Replacement Heating System

[ThePoplars]

Hopefully this question is in the right place.

We are looking to add a large extension to our detached house in the near future.  This will have a large garage and office on the ground floor, and living space above.  It will probably be built using SIPs.

  In the house there is a boiler which is 20+ years old, has required 6 call outs in 6 months, and needs a powerflush to get rid of the sludge in the system.  Rather than paying for the powerflush now and boiler whenever it gives up the ghost completely, we think we should have a replacement installed now.  Whatever system we choose has to have suitable capacity for our extension when it is built.

  We are on mains gas, and a condensing boiler (rather than combi) has been the recommendation for the replacement, however, if there was the potential for a renewable energy solution within our overall budget (with an incentive payment to offset the additional cost), I’d be interested in exploring that further. 

To give you further background, the house is currently approx. 160 square metres heated by 13 radiators with an additional 100 square metres of living space to be added in the extension.  I’d be happy to have underfloor heating in the extension, but most of downstairs in the house can’t be retrofitted with underfloor heating in the short term so we would probably have to use the existing radiators or upgrade them. 

Our roof faces East and West, and the extension roof will face East so I’m thinking photovoltaics are probably not worthwhile and I’ve no interest in maintaining a biomas boiler. I work from home, my chap will retire in 5 years, and we don’t want to move again till we are doddery. 

So, I’m thinking that we are limited to Air Source or Ground Source Heat Pumps if our circumstances really warrant their use.  We do have a South facing wall where an Air Source Heat Pump could be fitted but, I don’t think we would dig up the garden so, a borehole would have to be used if we considered a Ground Source Heat Pump.

My questions are:

1. Does anybody have any experience of retrofitting one of these pumps to an existing home as a replacement for a mains gas boiler and would they recommend it?
   

2. I’m assuming that the application for RHI would be better to be made when the extension is complete (if we are prepared to take the risk that the tariffs paid could be reduced before we make the application)?  I know we have 12 months to apply from commissioning, so if we have planning permission and have commenced the build, does anybody know if we can make an application claiming the final total size of house at that point or do we have to wait till the extension is complete?
   

3. Are there any other recommendations you could make for a replacement system?

Many thanks

Gillian

[mafaldina]

I'm sure others will be along in a moment with appropriate suggestions, but if you are keeping your rads (or even just the pipes) you will have to powerflush anyway.

 

[PeterW]

As quick answers ...

 

- if you keep all the rads the same size, a heat pump will struggle to get to the temperature needed to get them warm. 

- RHI is bonkers for new builds as it works in favour of energy usage not energy saving. It pays for retrofits but it becomes expensive when you have to get a registered installer involved. A £3k ASHP becomes £7k when you claim RHI so the installer makes his money too. You're paid for wasting energy as you get paid per kWh used and a C band house uses a lot more than an A band..!

- depending on DHW (hot water) load, you may be better off with a big combi boiler and lose the tank. 

 

If the Rads are also 20+ years old it may be worth renewing as if you go from a gravity system to a pressure system you may find leaks appearing.  

[ThePoplars]

Thanks Mafaldina - you're right; I was thinking of the cost saved by only draining the system once. 

 

PeterW - I'd say the radiators are at least the same age, and some look like they may have leaked in the past so that's something else to add to the cost benefit analysis. We do have to have an installer doing it all because we don't have the time/skills to do it ourselves.  

 

Whilst it would be great to make a profit, I'd be happy if we could break even. 

[ThePoplars]

Or at least, not pay much more than a like (ish) for like replacement boiler :-D

[Vijay]

If you have mains gas, what's your reason for wanting an ASHP?

[ThePoplars]

Hi Vijay

 

Sorry for the delay in replying (I don't think I can have set up my notifications correctly).

 

I'm thinking about it because we're having to replace our heating system soon and the extension will increase our living space by a large margin.  I wanted to make sure we weren't dismissing alternative heating mechanisms just because we are on mains gas. 

[Vijay]

The reason I ask is I believe mains gas is still a relatively cheap way of heating with a good boiler. So the extra expense of an ashp  would pay for a lot of gas bills

[SteamyTea]

As always, you need to look at the numbers involved.

So you need to know, in no particular order, the min, max and 3 standard deviations (99% of the time) for your space heating load.  This will help you choose a boiler of any sort.  Domestic Hot Water (DHW) then complicates it if you are running it off the same system.  Basically, it cannot do both at the same time, so you loose a bit of space heating while you are heating water.  So you go oversize a bit to accommodate this.

All that is fairly easy to establish either from first principles or from a local plumbers experience.

 

It gets harder when trying to work out the best options for supplying the energy and power (energy and power are different things).

Starting at one end you can have normal resistance heating, just an electric heater that warms the room or water.

Then you could have an infra-red heater for the rooms, and resistance for the water.

Resistance water heating can take two forms, stored and unstored, so a cylinder that is heated up, or an inline instantaneous water heater (like a cheap shower).

Moving up a step, you can get a solid fuel boiler, that can heat water to supply the space and DHW systems.  Or an oil one, or a gas one.

Then you could look at Air 2 Air Heat Pumps for space heating and resistance heating for water.

Or an Air 2 Water Heat Pump (ASHP) for either just space heating (works well) or for both space and DHW.  The DHW side is a bit trickier as it needs to deliver at a higher temperature, so can, when it is drawing cold air from outside, have a coefficient of performance (CoP) of 1 (one), which is, in effect, just a resistance heater.

ASHP also 'frost up', so need to be sized correctly (oversized) to reduce the periods that this happens.  Not a big issue (though people make it one), just a feature of the technology and physics.

One way around the lowing CoP is to heat and store DHW at a lower temperature, then boost it up with an inline heater.

Then there is Ground Source Heat Pumps (GSHP).  These work exactly the same as ASHP, but draw the heat from the ground (actually they tend to use the ground water that passes by).  They generally don't suffer from frosting, though it has been known to freeze the ground in some places (when the pipework is not deep enough and sized incorrectly).  Generally they are very good and reliable, but still fail to reach a high temperature for DHW, so usually have a resistance heater built in to help out (but this lowers the CoP).

Finally you can have a Water Source Heat Pump if you are by a large river or lake.  These are the best, but impractical for most people.

 

Now comes the harder bit.  The environmental damage that each technology causes.  This is usually based around two criteria, CO_2e and resource depletion/damage.

So Coal has a pretty bad environmental record.  It has high CO_2e emissions, is limited in reserves (though they are still huge) and wrecks the mining area.

Gas and oil are similar, but better, a lot better on all 3 counts.  Reserves are more limited, but still huge.

Large scale Hydro has low CO_2e (though the constant plant growth/decay adds to the CO_2e levels) but causes large amounts of damage locally and socially (a million people moved in China).

Nuclear has a very low CO_2e footprint, but is expensive and not very useful for variable loads (not a criticism, but a characteristic of the technology and physics)

Then moving onto the more 'renewable' technologies is Wind, a good method of generating energy, but not so controllable on the power though.  To be more useful on power delivery, we would need either, a lot more turbines (like millions of them) or a decent storage methods (and the best is pumped water storage, so see Hydro for issues).

Wind is not without problems, it causes great social issues (though not as great as climate change and not having electricity cause).  It is cheap to deploy though and can now be integrated quite well into very large national grids (once planning has been overcome).

Solar is similar, and oddly, in the UK, would use less land area (but in a different way,).  It has the advantage that is is quiet, can be easily scaled, and is easily moveable (it packs up into a truck).

Like Wind, it is low CO_2e but does rely on some mineral depletions (not a problem today, but could be in the future).

Finally there is Ocean Based Generation.  The best is probably tidal flow (turbines under the sea) and then tidal lagoons (hopefully one being built near Cardiff).

The worse are tidal barrage as they physical change the local environment , though can help with local flood control.  They are predictable though.  One problem is that when integrated into a grid, is that other systems need to be turned off.  This sounds like a good thing as coal fired generation could be turned off, but it is often easier to turn off Gas and Wind as they do not need long lead in times.  So not the ideal silver bullet that people thing.

The very worst is wave power.  They just don't last very long around our coast line.  They are still experimenting with them though (I like the Hayle Estuary, it has a cafe at the shallow end).

 

So all that, may or may not, help you decide which is the 'best' system to use.

If I had spare cash, I would got for an PV assisted ASHP, UFH, TS with IL RH for DHW.  Just had to get all those horrible abbreviations in.

But as it is I shall stick with my storage heating on E7.

Edited September 13, 2016 by SteamyTea

[ThePoplars]

19 hours ago, Vijay said:

The reason I ask is I believe mains gas is still a relatively cheap way of heating with a good boiler. So the extra expense of an ashp  would pay for a lot of gas bills

 

Currently, yes, but I know gas prices have risen higher than electricity over the last few years.  In 2009, I was setting up a large contract for gas supplies and had a conversation with an energy consultant who gave me all the reasons why we wanted to lock ourselves in to a long contract for the amount the company was buying.  I don’t have the same source of information now but my gut tells me that gap is going to continue to narrow.  I just wondered if there was somebody using the forum who had judged the retrofit away from mains gas under RHI to be worth it.  If nobody has, staying with a likeish for like replacement is maybe the best solution. 

[ThePoplars]

Thanks for such a comprehensive reply SteamyTea.  I probably should have limited my question to the alternative heating systems covered by the RHI.

Those abbreviations just trip off the keyboard – I’ve translated your recommendation as: Photo Voltaic assisted Air Source Heat Pump, Under Floor Heating, [TS] with In Line Resistance Heating for Domestic Hot Water, but you’re sticking with Economy 7.

TS is stumping me? :-D

[Redoctober]

3 minutes ago, ThePoplars said:

TS is stumping me? :-D

 

 

I believe it is Thermal Store.

[SteamyTea]

Yes

[ThePoplars]

You learn a new thing every day.  Thank you.