Heat Loss - Seems very low!

[Bramco]

  On 03/11/2021 at 09:47, Moonshine said:

allows you to get the RHI payments

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  On 03/11/2021 at 09:50, IanR said:

MCS provides the entrance to RHI.

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But if your build is highly insulated, airtight etc, then the RHI ain't going to be worth having as against the extra costs of the MCS install. But it's almost impossible to find installers who will do a non RHI install, Everyone adds on this chunk regardless - and some who behave like the old double glazing salesmen taking money from gullible people.

 

So I guess, where I'm going is - which are the installers out there who will give a proper quote based on supply and install with no frills?

 

Simon

[IanR]

  On 03/11/2021 at 17:31, Bramco said:

But if your build is highly insulated, airtight etc, then the RHI ain't going to be worth having as against the extra costs of the MCS install.

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As is often reported on this forum, SAP, and indeed the calcs MCS installers do, tend to over-estimate energy losses of homes that are built to, or close to, PH standards. The inbuilt defaults for cold bridging are an obvious anomaly, but I'm sure there are others. With the RHI grant based upon the SAP energy use, it's likely the figures used will be higher than what PHPP would calculate.

 

I've heard of people gaming the system by not air testing prior to ASHP commissioning, so that a default 10m³/m².h is used in the calcs. I wondered if this was the reason for a push towards RHI payments being linked to metered energy usage, rather than Design Calcs, although that has never become mandatory.

 

  On 03/11/2021 at 17:31, Bramco said:

Everyone adds on this chunk regardless

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That's not my experience of Eco East Anglia. I felt their (MCS) install at mine was reasonable value. I can't see that a chunk was added, just because the MCS allows an RHI grant to be obtained. Yes, I could have got it a bit cheaper, by selecting cheaper products, but I wasn't going to install it myself, so needed someone to install and don't feel I paid that much of a premium for an MCS install.

[ProDave]

  On 03/11/2021 at 17:31, Bramco said:

But if your build is highly insulated, airtight etc, then the RHI ain't going to be worth having as against the extra costs of the MCS install. But it's almost impossible to find installers who will do a non RHI install, E

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Several of us made that same conclusion and just bought an ASHP as cheap as possible and installed it ourselves.

 

I have done one no RHI install (just the electrics) for a customer.  There is no reason whatsoever that any plumber and any electrician cannot install an ASHP between them at their normal labour rate.  They just need to be able to read a manual, so that might rule some out.

[Bramco]

@IanR

 

Thanks Ian - we may not make it under the old scheme anyway but gaming the system sounds like a great way to avoid doing what I should be doing by building an expert system.....

 

Your experience with Eco East Anglia sounds like you found the right installer.

 

Simon

[Bramco]

  On 03/11/2021 at 18:07, ProDave said:

so that might rule some out.

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er yes.....

 

In the current house we installed a heat bank with solar thermal (long time ago, wouldn't dream of it now) and I had to draw a very simple diagram for the plumber to follow - which he did perfectly.  Electric install I did myself. It's been great especially since we installed PV and a diverter.

 

All the trades working on the extension basically looked at the heat bank as if it was a starship.   

 

Having said that, the house is now up for sale and I can see a lot of viewers building in the cost of ripping everything out and putting  a 'nice big' combi in....    And they'd be missing out on free hot water for 4 or 5 months a year...

 

Simon

[TerryE]

@Bramco, I see that you've just done a like on my Heating the Slab – an overview post.  After over 4 years the system works pretty much faultlessly.  The 3 storey house is heated by what is essentially and immersion heater.  The one tweak I have added is an oil filled rad in my office on the 1st floor which my home automation system turns on for a few hours overnight to add a bit of space heating for the upper floors (and more in cold snaps).  Without this the temperature can drop below 20°C which is my comfort threshold.   This actually works out cheaper as only need to supplement heating outside of E7 rates in the absolutely coldest weeks.  Using resistive heating like this isn't really efficient in terms of run-rate as it costs us about £700 p.a. for space heating, but the upfront installation cost was pin-money,  and zero maintenance because of no moving parts (I do have a spare Harvey heater for cold-swap if the current one fails.)  Adding an ASHP might drop this energy cost by £500 p.a. or so, but at an upfront one-off cost of £5-10K for installation of a piece of kit that might just have a 10-year life.  There is no economic case here.

 

Our slab input temperature rarely gets above 35°C and if we were spreading our heating throughout the day then an ASHP with <5kW output in the range 30-35°C would be fine.  Most installers wouldn't understand a house that operates in this domain.

 

Our house pretty much exactly performs as the simple heat calc predicted -- we've got maybe + 20% on the slab losses due to a thermal bridging flaw discussed in the blog, but this is small-beer overall.

 

And yes, air heating of leak churn is a big component.  IMO using MVHR is pretty essential.  This allow you to have an air-tight house but at the same time keeping it fresh and avoiding the damp + stuffiness that you would get without it.

[RomyD]

Tks for the advice everyone. I think I may have made a mistake on the heat loss calcs. I used 21c and 10C for the delta on the floor but I suppose I should have used the UFH set temperature. Is that right?

[SteamyTea]

  On 19/11/2021 at 19:18, RomyD said:

I used 21c and 10C for the delta on the floor but I suppose I should have used the UFH set temperature. Is that right?

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Not sure what you mean.

But you need to use the mean flow temperature of the UFH system as the floor temperature and use 8°C for the ground.

So your delta T may be around 30 K, rather than 12 K for a radiator central heating system.

This is what a lot more floor insulation is needed than building regs suggest.

[RomyD]

Yes I get it now. Tks!