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Problems with sizing a replacement for a Viessmann Vitocal 200-S

jon · 2026-08-31T15:19:20Z

Apologies in advance for the long post! My house has run on a hybrid system since it was built in 2013: a Viessmann Vitocal 200-S air source heat pump working alongside a Viessmann Vitodens 200 gas boiler, installed together on Viessmann controls. The heat pump is type AWB-AC 201.B13, a 13 kW split, with the boiler covering the peak and doing all of the hot water as a design decision.The heat pump has now reached end of life, with Viessmann confirming that spare parts are no longer available. Even though I had the fan motors and control PCBs replaced three years ago, I am worried that something else will break. Rather than waiting for this to happen, I would like to replace the unit and take the opportunity to remove the boiler at the same time to come off gas completely. I need to find a unit that will carry the entire load on its own and also do the hot water.That makes the design load figure the whole question and it is where I have run into trouble. I have two answers that differ by a factor of 1.7, and I would like someone to tell me whether I have got this wrong before I commit to equipment.The houseIt is a developer-built detached house completed in 2013 in Surrey, on four storeys including a basement and rooms in the roof, with around 550 m2 of floor area excluding the garage. There is underfloor heating on three floors, radiators on the second and MVHR throughout.The as-built U-values are 0.28 for the external walls, 0.16 for the pitched roof, 0.21 for the flat roof, 0.22 for the ground floor and 1.8 for the windows. Air permeability is 8.2, according to the EPC. The dataThe hybrid arrangement has given me two distinct measurement regimes, which is the reason this post might be useful to anyone else working a design load out from consumption figures.The heat pump was switched off entirely from 2014 until February 2021 and the boiler carried the whole load, which gives a clean gas-only record; my meter readings cover 2015 to 2018 within that period. Since 2024 I have had daily gas consumption metered separately for hot water, along with heat pump output logged into InfluxDB, which gives a period where both heat sources are running and can be separated from each other. Three methodsFirstly, I regressed twenty gas meter intervals from 2015 to 2018 against heating degree days computed hourly from ERA5 data at my coordinates. The hot water baseline was taken from each year's measured summer consumption rather than assumed, and the fit was forced through the origin so that only one parameter remained free. That gives 815 W/K at a fitted base of 13 C, with an R2 of 0.96.Boiler seasonal efficiency is taken as 90 per cent gross, which is reasonable for a Vitodens 200 condensing on underfloor return temperatures. It is worth noting that gas meter readings convert on gross calorific value, so the 98 per cent net figure manufacturers quote is not the number to use here. Varying it between 85 and 92 per cent moves the result by about 1 kW.Secondly, I took the 2024-25 season and combined heat pump output with boiler heat to give total delivered heat. That gives 814 W/K.Thirdly, and this is the one I trust most, the heat pump saturates at around 7 kW below about 5 C and its output barely rises as the weather gets colder, so the boiler supplies almost all of the additional load. That makes the slope of boiler output against outside temperature a measurement taken entirely from the gas meter, with no heat pump instrumentation involved at all. The boiler contributes 538 W/K and the heat pump 276 W/K over the same weeks, giving 814 W/K combined. This is only possible because it is a hybrid, and it is the strongest evidence I have.Where the calculator disagreesThe de Podesta HTC estimator gives 474 W/K and a 10.2 kW heat pump from the same consumption figures.The entire difference comes from one assumption. The estimator places the balance point 3.5 K below internal temperature, which for me gives a base of 18.5 C and roughly 2,550 degree days a year. Fitting the base to the data instead gives 13 C and about 1,350 degree days. Dividing the same annual consumption by half as many degree days doubles the coefficient, so neither of us has made an arithmetic error, we have simply used different denominators.The tiebreakerIn the week ending 13 January 2025, at a mean outdoor temperature of 0.8 C, the house consumed 12.75 kW continuously for seven days. Of that, 5.01 kW came from the boiler, which is a gas meter reading with no modelling in it whatsoever. The calculator's 474 W/K predicts 8.4 kW at that temperature, so the house measurably used half as much again as the model says it can.Where I think I am weakestA base of 13 C against a 21 C setpoint implies around 6.5 kW of free gains, which is considerably more than a normal house. Some of that is real, since there is a lot of south-west glazing and a secondary hot water loop was dumping about 900 W into the fabric around the clock until I put it on demand. But some of i...

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Air Source Heat Pumps (ASHPs)

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jon

(@jon)

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Topic starter 10/09/2026 7:34 pm

@judith Thanks very much for this! My room by room calculation, done on my own spreadsheet, comes out at about 18 kW. I've also found the original Nu-Heat UFH design sheets, which also show around 18 kW (although it gets some rooms completely wrong, mostly as they get too hot).

Then I worked out the flow rate from the secondary pump on the internal unit, which feeds the buffer cylinder. As I don't have a flow meter, I did this by charging the buffer with the flow and return to the UFH and rads closed. I used this to recalculate the measured delivered heat by source for the 2024 - 2025 season:

This pretty much agrees with the previous calculations in the colder bands, but diverges as it gets milder. This is a much better fit and again agrees with the gas only regression which gave around the same figure at a design temperature of -3. So the house draws around 13 kW at -3 with gains present. The 12 hour sustained peak was 13.7 kW. Adding in a margin for the coldest bin, the boiler efficiency being 90% and the house actually being heated to 23.5 C (rather too hot, I know!), gives a heat pump sizing of 16 kW delivered at A-3/W35.

Posted by: @judith

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Your house ought to be more thermally efficient from its build standard so you probably won’t need 2.5 times our usage to heat it (ratio of floor areas).

Your 5 January figure, corrected by your own 5 per cent, is 169 kWh over 24 hours in 200 m2. That is 7.0 kW average, or 35.2 W/m2, at 21 C internal.

My nearest equivalent is 147 hours in the -2 to 0 band last season, at 11.2 kW. Over 571 m2 that is 19.6 W/m2, while holding the house at about 23.5 C rather than 21. That's around 1.53 W/m2K conductance for yours against 0.82 for mine.

Posted by: @judith

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Your extrapolation to -3/-4 is indeed eye watering and perhaps you might consider plotting error bars on the data and slope?

I haven't done this yet, but will post again once I have!

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