terrace house which currently has all-electric heating (storage heaters, with electric underfloor in kitchen and bathroom). We are about to start an extension and at the same time we are going tor an ASHP.

The insulation is not bad (cavity walls are insulated and the loft has decent insulation) but let down at the moment by a 1980s sliding patio door with bad heat bridging issues and some draughty windows. Those will be replaced as part of the build and we are putting wet UFH throughout the ground floor.

We wanted to upgrade from the electric heating to proper central heating and it just seemed like a backward step to be putting in a new gas system in 2022.

Ideally I would be adding solar PV at the same time but I don't think the budget stretches to it on top of everything else. Maybe in a year or two.

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3 hours ago, Bob77 said:

The insulation is not bad (cavity walls are insulated and the loft has decent insulation) but let down at the moment by a 1980s sliding patio door with bad heat bridging issues and some draughty windows. Those will be replaced as part of the build and we are putting wet UFH throughout the ground floor.

We wanted to upgrade from the electric heating to proper central heating and it just seemed like a backward step to be putting in a new gas system in 2022.

Ideally I would be adding solar PV at the same time but I don't think the budget stretches to it on top of everything else. Maybe in a year or two.

 

forget about the PV and sort out insulation underneath your new wet UFH. Now is your only chance to do that. As you say, you can come back later and add PV

 

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Ask yourself "Why is it cold in winter?".  The answer is because we get very much less solar energy in winter than we do in summer.  This makes solar PVs a very bad fit to any form of electrical heating; you get the least amount of solar electricity just when you need it the most.  

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Posted (edited)
12 minutes ago, ReedRichards said:

The answer is because we get very much less solar energy in winter than we do in summer.

Not strictly true as in the UK, which is in an odd location, we are more affected by sea surface temperatures and wind direction.

The reason that solar irradiation is reduced is because the sun's photons have to pass though a longer path through the atmosphere due to the tilt of the Earth, and they then hit the land/ocean at an oblique angle, which, in effect, spreads the power out over a larger area.

Cloud cover also makes a huge difference, clouds can form in a number of ways, but are usually a combination of tempratre,, air pressure and humidity (why exceptionally warm oceans can cause more clouds and rain at higher latitudes). The solar constant, 1.361 kW.m-2 does not vary much (actually goes up slightly during the Norther Hemisphere winter, and is not a constant in the true scientific term, just an average, adjusted to 1 AU). This is why having a steep angle on the roof can be useful for winter PV production.

Edited by SteamyTea
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18 hours ago, joth said:

 

forget about the PV and sort out insulation underneath your new wet UFH. Now is your only chance to do that. As you say, you can come back later and add PV

 

Exactly what I did.

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Just now, SteamyTea said:

Not strictly true as in the UK, which is in an odd location, we are more affected by sea surface temperatures and wind direction.

The reason that solar irradiation is reduced is because the sun's photons have to pass though a longer path through the atmosphere due to the tilt of the Earth, and the then hit the land/ocean at an oblique angle, which, in effect, spreads the power out over a larger area.  This is why having a steep angle on the roof can be useful for winter PV production.

My comment was meant as a general observation about winter (you're right about the reason why).  In the UK there are mitigating factors due to our proximity to the Gulf Stream; otherwise winters would be even colder!

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1 minute ago, ReedRichards said:

In the UK there are mitigating factors due to our proximity to the Gulf Stream

I wish the Gulf Stream would stop, cloudy this morning.

Atlantic Meridional Overturning Circulation is the posh term.

Clouds are interesting as we think of them as cooling the UK, they cool between the tropics more, and we would be in real trouble if there was less of them.

clouds.jpg

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Posted (edited)
19 hours ago, joth said:

 

forget about the PV and sort out insulation underneath your new wet UFH. Now is your only chance to do that. As you say, you can come back later and add PV

 

 

Yes the whole ground floor is being dug out and insulation being added. The exact details of this are still to be fixed - I am open to any recommendations as to the standard of insulation I should be going for so I can make sure it is done properly.

As you say this is the one chance to do that so I am keen to get it right!

Edited by Bob77
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Just now, Bob77 said:

open to any recommendations as to the standard of insulation I should be going for?

The thicker the better.

Are you going to be installing UFH as well?

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1 minute ago, Bob77 said:

 

Yes the whole ground floor is being dug out and insulation being added. The exact details of this are still to be fixed - open to any recommendations as to the standard of insulation I should be going for?

As you say this is the one chance to do that so I am keen to get it right!

200+ PIR,  UFH tied to the rebar mesh, 100mm conc

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13 minutes ago, HughF said:

200+ PIR,  UFH tied to the rebar mesh, 100mm conc

 

If you have the space I'd prefer EPS. 

For reduced labour use long staples into the EPS for the UFH and fiber impregnated concrete instead of mesh. 

 

However your SE will advise if it's needed. 

 

 

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6 minutes ago, SteamyTea said:

The thicker the better.

Are you going to be installing UFH as well?

Yes, the current house has a concrete slab. I have been reading about insulation with UFH and have come up with conflicting opinions - some sites telling me that insulation should be under the slab so the UFH heats up the concrete and acts as a thermal store. Others saying don't do that, put insulation over the slab and then UFH on top of that in a thinner screed layer.

 

Option 1 is going to be a lot more expensive but if that's the right way then I guess it's worth it. On the other hand I don't mind losing a bit of ceiling height with insulation over the top, and as the whole house is being done it shouldn't create a problem with differing levels.

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5 minutes ago, Bob77 said:

Yes, the current house has a concrete slab. I have been reading about insulation with UFH and have come up with conflicting opinions - some sites telling me that insulation should be under the slab so the UFH heats up the concrete and acts as a thermal store. Others saying don't do that, put insulation over the slab and then UFH on top of that in a thinner screed layer.

 

Option 1 is going to be a lot more expensive but if that's the right way then I guess it's worth it. On the other hand I don't mind losing a bit of ceiling height with insulation over the top, and as the whole house is being done it shouldn't create a problem with differing levels.

The whole slab->insulation->screed design seems really backwards to me, and something from the 70's. Concrete finishing isn't hard, just use the 100mm conc slab as the finished floor (ready to accept your surface covering) and stuff the gap underneath with insulation. 

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50 minutes ago, Bob77 said:

Yes, the current house has a concrete slab. I have been reading about insulation with UFH and have come up with conflicting opinions - some sites telling me that insulation should be under the slab so the UFH heats up the concrete and acts as a thermal store. Others saying don't do that, put insulation over the slab and then UFH on top of that in a thinner screed layer

I think they are probably starting from a different place.

 

Option 1 for ASHP (use that slab as a store and stop heat going to the ground).

Option 2 with gas boiler running at relative high flow temperatures.

 

Latest regulation will force you to have Option 1 anyway (low temperature systems).

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1 hour ago, HughF said:

The whole slab->insulation->screed design seems really backwards to me, and something from the 70's. Concrete finishing isn't hard, just use the 100mm conc slab as the finished floor (ready to accept your surface covering) and stuff the gap underneath with insulation. 

 

conc slab on top of insulation can be fine for a clean sheet new-build, but on a retrofit where you're digging out an existing concrete slab, you may have challenges getting a SE and BC to sign this off.  We certainly did.

 

As we were digging down deep  there's a risk of disturbing the existing strip foundations (that were unknown depth/quality until we'd already committed to work, i.e. post BC design sign off),  so all they would approve at the design stage was MOT->slab->insulation->screed.

So we ended up with UFH in 80mm screed on top of 160 - 260mm of insulation (thickness dependent on the depths we could dig to in various places).

Not ideal, but this is retrofitting for you.

 

We also tried to do a raft foundation in the extension, but this was rejected after doing all the ground works as the BI decided the raft might float away from the existing house strip foundations.

 

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