The grant decides which box you buy. The building decides whether it works
When the talk turns to replacing a heat source, the conversation almost always starts with the subsidy and ends with a brand. That is understandable: the voucher and the grant cap are concrete numbers you can compare across quotes. The trouble is that none of those numbers says anything about whether the new unit will work well in your particular house. The subsidy decides which box you buy. The house decides whether that box will spend the whole winter running in the regime its manufacturer designed it for.
That is the difference between the supplier's view and the designer's view. A supplier sells equipment and his responsibility ends with the commissioning report and handover. A designer looks at the house as a system: envelope, emitters, flow temperature, controls, and only at the end the source. Reverse that order and you get a unit correctly installed, partly reimbursed, and still a disappointment. This piece is about avoiding that, and about what the two Slovak support routes actually offer.
Envelope and water temperature first, source second
A heat pump dropped into an uninsulated house with oversized cast iron radiators and an original 70/55 design regime is not faulty. It is doing exactly what the system asks of it. To deliver water that hot, the compressor has to work against high pressure and efficiency falls. The same unit that reaches a very good seasonal SCOP at a 35/30 regime drops, at seventy degrees, into a band where running it costs more than the gas it replaced. The owner then blames the technology, although the mistake was made in the order of decisions, not in the equipment.
The sequence that works is boring and fixed:
- Cut the heat demand: insulate the envelope, replace openings, deal with airtightness.
- Recalculate room by room heat losses from scratch, after the renovation, not from the original drawings.
- Find out at what water temperature the existing emitters cover the new, lower loss.
- Only now choose a source that runs economically at that temperature.
Step three is usually a pleasant surprise. Radiators in houses from the seventies and eighties are often heavily oversized, and once the house is insulated they heat the same rooms at a substantially lower water temperature. Swapping two or three critical units for larger ones, plus hydraulic balancing, often moves the house into low temperature system territory without breaking open a floor. That costs less than stepping the pump up one output size, and it improves shoulder season comfort too.
If somebody offers you a heat source without having seen the post renovation heat loss calculation, they are selling equipment, not a solution.
Which source suits which house
There is no single correct source. There is a source that fits the envelope, the available emitter surface, the electrical connection, and how much hands on attention the household will tolerate. The table below is what I use at a first consultation, before any calculation.
| Source | Who it suits | Heating water temperature | Electrical connection | Character of running costs |
|---|---|---|---|---|
| Air to water heat pump | Insulated house with low temperature emitters or oversized radiators | Best up to 45 degrees, loses ground quickly above 55 | Usually a larger breaker, often a three phase supply, headroom must be checked | Moderate and fairly stable, sensitive to electricity price and design quality |
| Ground source heat pump | House with enough land, machine access and a long horizon | Up to 45 degrees, output stable even in hard frost | Similar to air source, but with a lower peak draw | Lowest and least volatile, at the price of a high upfront investment |
| Pellet boiler | Older house with no insulation or with high temperature radiators | Handles 70 degrees with no loss of efficiency | Ordinary single phase, only for the auger and controls | Tied to the pellet price, needs dry storage and regular attention |
| Log boiler with a buffer tank | House with its own firewood and a willingness to fire it by hand | High temperature, mixed regimes possible with a buffer | Ordinary single phase | Lowest with own fuel, highest demand on the operator |
| Condensing gas boiler | House with an existing connection where envelope work is still ahead | Condenses reliably with return water below 55 degrees | Ordinary single phase | Directly tied to the gas price, lowest upfront investment |
Two notes the brochures omit. A ground source pump is technically the most stable choice, but a borehole or collector needs land, machine access and permits, so on a narrow urban plot it drops out on spatial grounds more often than economic ones. And pellets are not maintenance free: the hopper needs filling, the ash pan emptying and the fuel dry storage, which for an older household often outweighs the running cost difference.
Two support routes: a SIEA voucher and a SAZP package
Slovakia has two distinct routes to public support for replacing a heat source, and they differ not only in amount but above all in character. Zelena domacnostiam is a voucher scheme administered by SIEA. It deals with one specific device, it is administratively simpler, and it does not require you to do anything to the building envelope at the same time. Obnov dom, by contrast, is a renovation programme run by SAZP, in which the heat source is one item inside a package of measures assessed as a whole against the primary energy saving achieved.
| Aspect | Zelena domacnostiam (voucher) | Obnov dom (renovation package) |
|---|---|---|
| What it funds | One device from the list of supported equipment, for example a heat pump or a biomass boiler | A set of measures at once, where the heat source sits next to envelope, ventilation and renewables |
| Who administers it | SIEA | SAZP |
| What the applicant must do | Obtain the voucher, redeem it with an installer from the approved list, pay the balance | Supply an energy assessment and design documentation, sign a contract, settle the accounts after the work |
| How the amount is set | A percentage of eligible costs up to a cap fixed for that device | A percentage of eligible expenditure up to a cap graded by the energy saving demonstrated |
| When it makes sense | A standalone source swap with no construction work on the envelope | A house heading into a deep renovation anyway that wants the source counted into the saving |
The practical consequence: double funding of the same item from both schemes is generally not possible, so the decision comes early and has to be deliberate. If the house is heading into a comprehensive renovation, keep the source inside that project, because it feeds the saving that sets the support band. If you are only swapping a boiler and have no facade plans for five years, the voucher is simpler. The mechanics of the renovation programme are covered in detail in the Obnov dom subsidy guide, which is also the only place on this site where I keep specific current figures up to date.
The mechanics that applications trip over
The installer has to be on the list
In a voucher scheme it is not the case that you find any plumber and redeem the voucher afterwards. The device must appear on the list of supported equipment and the work must be done by a contractor on the approved list. The neighbour with a trade licence who does it cheaper is a legitimate choice, it simply means no support. Check this before collecting quotes: it narrows the field of firms worth talking to and saves a round of pointless site visits.
The order, apply then install, is not interchangeable
This is the most expensive mistake available in this subject. A device installed before the voucher is issued, or before the contract is signed under the renovation programme, generally cannot be reimbursed retrospectively. It does not help that the invoice is in order or that the house genuinely achieved the saving. If your boiler dies in January, admit plainly that an emergency replacement and a subsidised one are different things, and decide accordingly. The paperwork also differs between schemes, so read the procedure in the current call, not in an article from a year ago.
You are reimbursed less than the invoice says
Support is not a percentage of the price a firm quotes you. The schemes work with maximum unit prices and with a cap per device or per house, so the reimbursed sum is limited twice over, by the percentage and by the cap. The voucher route reimburses up to 50 percent of eligible costs, the standard renovation call up to 75 percent of eligible expenditure, with the cap graded by the saving demonstrated. The specific caps for individual devices change between calls, so always read them in the current call. The gap between the invoice and the grant is yours to pay and belongs in the budget from the start, not at the settlement stage.
What to do if you are not eligible or the call is closed
Calls have a capacity and in popular rounds it fills fast. A closed call is no reason to panic, and no reason to buy a worse unit simply because it happens to be in stock right now.
- Do the parts of the renovation that need no support and help the future source: hydraulic balancing, thermostatic heads, insulating pipework in unheated spaces, insulating the ceiling above the top floor. All of them lower the water temperature the house needs.
- If the ageing boiler is still holding, wait for the next round and use the time for design preparation. Documentation already in hand is often the difference between an application filed and one missed on day one.
- If the boiler is not holding and replacement is unavoidable, choose a solution that does not close off the future: a genuinely low temperature system and enough electrical headroom mean a heat pump can be added later, even without support.
- Consider a hybrid arrangement in which the existing condensing boiler stays as backup for the coldest days and the heat pump covers most of the season. The upfront investment is lower and the house is never left without heat.
Conclusion
Subsidies are a good instrument and in Slovakia today they genuinely move households towards better heat sources. But they are an instrument of financing, not an instrument of design. Let the size of the voucher decide what goes into the house and you may secure maximum support for a solution that never works the way the catalogue promised. The order worth keeping is simple: cut the heat demand, find out at what water temperature the house can be heated, choose the source, and only then look at which scheme pays for it best. Always verify amounts and conditions in the current call at SIEA or SAZP; the mechanism described above holds regardless of which round happens to be running.