ᐅ Replacing the Heating System in an Older Home: Which Option Is Best?

Created on: 4 May 2020 14:51

Hello everyone,

I need your advice on choosing the heating system for the planned replacement of our heating installation.

Facts:

- Semi-detached house with two separate living units, built in 1972, purchased by us in 2014
- Approx. 220 sqm (2370 sq ft) of heated space, 186 sqm (2002 sq ft) of living area, 95 sqm (1023 sq ft) on the ground floor, 91 sqm (979 sq ft) on the upper floor
- Building envelope insulated with 10 cm (4 inches)
- New triple-glazed windows, conservatories with full glazing on both ground and upper floors
- Roof not insulated
- Heating distribution: (We are currently planning to replace the old tube radiators with low-temperature radiators)

Basement: One old tube radiator and about 40 sqm (430 sq ft) of underfloor heating (old system – probably pipes will be reused)
Ground floor: Underfloor heating with 9 loops, installed in 2014, conservatory still with old underfloor heating system like in the basement
Upper floor: Six old tube radiators

- Replacement of old oil heating from 1989 (low temperature), oil tank with 6000 liters (1585 gallons) steel tank, current consumption about 2500 liters (660 gallons) per year
- No gas connection available
- Additional planned project: photovoltaic system with about 8 – 9.9 kWp on the south side of the roof with a 10 kWh battery storage
- Ideally BAFA subsidy of 40% – 45% per system utilized

There have been many considerations regarding the options for the new heating system:

Option 1: Gas hybrid with solar thermal system of 8.5 sqm (91 sq ft) and 600-liter (160-gallon) water buffer tank
(Offers from Buderus or BOSCH systems)

Option 2: Gas hybrid with 400-liter (105-gallon) water tank and small supplementary air-water heat pump (possibly combined with the rooftop photovoltaic system)
(Offers from WOLF systems)

Option 3: Air-water heat pump with integrated domestic hot water preparation in the indoor unit and outdoor unit (possibly combined with the rooftop photovoltaic system)
(Offers from Vaillant systems)

Each option has its pros and cons, but I am also interested in your opinions and experiences. In particular, I am uncertain about the noise level of the outdoor units of the heat pumps. How loud are they really?

Or maybe some of you are currently planning a replacement with a similar setup. I would be glad to hear your experiences with these systems and which you would recommend for my situation.

Thank you very much for your help!!!

Best regards,
Chippy

tiborb10 Jun 2020 18:20

#19

Here is an update:

Another company has now provided me with an offer for a solution using only an air-water heat pump, including calculations. Here are some key facts:

Heated floor area: 200 m² (2,150 sq ft)
Standard heating load: 21.86 W/m² (6.7 BTU/h·ft²)
Total heat generator capacity: 11 kW (37,500 BTU/h)
Building heating load: 11 kW (37,500 BTU/h)
Heat pump coverage ratio: 100%
Heat pump output share: 78.36%

Annual performance factor (heating): 3.9 without auxiliary heater, 3.9 with auxiliary heater
Annual performance factor (domestic hot water): 4.0 without auxiliary heater, 4.0 with auxiliary heater
Annual performance factor according to VDI 4650: 3.9

Flow temperature: 55°C (131°F)
Hot water storage temperature: 50°C (122°F)

Performance coefficient COP A-7/W35: 2.71
Performance coefficient COP A2/W35: 4.28
Performance coefficient COP A7/W35: 5.61

We are possibly planning to install a photovoltaic system with 8.16 kWp and a 9.6 kWh battery on our south-facing roof.
Calculated grid feed-in: approx. 9262 kWh per year.
Current own electricity consumption is about 3000 kWh, likely increasing to 4500 kWh later.

Electricity consumption:
For electricity consumption, the heating load divided by the annual performance factor, multiplied by 2000 (heating hours), gives the following electricity demand:
11 kW / annual performance factor 3.9 × 2000 = 5641 kWh, which means at an average rate of about 30ct per kWh, approximately 1692 EUR / year (141 EUR / month) electricity cost for the air-water heat pump.

---

Questions:

1.) Do you think the company's calculation is realistic, with an annual performance factor of 3.9 for an air-water heat pump?
2.) Are the electricity costs possibly too high?
3.) If the electricity costs turn out to be uneconomical, would the mentioned combination with the photovoltaic system make it sensible and profitable again?
4.) Is it allowed to ask for experiences with air-water heat pump models here, as long as no prices or advertising are mentioned?

Thank you for your feedback.

Best regards,
Mr. Wirtz

nordanney10 Jun 2020 18:49

#20

tiborb schrieb:

Performance factor COP A-7 / W35 -> 2.71
Performance factor COP A2 / W35 -> 4.28
Performance factor COP A7 / W35 -> 5.61

Uh – W35 means heating water at 35 degrees Celsius (95°F), right? And you want to operate with a supply temperature of 55 degrees Celsius (131°F)? I wonder what the COP will look like then? And wait until you see your electricity bill. LOL

tiborb schrieb:

We are possibly planning to install a photovoltaic system with 8.16 kW peak output and a 9.6 kWh battery storage on our south-facing roof.
Estimated net feed-in about 9262 kWh per year.
Our current electricity consumption is about 3000 kWh, likely increasing to 4500 kWh later.

I think photovoltaic systems are great. Battery storage? Not really, or at least not economically viable at the moment. How much is that going to cost?

Just to quickly summarize your investment: you want to replace an oil heating system that costs you about €1150 per year in heating expenses with a highly efficient heat pump for which you expect €1700 per year in heating costs. Spot the mistake!

You should have some specialists run the numbers...

Joedreck10 Jun 2020 19:01

#21

The supply temperature is definitely set too high at 55 degrees Celsius (131°F). This is even too high for a gas condensing boiler. In my old building with radiators, I operate at around 40 degrees Celsius (104°F) under natural draught conditions.

tiborb10 Jun 2020 19:54

#22

nordanney schrieb:

Uh – W35 means heating water at 35 degrees Celsius (95°F), right? And you want to run the system with a flow temperature of 55 degrees Celsius (131°F)? How do you expect the coefficient of performance (COP) to look then? And just wait until you see your electricity bill. LOL

I think photovoltaic systems are great. Is a battery storage useful? Probably not, or at least not really cost-effective at the moment. How much does the whole setup cost?

Just a quick summary of your investment: you want to replace an oil boiler—which costs you about €1,150 per year in heating—with a high-efficiency heat pump, but you expect heating costs of €1,700 per year. Spot the mistake!

Time to get some experts to do the math...

Yes, that’s right, 35°C (95°F) is the standard flow temperature. I just checked the manufacturer’s website and they don’t provide a specific value for 55°C (131°F). Is it possible to calculate or estimate it somehow? Would it be better to operate the system only at 35°C (95°F) flow temperature?

Also, why is the calculated seasonal performance factor (SPF) for us not a reliable indicator for electricity consumption? It was calculated by the heating company as 3.9, wasn’t it?

The photovoltaic system, including the subsidy for battery storage, should cost around €19,000 in total.

Joedreck schrieb:

The flow temperature is definitely calculated too high at 55°C (131°F). That’s even too high for a gas condensing boiler. I run an old building with radiators at about 40°C (104°F) on the supply line with natural convection.

Our situation is a bit special because the first floor and basement have underfloor heating (separate heating circuits), and the first floor also still has tubular radiators. We want to replace those with low-temperature radiators designed for a flow temperature of 55°C (131°F). Could we possibly operate those radiators at just 35°C (95°F) flow temperature instead?

Thanks for your help.
Best regards

nordanney10 Jun 2020 20:06

#23

tiborb schrieb:

Yes, that’s the standard value for a flow temperature of 35°C (95°F). I just checked the product provider’s website, and there is no specified value for 55°C (131°F). Is there any way to calculate or extrapolate this? Would it make sense to run the system only at a 35°C (95°F) flow temperature?

That should be calculated by someone who is knowledgeable in this area. Unfortunately, I can’t do that.
From my experience, I can tell you that you will have real problems with a low flow temperature during winter. Some poorly designed systems in new buildings only manage the winter with an electric heater as backup, achieving a coefficient of performance (COP) of 1 at lower temperatures. And those systems only require 30–35°C (86–95°F) flow temperature with a full underfloor heating system.

tiborb schrieb:

The photovoltaic system should cost about 19,000 EUR net including subsidy and storage. 8.16 kWp should cost around 10,000 EUR plus storage minus subsidies.

19,000 net and the subsidy is already deducted? That’s not worth it and is also too expensive.

What do you think about my calculation comparing your electricity costs vs. oil costs without renovation?

tiborb10 Jun 2020 20:30

#24

nordanney schrieb:

....What do you think about my calculation comparing your electricity costs versus oil costs without renovation?

Yes, there seems to be some kind of system error :-O But what exactly is it? Could it be that the heating load was calculated too low at 11 kW?

The heating company said that the proposed air-to-water heat pump (two outdoor units) provides sufficient heating capacity even without the electric heating rods.
And that I will have about 4000 - 5000 kWh electricity consumption.

ᐅ Replacing the Heating System in an Older Home: Which Option Is Best?

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