[another] going greener with Viessmann 200W / efficient heating system design questions

delgarno

As inspired by the excellent thread started by @Screwdriva I’m starting to plan a new heating system as part of refurb work I’m doing.  Still in the planning stage so more questions than answers.  Some thoughts, considerations, and questions below.

Thoughts, feedback and challenge welcome to help me improve the design.

• Current setup
  • 5 bedroom, 4 bathroom, 3000 sq ft, semi-detached, Victorian 1890’s build
  • Family of four, kids 10 and 7, will be busy times for showers in the morning before school / work and evenings before bed
  • Heating: ~20 radiators, about 15 recently replaced on upper floors, attempted to oversize for T40-ish
  • Current system: 37 kW system boiler, 250 litre unvented cylinder located in kitchen
  • most external walls are already insulated (1 inch ish polystyrene), we will put in new double glazed windows everywhere, and I have been adding insulation to the loft with the goal to get 200mm
• Planned works / reason for change
  • We are doing a side return extension which will open up the kitchen and force us to move the boiler & cylinder
  • Adding underfloor heating to kitchen (about 50 sq m) and currently unheated basement (about 40 sq m)
• Goals
  • Balanced cost to install / cost to run – economics need to make sense
  • Reduce energy usage
  • Future proof
  • Re-use where possible, e.g. existing cylinder
• Options considered
  • Heat pump – was excited about this but really could not find a way to make the economics work
  • Electric boiler – currently on single phase, and even if we convert to three phase still not enough output, as I understand a ceiling of 100% efficiency would be v expensive at UK prices compared to gas
  • Move to combi – feels like a step back
  • Move to combi storage – I like the looks of the Viessman 111 or 222, but again seems like a step back
• What I’m thinking
  • Re-use existing cylinder (but it’s giant, perhaps oversized??) but if no reason to change we will keep
  • Replace boiler with Viessmann Vitodens 200W – seems like a good solution for tech, weather compensation, modulation, efficiency
  • Move both to basement if the flue works, else keep the boiler in the kitchen and move the cylinder to basement
  • Leverage all the tech in ‘advanced’ weather compensation, no room controls
  • Hot water priority
  • Two or three zones with different heating curves: 1) upper floor rads, 2) kitchen UFH, 3) basement UFH
  • After doing a bunch of research I came across this excellent video from Urban Plumbers which seems to describe almost exactly what I was thinking: 
• Questions
  • What size boiler do I need – all the boiler companies are telling me the 32kW which I fear could be overkill, and also in this boiler I think would not modulate as low?
  • Are there any other boilers other than the 200W I should consider?
  • Should I split zones for kitchen UFH and basement UFH? Kitchen will be used many hours a day, and basement much less.
  • Any downside to only using weather compensation?
  • Any recommendations on who could do this work for me, I’m in London – SE14
  • What else should I think of?

Screwdriva

Thanks for the kind words. As my thread has highlighted, the V200 has saved us over 31% gas consumption over our previous 5 year old Worcester Bosch, which really helped during the energy "crisis. That said, there are a few things I would do differently if I had a DeLorean time machine. 

1) I would go with a Mixergy cylinder (ideally the iHP with the built in heat pump) instead of the Megaflo we invested in, especially with Solar PV.

2) You won't need a 32 kW boiler especially with the insulation improvements you are undertaking. You will need a heat loss survey done to correctly size your boiler but I suspect you will be fine with a 19kW unit. That said, if you were to oversize the boiler, the V200 can still modulate down effectively minimizing this miscalculation - a huge plus!

3) I would maximize the emitter surface area of the entire area under renovation if I could. This means the 700mm tall X maximum width possible K22 or K23 Stelrad radiators or top tier wet UFH.

4) I would ensure 300mm of loft insulation + ensure my door/ garage door/ windows were new/ thick and airtight. 

5) No other boiler can modulate as low as the V200. That said, if you struggled to find a Viessmann installer, your next best bet would be a Vaillant. 

1961Nick

I'd also consider smart controls for your heating such as Honeywell's Evohome or the Drayton Wiser system. This would give you individual control of every room in your house as well as DHW.

My experience of changing to a Weissmann boiler & Honeywell Evohome in 2020 is a reduction in gas consumption from 27,000kWh to 10,000kWh. Additional use of an air/air heat pump has also made a contribution to the reduction.

delgarno

Thanks @Screwdriva super helpful suggestions!

I have an existing cylinder (was there when I bought the house a year ago), which my plan is to re-use.  Unless there is a good reason (cost / benefit) to replace.  

I’m thinking of wet UFH for all the areas of heavy renovations, which happen to be the lowest levels of the house (kitchen in outrigger in the back) and basement in the front.  This seems like it would be ideal to heat from the ground as that rises?

I need to do a proper heat loss calculation to side the boiler.  Any idea where to start with this?

Good shout on the loft insulation.  When it cools down a bit in the fall I’ll try to get 300mm up there.  I have been buying a roll at Selco every time I go for parts for other projects.

delgarno

Thanks @1961Nick just so I understand are you suggesting this in addition to weather compensation, or instead of?  I already have TRVs on all of the rads.

My understanding of weather compensation is that you let it do all (most) of the work once you get the curves dialled in so no need for room controls.  Or did I get that wrong?

1961Nick said:

I'd also consider smart controls for your heating such as Honeywell's Evohome or the Drayton Wiser system. This would give you individual control of every room in your house as well as DHW.

My experience of changing to a Weissmann boiler & Honeywell Evohome in 2020 is a reduction in gas consumption from 27,000kWh to 10,000kWh. Additional use of an air/air heat pump has also made a contribution to the reduction.

Screwdriva

delgarno said:

Or did I get that wrong?

You didn't - We have no room influence and use pure WC and only the Viessmann app. As I understand it, the V200 doesn't like 3rd party controllers. 

delgarno said:

I need to do a proper heat loss calculation to side the boiler.  Any idea where to start with this?

Your Viessmann installer should do this by default. 

1961Nick

delgarno said:

Thanks @1961Nick just so I understand are you suggesting this in addition to weather compensation, or instead of?  I already have TRVs on all of the rads.

My understanding of weather compensation is that you let it do all (most) of the work once you get the curves dialled in so no need for room controls.  Or did I get that wrong?

1961Nick said:

I'd also consider smart controls for your heating such as Honeywell's Evohome or the Drayton Wiser system. This would give you individual control of every room in your house as well as DHW.

My experience of changing to a Weissmann boiler & Honeywell Evohome in 2020 is a reduction in gas consumption from 27,000kWh to 10,000kWh. Additional use of an air/air heat pump has also made a contribution to the reduction.

I'm suggesting that smart radiator valves could save you multiple times what weather compensation will. Every room  is a zone & each valve can demand heat so you can heat just one room if required - for example just the occupied bedrooms overnight - or if you work from home just the office during the day.

Reed_Richards

delgarno said:

• • Any downside to only using weather compensation?

The advocates of just using Weather Compensation also advocate that you shouldn't use zones.  If you have no internal thermostats then Weather Compensation relies on your entire house behaving as a single unit and will get confused if the heat loss from one zone varies because another zone has a temperature that varies.

Reed_Richards

Screwdriva said:

3) I would maximize the emitter surface area of the entire area under renovation if I could. This means the 700mm tall X maximum width possible K22 or K23 Stelrad radiators or top tier wet UFH.

I'm all in favour of the sentiment but I think you ought to be a bit more scientific and try to match the size of the radiator to the heat loss from your room.  What you are trying to do is to run your boiler at as low a temperature as possible in order to achieve maximum efficiency so you should size each radiator (or set of radiators) to keep each room at 20 C when it's, say, - 4 outside and have the boiler heating the radiators to, say, 40 C input temperature.  Choice of input numbers may vary but the point is to do some calculations rather than just finger-in-the-air radiator sizing.

For wet UFH you should be aiming for a lower input temperature than 40 C.

Reed_Richards

delgarno said:

• Options considered
  • Heat pump – was excited about this but really could not find a way to make the economics work
  • Electric boiler – currently on single phase, and even if we convert to three phase still not enough output, as I understand a ceiling of 100% efficiency would be v expensive at UK prices compared to gas
  • Move to combi – feels like a step back
  • Move to combi storage – I like the looks of the Viessman 111 or 222, but again seems like a step back

At current fuel prices the running cost of a heat pump should be about the same as that of a gas boiler but an electric boiler, however cheap to install, would cost about 3x as much to run.  You can look at how much you currently pay per annum for gas and multiply that by 3 for an electric boiler.

Given that a heat pump would cost about the same to run as a gas boiler, does it have any advantages?  There's no cost advantage, a heat pump system would be more expensive to buy than a gas boiler (probably even after the £5k grant) because you would have to replace your water cylinder with one with a larger area coil.  if your house really needs 35 kW of heat in cold weather (and so you have a massive gas bill) then you could struggle to find a heat pump large enough and siting the heat pump somewhere outside could be an issue.  The only advantage is that its greener; even though some of your electricity will come from inefficient gas-fired power stations, you will emit less carbon with a heat pump than with a gas boiler.  

Screwdriva

Reed_Richards said:

I'm all in favour of the sentiment but I think you ought to be a bit more scientific and try to match the size of the radiator to the heat loss from your room.  What you are trying to do is to run your boiler at as low a temperature as possible in order to achieve maximum efficiency so you should size each radiator (or set of radiators) to keep each room at 20 C when it's, say, - 4 outside and have the boiler heating the radiators to, say, 40 C input temperature.  Choice of input numbers may vary but the point is to do some calculations rather than just finger-in-the-air radiator sizing.

Every single heating engineer (there have been a few) I have spoken with about the subject, including the Heat Geek who designed our system, have said the following "there is no such thing as too large a radiator for low temperature heating". 

Reed_Richards said:

At current fuel prices the running cost of a heat pump should be about the same as that of a gas boiler but an electric boiler, however cheap to install, would cost about 3x as much to run.  You can look at how much you currently pay per annum for gas and multiply that by 3 for an electric boiler.

I haven't crunched the latest numbers but I understand that Octopus Gas tracker averages around 4p per kWh. Their Agile incoming is currently ~.20p or thereabouts. Not sure if that factored into your calculations of 3X the cost of running but currently I believe gas is 4X cheaper than electricity, if not more.