Viessmann 200-W Gas Boiler - The most efficient boiler sold in the UK?

QrizB

JSHarris said:

40°C to 45°C flow into the UFH seems high.

Heat output from UFH depends on both the flow temp and the density of UFH pipes. MeteredOut's UFH installer might've laid a lower density of pipes than you did, expecting it to be fed by a combustion boiler wher high flow temps aren't a huge issue.

The difference between 35⁰C flow and 45⁰C is what, a couple of percent of boiler efficiency? In the old days, when gas was 3p/kWh, that would be £10 a year?

MeteredOut

JSHarris said:

MeteredOut said:

JSHarris said:

MeteredOut said:

Reed_Richards said:

Screwdriva said:

... despite having underfloor heating on all 3 floors ...

Obviously you would love to have your neighbour verify the capabilities of your boiler model but with underfloor heating throughout, they really ought to consider the option of a heat pump.  Needing only low water temperatures for heating, they could aspire to an SCOP of 4-ish which should be cost-competitive with gas and will go a little way towards saving the planet.  

Does that also depend on insulation levels? We have UFH but don't have great in-wall insulation and I'm not convinced swapping our has boiler for a HP would work for us.

Heat pump efficiency is very largely driven by flow temperature, and UFH allows a much lower flow temperature than radiators.  The problem is hot water, as that needs similar temperatures to radiators, so the saving from a heat pump is greater if it only runs UFH heating.  Maximum UFH temperature for a poorly insulated house with a high heat demand is unlikely to exceed 30°C to 35°C.  Ours runs at about 25°C UFH flow (it's mixed down to that from 35°C).

There is a sort of fix for hot water that I used, which is to have a buffer tank heated to the heat pump flow temperature (35°C in our case) and then use that through a heat exchanger to pre-heat the hot water system.  I installed a thermal battery for hot water, which instantly heats it from a cold input, like a combi.  I feed it with pre-heated water from the heat exchanger on the buffer, so it only has to raise the temperature by a few degrees.  This works well, as although the thermal battery uses direct electric heating (at the off-peak rate) the usage is so low that it's not worth worrying about.  It's also simple, with no moving parts.

That's the issue we have. The UFH was already in place when we purchased this property, and was commissioned with a 55C flow (at the mixer, as documented in the installers notes  that were left behind). I've managed to reduce that to 40-45C (to help with condensing), but any lower and the house does not get/stay up to temperature in the colder winter days.

40°C to 45°C flow into the UFH seems high.  If I push ours over about 27°C the house has a big overheat swing that takes a day or two to cool off.  The heat pump flow is fixed at 35°C, as it needs to be at least 5°C to 6°C above the highest UFH flow temperature to regulate properly, plus it's handy to pre-heat the buffer to 35°C as the heat pump still works very efficiently at this temperature.  I have heard that there are now electronic mixer valves that allow a lower temperature differential, IIRC they work OK down to about a 2°C difference, which allows the heat source flow temperature to be reduced a couple of degrees or so further.

I know, but we have challenges with keeping our living are up to temp. at flows below that. It's a double height open plan living area, so a big space to heat.

I stated poor insulation above, but we also have a gas fire with a boxed-in flue that goes 4 meters up and out through the roof.  On windy days you can feel the drafts in the room very easily. I've spent a lot of time in the past trying to tweak the settings to reduce the flow as much as possible, balance that with the boiler flow setting, and keeping the rooms up to temp.

(we also have 2 upstairs bedrooms with "normal" sized radiators, so the flow also needs to be a big higher if we want to heat these rooms, but they are only used when we have family/visitors staying).

JSHarris

QrizB said:

JSHarris said:

40°C to 45°C flow into the UFH seems high.

Heat output from UFH depends on both the flow temp and the density of UFH pipes. MeteredOut's UFH installer might've laid a lower density of pipes than you did, expecting it to be fed by a combustion boiler wher high flow temps aren't a huge issue.

The difference between 35⁰C flow and 45⁰C is what, a couple of percent of boiler efficiency? In the old days, when gas was 3p/kWh, that would be £10 a year?

Our pipes are on a 200mm spacing, which is slightly on the wide side.  150mm is about as tight as most installers go.  If the UFH is in decent screed or concrete, or has decent thickness spreader plates, then the spacing isn't super critical, as heat will conduct out of the pipes quickly and heat the floor surface evenly.  The heat output power per unit area is only really a function of the mean temperature differential between the floor emitting surface and the room  temperature, which isn't that easy to calculate super accurately, but can be roughly approximated from this equation:

Heat output per m²= (8.92 * ΔT)^1.1

Where ΔT is the temperature differential in K and the heat output is in W/m².  It's handy to have an idea of this, just measuring the floor surface temperature with an IR thermometer at a few locations and subtracting the room temperature from the mean value measure is good enough to get a number to slot into the formula, which in turn can be used to estimate the heat output power for the whole house.  I'll stress this is an estimate, as there are fudge factors in this formula to simplify it a lot, but it'll be within a few percent for normal UFH installations.

For a boiler the difference between 35°C and 45°C flow will be lost in the noise, but for a heat pump the efficiency difference will be a great deal higher.  At 45°C flow my heat pump will drop about 25% in efficiency in cold, wet weather when compared to running at 35°C flow.

MeteredOut

QrizB said:

JSHarris said:

40°C to 45°C flow into the UFH seems high.

Heat output from UFH depends on both the flow temp and the density of UFH pipes. MeteredOut's UFH installer might've laid a lower density of pipes than you did, expecting it to be fed by a combustion boiler wher high flow temps aren't a huge issue.

The difference between 35⁰C flow and 45⁰C is what, a couple of percent of boiler efficiency? In the old days, when gas was 3p/kWh, that would be £10 a year?

I doubt that's the case here. Here's a thermal image. Installation was around 15 years go. I don't know floor makeup under our tiles, but I suspect we're losing quite a bit of heat to the ground too.

JSHarris

Ouch!  Looks like no spreader plates or screed was used, that's the primary reason you need to up the flow temperature so much, as most of the floor surface isn't being heated to close to the flow temperature.  When I looked at our floor with a thermal camera I couldn't locate the UFH pipes, the whole floor surface was the same temperature (within the measurement accuracy of the cheap thermal camera).  It was annoying at the time, as I was trying to plot out exactly where the pipes were and couldn't find them!

QrizB

JSHarris said:

Ouch!  Looks like no spreader plates or screed was used ...

Ok, so they cut corners, just not the one I had expected them to cut!

JSHarris

Found an old photo of part of our UFH going in, I really regret not taking more photos showing all of it, as there have been a few occasions where I've wanted to know exactly where the pipes are.  The foam insulation is 300mm thick in the centre, 400mm high around the outsides (where the membrane is wrapped over).  This was all filled with C35 concrete, so the pipes end up pretty much in the middle of the 100mm slab thickness.  No doubt the fact that they are cable tied to the steel rebar helps a bit with distributing heat, too.

MeteredOut

JSHarris said:

Ouch!  Looks like no spreader plates or screed was used, that's the primary reason you need to up the flow temperature so much, as most of the floor surface isn't being heated to close to the flow temperature.  When I looked at our floor with a thermal camera I couldn't locate the UFH pipes, the whole floor surface was the same temperature (within the measurement accuracy of the cheap thermal camera).  It was annoying at the time, as I was trying to plot out exactly where the pipes were and couldn't find them!

We have 2 cats. If I didn't have a thermal camera, I'd simply map out where they sit over the period of a few weeks

Screwdriva

Real world efficiency report on the V200-W

https://www.youtube.com/shorts/MgqZbYWYEZs

MeteredOut

Screwdriva said:

Real world efficiency report on the V200-W

https://www.youtube.com/shorts/MgqZbYWYEZs

Pardon my skepticism, but he just happened to be filming the conversation?

Who is the person giving the glowing "report"?