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Heating conundrum - gas or ASHP


SuperPav
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Right, the other half and I are going round and round in circles on this to the point where any sanity has left the building...

 

The builders are about to start the roof on the bungalow second storey, as soon as they've sheathed it, we're in digging up the old floors for insulation/screed/UFH and running 1st fix.

 

One thing we still haven't decided on is what to do with the bl**dy heating! The property has gas, currently a meter inside on a wall that will be in the living room, so the meter has to go. The other complication is that we've bought a gas range cooker, and that is a non-negotiable with the boss, so need some form of gas for cooking.

 

It's a 50's solid wall bungalow, new second storey will have insulated cavity walls. All floors (suspended timber and loosefill screed) dug up and UFH in liquid screed laid over ~200mm of EPS/PIR build up.

 

The layout means the new boiler/cylinder/cooker is on the other side of the house from where the current connection is.

 

Option 1: Keep gas for both heating and cooking - move meter to the outside of the current wall (WW Utilities will charge £750 for the privilege), run trac pipe either under the floor, or around the garden, or through the posijoists to the new boiler and cooker (which is easier??). With a 40kW combi boiler + 12.7kW cooker connection I imagine means a fairly hefty tracpipe (35mm?) as it'll be about 15-20m long.

 

Option 2: Keep mains gas, but install 300l UVC with system boiler, to allow future switching over to ASHP. Otherwise same as above, what's the most painless way to run the gas from one side of the house to the other?

 

Option 3: Keep gas for cooking only, install 8kW-12kW ASHP and a 300l cylinder (£7k). If we're doing this, the pipe can be much smaller, but still needs the meter relocating and trac pipe running through the house.

 

Option 4: Cap gas off completely, install ASHP + Cylinder (£7k?), + find location on other side of house for a LPG tank, and pipe that to the cooker.

 

 

Literally depending on the day of the week, we're undecided on any of the above options. I'd prefer an ASHP from a "green-ness" point of view, and we'll be upgrading the supply to 3-phase, but at the same time it seems like it'll be a significant expense + we still need to keep some form of gas for cooking.

 

What are your thoughts? Diagram showing layout and gas run conundrum attached..

 

1676477709_gaslayout.thumb.png.4002e21a93a2f0f7312073097c84e06b.png

Edited by SuperPav
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We use cylinder gas for our hob. Cost in terms of £ per kWH its high but in absolute terms its not expensive. A 47kg cylinder lasts at least 18 months. We have two cylinders feeding a 6 ring hob Britannia range with electric oven. 

 

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8 minutes ago, Temp said:

Solid uninsulated walls on ground floor ? Stick with mains gas.

 

Hi @SuperPav

 

Sadly +1 to temp's suggestions above.  ASHP is good if you have great thermal resistance and airtightness because its strength is being able to keep the temperature stable.

 

But if you have heavy thermal loss the ASHP has to work much harder (less efficiently) to raise the temperature.

 

Put as much insulation in the loft as you can. We have about 100m2 bungalow with the combo of an ASHP, MVHR, PV and an EV. This works very well. But we have a timber framed 1970 renovated and greatly insulated building with about 200mm rigid insulation in the floor, 150mm insulation in the walls and 400mm fluffy in the loft, no trickle vents, no cat flap and reasonable airtightness.

 

Good Luck

 

M

 

 

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11 minutes ago, Marvin said:

But if you have heavy thermal loss the ASHP has to work much harder (less efficiently) to raise the temperature

Only if it is sized I correctly.

Put a gas boiler in that is too small and you would have the same problems.

It really is time we stopped thinking that say an 8 kW HP is a direct replacement for a 24 kW gas boiler.

 

48 minutes ago, SuperPav said:

builders are about to start the roof on the bungalow second storey

Are you having PV fitted. That would change the economics of running a heat pump.

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It'd be Option 1 for me, or option 2 if you have more showers in the new second story and think multiple showers need to run at the same time, or the DHW runs will be very long to those showers (a UVC allows for secondary circ)

Presumably you don't have a working boiler already that could just be moved?

Can you get any EWI onto the downstairs walls now? (or as a separate  job in future, which maybe VAT 5% if not done as part of the larger works). It'd make the ASHP future that much more attractive, and reduce running costs/impact of staying on gas. And, if at least designed into the profile of the new upstairs wall the EWI can be done in a less obtrusive way.

 

Edited by joth
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Whatever boiler you use insulate as much as possible, you only buy insulation once. My other half insisted on gas for the hob, 19kg bottle lasts nearly a year. Instal a large DHW tank and PV ready for an ASHP if you decide on gas for the short term.

Edited by joe90
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Thanks all, all thoughts along similar lines to ours.

 

The downstairs walls which are remaining will be solid, but with 40mm-50mm insulated PB on the inside. It's Cotswold stone exterior in a very sensitive area, so no chance of EWI!  Porch + Bays are new, as is the garage (which will be insulated timber frame), so it's not the full perimeter that is solid wall.

 

Upstairs will have 125mm full fill of Icynene or equivalent in the walls, vaulted ceilings with 200mm wool between rafters + 40mm insulated PB under.

All windows are rationel triple glazed. The few skylights we'll have are velux TG. Flat roof will be 150mm PIR warm roof.

 

Existing boiler has long since been converted to tins of tuna or baked beans. 

 

Small PV will go on south facing slope of garage, but depending on budget this may not happen right now. 

 

I guess our biggest headache is to whether just take the plunge to ASHP now, not bother at all (combi), or get it ready for ASHP/PV (gas system+UVC) so it's an easy swap over in the future.

 

 

We're leaning towards gas, but I'm still not sure how easy it will be to route a 35mm tracpipe through the fabric of the house!

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1 hour ago, Temp said:

Solid uninsulated walls on ground floor ? Stick with mains gas.

 

Plus one

 

We had this to decide on our previous build Gas all day long 

Much cheaper to install 

Cheaper to run 

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13 minutes ago, SuperPav said:

I'm still not sure how easy it will be to route a 35mm tracpipe through the fabric of the house!

 my gas pipe for the hob had to run around the outside .

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1 hour ago, SuperPav said:

With a 40kW combi boiler + 12.7kW cooker connection I imagine means a fairly hefty tracpipe (35mm?) as it'll be about 15-20m long.

 

Why a 40kW combi? What is your mains supply, pressure and flow rate? How many bathrooms are you planning? Check all these things first as otherwise you may end up with an oversized boiler - it's worthless if you don't have the mains flow rates to use the boiler capacity. It's worth checking your heat loss calculations to make sure the boiler you've chosen can modulate CH output low enough to prevent cycling.

 

The combination of boiler and cooker you've mentioned will require 35mm pipe but at least the installation volume will be below the commercial threshold at 20m length assuming it's a standard U6 type meter.

 

Have you considered an induction hob instead of gas cooker?

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3 bathrooms. 3 showers unlikely to be used all simultaneously, but 2 certainly. Had a vaillant 938 in a previous house and that was decent enough, price difference vs a smaller was insignificant. Obviously if I go for option 2 and get a UVC instead of a combi, I'm assuming that the boiler can be downrated somewhat!

 

To be clear, the 12.7kW is the max the cooker can draw (so presumably that's what needs to go into the sizing of the pipework) it's rated at 7kW nominal, and frankly I think the whole house would cook if all 8 burners were on full whack! The cooker can't be replaced by induction, that was one of the must-haves for the kitchen for the missus. We've had both in the past, the cooker stays :)

 

How easy is it to run a 32-35mm tracpipe? never handled the stuff myself. Obviously it would be laid by the gas man, but I'd be doing all the trenching/prep, so just trying to work out the best route, whether through the house in the floor insulation, through the house via the ceiling joists, or outside the house in the garden, trying to avoid drains and other mains....

 

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12 minutes ago, SuperPav said:

The cooker can't be replaced by induction, that was one of the must-haves for the kitchen for the missus.

Why, can't she cook.

There is a reason that modern commercial kitchens mostly use induction hobs now.

13 minutes ago, SuperPav said:

the whole house would cook if all 8 burners were on full whack

 

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Well I won't post on a public forum (where she's a member) about her cooking abilities :D

but it was a must-have, and as much as I like induction I do also prefer gas for cooking so it's a done deal (and the cooker has been purchased already).

 

The cooker itself is not that much of an issue though, as it can easily run off LPG if needed.

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49 minutes ago, SuperPav said:

3 bathrooms. 3 showers unlikely to be used all simultaneously, but 2 certainly. Had a vaillant 938 in a previous house and that was decent enough, price difference vs a smaller was insignificant. Obviously if I go for option 2 and get a UVC instead of a combi, I'm assuming that the boiler can be downrated somewhat!

 

What mains water pressure and flow rate do you have?

 

50 minutes ago, SuperPav said:

The cooker can't be replaced by induction, that was one of the must-haves for the kitchen for the missus. We've had both in the past, the cooker stays :)

 

 

I used to do most of the cooking in my house on gas and never thought I'd move away from it. To try out induction, I bought a couple of cheap portable inductions hobs from Ikea to test for a while. I will never go back to gas. No POCs, better control, automic switch off if you leave the hob on, heats up much faster, and much easier to clean. I now have an induction hob that senses where the pan is a puts the heat there, what a dream. But we all have our preferences.?

 

56 minutes ago, SuperPav said:

How easy is it to run a 32-35mm tracpipe?

 

It's very straightforward by all accounts although I haven't used it personally. Joint free continuous runs and the ability to hand bend are the main attractions of course. It ain't cheap.

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2 hours ago, joe90 said:

 my gas pipe for the hob had to run around the outside .

 

Joe, did you just end up running it in gas plastic then, rather than trac pipe if through the garden? Is there a document that shows what the restrictions are in terms of how close to structures/other services/drains/trees it can go? I recall seeing something about trench depths but not the other constraints, as we only have a small strip of a garden all around the house...

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24 minutes ago, SuperPav said:

Joe, did you just end up running it in gas plastic then,

Yes, yellow stuff, just ran it round the house in a Trench, sand and dig tape over the top. Local gas guy connected it up and signed it off. 

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Our gas pipe runs through insulated Posi rafters and is run in Copper (22mm).

 

New gas boiler circa £500 to £1k, make the heating system low temperature, to future proof.  You may need a buffer tank.

 

Combi or system boiler up to you.  If you do use a combi, get one that take preheated water such as an Atag, then you can have a cylinder upstream of the DHW that allows to use to solar preheated water later if required.

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4 minutes ago, JohnMo said:

Our gas pipe runs through insulated Posi rafters and is run in Copper (22mm).

Yes plastic is not allowed indoors in case of fire melting it, mine had to be converted to copper as it entered the house.

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@JohnMo running it in copper through the posi's is an option. Because of the bends in the copper though, I suspect that it'll be *at least* 35mm pipe to maintain the pressure drop within regs, which starts becoming quite expensive in pipe and fittings. All the joists are exposed so labour might be cheap enough if it's something he can do in a day...

 

When running pipe through posi's does there need to be any venting? Obviously trying to avoid venting the void between ceiling and upstairs floor for sound transmission and insulation...

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3 hours ago, SimonD said:

It's very straightforward by all accounts although I haven't used it personally. Joint free continuous runs and the ability to hand bend are the main attractions of course. It ain't cheap.

 

A kit of Tracepipe 22mm 15m long is about £160 including fittings each end. certainly cheaper than altering the mains gas pipe, even with labour!

 

 

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