Going green(er) with a Viessmann 200W (& heating efficiency home improvements)

Screwdriva

coffeehound said:

Hi @Screwdriva, do you have a ref to this figure?  Looking at Viessman's info, they just quote a minimum figure of 2.9 - 3.2 kW, dependant on flow temperature.  In comparison, your WB has a quoted minimum output of 5.54 kW, so yes the Viessman can go significantly lower.

I calculated 2kW using the 1:10 ratio but you're correct - Viessman clearly state a ~3kW minimum. To your follow up point, still > 42% lower than the WB. With the weather/load compensation and UFH, we are hoping to be able to take advantage of this modulation to net a higher real world improvement.

On quoted efficiency, I believe aluminum heat exchangers experience a more rapid drop in efficiency (vs. stainless steel) over time - it's likely that a 5 year old WB boiler won't be achieving those quoted figures today, but I have no way to prove this. 

We have a 250L Megaflo indirect tank with an immersion heater. We don't have a diverter but plan to manually engage immersion heating when the PV system exceeds 3kW generation (usually from 11:15-1:15-pm).

What do you think the real world improvement will be?

ed110220 said:

Sorry to pile in on the negativity but I'd also question installing a new gas boiler when it's technology that's being phased out. Maybe if your previous one failed and a heat pump isn't for you right now, but I wouldn't want to be replacing one newish gas boiler with another when it's likely to be legacy technology soon.

No worries and thanks for posting - I was expecting this point would be raised. We did a ton of research about Heat pumps and reviewed several quotes. We even visited a couple installations and decided it just wasn't compatible with our application. Maybe after a decade of innovation, when they're quieter, cheaper and more reliable?

For now a 20% H2 ready gas boiler with the above efficiency improving measures is as good as it's going to get for most people. Despite our best intentions, really can't see a more efficient way to heat our homes.

Reed_Richards

Screwdriva said:We did a ton of research about Heat pumps and reviewed several quotes. We even visited a couple installations and decided it just wasn't compatible with our application. Maybe after a decade of innovation, when they're quieter, cheaper and more reliable?

I replaced a 2012 external oil boiler with a heat pump in December 2020.  So far the heat pump has not gone wrong, which makes it a good deal more reliable than the oil boiler which went wrong at least once a year.  The noise the heat pump makes seems to me to be of a similar volume to that made by the oil boiler, although the sound is different.  But it was fiendishly expensive by comparison; thank heavens that was in the time of the RHI subsidy.

ABrass

How well insulated is your floor?

Screwdriva

ABrass said:

How well insulated is your floor?

At this stage, not very. It's a solid concrete foundation on top of which sits a membrane and then tiles. The new polypipe UFH system will be installed using S2 flexible adhesive and the tiles will be installed on top of this. 

Reed_Richards said:

I replaced a 2012 external oil boiler with a heat pump in December 2020.  So far the heat pump has not gone wrong, which makes it a good deal more reliable than the oil boiler which went wrong at least once a year.  The noise the heat pump makes seems to me to be of a similar volume to that made by the oil boiler, although the sound is different.  But it was fiendishly expensive by comparison; thank heavens that was in the time of the RHI subsidy.

We've been very fortunate with the WB. No boiler related issues since installation in 2017. Modern Viessman 100-W's heat only boilers are also supposed to be very simple/ reliable. 

We live in a lake house surrounded on all sides by water, which will carry the external noise from ASHPs much further than normal. (We have looked at water source heat pumps as an option for the future).

Your oil boiler replacement is a great example of a use case where heat pumps make complete sense. Would be good to know what your real world savings are. 

ABrass

Screwdriva said:

ABrass said:

How well insulated is your floor?

At this stage, not very. It's a solid concrete foundation on top of which sits a membrane and then tiles. The new polypipe UFH system will be installed using S2 flexible adhesive and the tiles will be installed on top of this. 

Reed_Richards said:

I replaced a 2012 external oil boiler with a heat pump in December 2020.  So far the heat pump has not gone wrong, which makes it a good deal more reliable than the oil boiler which went wrong at least once a year.  The noise the heat pump makes seems to me to be of a similar volume to that made by the oil boiler, although the sound is different.  But it was fiendishly expensive by comparison; thank heavens that was in the time of the RHI subsidy.

We've been very fortunate with the WB. No boiler related issues since installation in 2017. Modern Viessman 100-W's heat only boilers are also supposed to be very simple/ reliable. 

We live in a lake house surrounded on all sides by water, which will carry the external noise from ASHPs much further than normal. (We have looked at water source heat pumps as an option for the future).

Your oil boiler replacement is a great example of a use case where heat pumps make complete sense. Would be good to know what your real world savings are. 

Surely that will result in a decent fraction of your heat vanishing into the ground, more importantly a greater fraction than  currently does. The retrofit systems I've seen all promise insulating mats, but they're all wafer thin and never give R values.

Screwdriva

ABrass said:

Surely that will result in a decent fraction of your heat vanishing into the ground, more importantly a greater fraction than  currently does. The retrofit systems I've seen all promise insulating mats, but they're all wafer thin and never give R values.

The Polypipe kit being used comes with grooved insulation boards that are 18mm thick. No idea if a superior thermal insulation option exists?

ABrass

Screwdriva said:

ABrass said:

Surely that will result in a decent fraction of your heat vanishing into the ground, more importantly a greater fraction than  currently does. The retrofit systems I've seen all promise insulating mats, but they're all wafer thin and never give R values.

The Polypipe kit being used comes with grooved insulation boards that are 18mm thick. No idea if a superior thermal insulation option exists?

It says extruded polystyrene. That means an R value of around 3.5 for 18mm. In reality you'll get half that or less as it's notched for the pipes. If the pipes are 12mm then you'll have 6mm of material between your hot water and the slab.

Working out the heat loss calculation isn't trivial but there's going to be a lot being dumped into your concrete slab. I'd see if I could find someone who can model it before I spent money on it.

XPS is good insulation, PIR is better but the difficulty is the thickness you need to get near modern standards for a retrofit property.

Reed_Richards

My concrete floor is topped with a layer of something (screed?) which is not well-adhered to the slab underneath so comes off quite easily.  It's about an inch/25 mm thick.  In your situation I would remove that, perhaps put down a thin layer of self-levelling compound then add more insulation to go underneath your polypipe kit.  The more insulation the better, if you don't mind banging your head on the ceiling. 

Screwdriva said:

We live in a lake house surrounded on all sides by water, which will carry the external noise from ASHPs much further than normal. (We have looked at water source heat pumps as an option for the future).

Oh but think of the fun you could have with a water source heat pump.
       

Screwdriva

Reed_Richards said:

My concrete floor is topped with a layer of something (screed?) which is not well-adhered to the slab underneath so comes off quite easily.  It's about an inch/25 mm thick.  In your situation I would remove that, perhaps put down a thin layer of self-levelling compound then add more insulation to go underneath your polypipe kit.  The more insulation the better, if you don't mind banging your head on the ceiling. 

Screwdriva said:

We live in a lake house surrounded on all sides by water, which will carry the external noise from ASHPs much further than normal. (We have looked at water source heat pumps as an option for the future).

Oh but think of the fun you could have with a water source heat pump.
       

Yes, leveling compound is also planned. As far as the water source heat pump, very tempting. 

BUFF

Screwdriva said:

Reasons for going with Viessman boiler are:

1) Ability of the boiler to modulate down to < 3 kW (1:10 modulation ratio), which is quite incredible! 
2) Weather compensation 
3) Load compensation via Nest 
4) Hot water prioritization
5) 10 year warranty for a relatively well made German boiler that is H2 compatible. 

Absolutely nothing wrong with our 5 year old Worcester Bosch 18Ri heat only boiler but it cannot achieve this level of theoretical efficiency, as it lacks the above functionality.

OK, the Worcester 18Ri can't modulate down to the same level as that Viessman but it can certainly do load & weather compensation with the right controls (e.g. WB EasyControl).
If it was installed according to the specified conditions (Viessman will have similar) a 10 year warranty was available & certainly current 18Ris are compatible with a 20% H2 blend (same as the 100-W). Your WB will have been made in England though, albeit assembled from many parts that probably were not.

It just seems to me that you are probably spending £2k plus junking a perfectly serviceable boiler (with it's embodied carbon) for something which whilst theoretically a bit better is still going to take many years to pay for that minor improvement?