In my apartment, hot water in the faucet is heated by an electric heater, while the hot water in the radiators for heating the rooms comes from a central gas boiler in the building. To me it seems like both systems solve exactly the same problem: producing a stream of water at around 60 degrees Celcius, so why would they be separate and use different energy sources?


9 Answers 9


The heating system is generally a closed loop, the water is circulated. Some possible reasons for not mixing it with drinking water.

  • If the temperature drops below 60°C, bugs such as legionnaires disease can reproduce.
  • The radiators and the heating system are not going to be food grade.
  • The water can have an additive to kill bug growth.
  • The heating of the water releases air, which needs to be bled in some fashion. Constantly introducing fresh water into it will require more bleeding. This would also lead to more energy loss.

This is based on my experience with my underfloor heating system. In my country, this water had to be isolated from the hot water going to the showers. So, I had to have a heat exchange system inside my 1000 litre boiler.

It doesn't mean that they can't use the same water, but as pointed out above there are barriers and conditions.

Others are quoting temperature difference, this can be achieved with valves and flow rates.

Just linking first articles I found,

Knowing how to bleed a radiator is essential to keeping them working well.

From How do radiators work? Your guide to understanding radiator heaters

How often should I check my glycol system?

From FAQ - Radiant Heaters Note, not all radiators are filled with anti-freeze, but they can be. When I went to school. they were filled with it.

  • 1
    Could add that hot water for radiators and hot water for tap are different temperatures for safety reasons (at least in some jurisdictions). Commented Jun 13, 2023 at 14:52
  • 3
    This doesn't necessarily explain why the closed loop system couldn't be used to heat the drinking water indirectly, by running the heating pipes around per-apartment hot water tanks, achieving the same basic effect as a regular hot water heater, but using the (presumably more efficient) central gas boiler, which is already piping water nearby for heating the air anyway. (I don't think you're wrong, it's just that the focus on why you don't drink the water out of the heating system ignores other possibilities) Commented Jun 14, 2023 at 0:56
  • 5
    None of this is valid - it is VERY common in the UK for the same fuel-fired boiler to be used for both radiators and for taps, with separate circuits - on closed-loop and one open-ended.
    – MikeB
    Commented Jun 14, 2023 at 10:58
  • 1
    Hot tap water isn't generally regarded as drinking water anyway
    – Chris H
    Commented Jun 14, 2023 at 11:03
  • 2
    @ChrisH Doesn't that depend on the plumbing? Traditionally, hot water was stored in a tank, which often made it non-potable (especially if the tank was not sealed); but some more recent systems dispense with a hot-water tank and generate hot water on demand, which can be just as safe as the cold water it's generated from.
    – gidds
    Commented Jun 14, 2023 at 12:26

This saves water. Using warm tap water requires running the tap for a while to flush out water that's cooled down. By using electric heating close to the tap point, there's less water between the tap and the heater.

For room heating purposes, the water is in a closed loop and this does not matter. However, the heating demand is significantly greater, so efficiency matters more. And the Dutch historically had a lot of cheap gas.

  • 1
    This is probably the main driver. Many ways of running hot water from a central boiler also mean very variable pressure, either as you go up the building, or when other users turn their taps on. Or very long dedicated runs of pipe, which maximise the heat loss. Hotels etc. often use secondary hot water circulation, in which insulated pipes form a loop, with a pump running all the time to keep the hot water available everywhere all the time. That can get quite costly
    – Chris H
    Commented Jun 14, 2023 at 11:06
  • 1
    Indeed. And of course, in many European countries, the central heating doesn't actually run for most of the year - there is a "heating season" when it's active, and the rest of the year it's completely cold. Most people want warm water even when it's warm outside :)
    – Luaan
    Commented Jun 16, 2023 at 6:13

In general, hot water for heating and for hot water supply are usually separate systems. There are systems that combine them but they are not often used.

Since they are separate, the question of why either gas or electricity would be chosen for either one is independent. You just want to know why your water supply is heated electrically and not with gas. Maybe because the total available gas supply or total available electricity supply to the building is inadequate for both. Maybe because hot water supply is paid for by the occupants while heating is paid centrally, and there is no gas supply to individual apartments. Maybe because small point-of-use heaters were convenient or appropriate for your building, and those are always electric.

There is an efficiency argument. During heating season, the boiler in the basement and the pipes running all over the building help to heat the building. The "lost" energy isn't really lost. But in summer time, a basement boiler and pipes running from there to all the units would heat the building which is both undesirable and wasteful.

  • 5
    80% of UK housholds have combi boilers - source uswitch.com/energy/boilers/boiler-statistics
    – Tetsujin
    Commented Jun 13, 2023 at 14:18
  • @Tetsujin hm ya, I guess that statement was a bit US-centric, maybe even NE US-centric. What about larger buildings in the UK? As I recall, heating supplied by a central common system is very rare?
    – jay613
    Commented Jun 13, 2023 at 14:24
  • I'm not really up on large builds, but from what I recall of visiting people in them, they each have their own systems. I don't have any reliable figures, tbh & not sure where to look for them.
    – Tetsujin
    Commented Jun 13, 2023 at 14:28
  • @Tetsujin later in the same article, it's 80% of sales are combi boilers, not 80% of existing installations. Based on what I've seen, I think the summary is wrong, and 80% of sales is right. However combi boilers use separate flows in one appliance, while stored hot water systems like mine have a single primary loop and a heat exchanger.
    – Chris H
    Commented Jun 14, 2023 at 11:09
  • 1
    @ChrisH - it turned out the point was moot anyway. [I binned all my other comments/answer, but missed that one.] OP seems to be talking about some larger building, apartment block, where it's far more logical; one shared heating bill, individual hot water bill. Maybe people are rushing to get their gas combis installed before they're banned ;))
    – Tetsujin
    Commented Jun 14, 2023 at 11:24

Normal heating radiators are made of steel.

Tap water contains dissolved oxygen. Oxygen + steel = rust.

Maintaining a closed circuit for the heating system reduces the oxygen content (it comes out of solution and is vented either automatically or manually when you bleed the system) and keeps the rust out of your domestic hot water. It also allows chemicals to be used to reduce the corrosion. If you've ever drained an old central heating system you would be quite shocked at how dirty it is.

Having entirely separate heating and hot water systems is not at all uncommon.

There is another reason specific to an apartment building: by using flow reducers, the maximum output from a communal heating system can be limited for each apartment. It's much harder to limit the volume of domestic hot water per apartment, leading to unpredictable costs and accusations of unfairness. The capital cost of a central system which satisfies everyone's demand for morning baths and showers might well be more expensive than one system per apartment.

  • 2
    Upvote for the most correct answer so far. Valves, circulators, air traps, near-boiler plumbing, expansion tanks, and boilers are also usually made of steel and cast iron components to limit costs. Copper and lead-free bronze components cost roughly double their iron counterparts. Stainless steel components are astronomically priced. Commented Jun 16, 2023 at 5:49
  • Plus rust inhibiting chemicals are added to the heating system. Mine includes a powerful magnet (near the boiler) which can hold on to any iron sediments that try to circulate through the system. Cleaning it is part of the annual boiler service. Commented Jun 24, 2023 at 22:07

In the Netherlands, combined gas heaters (combi-ketel) are absolutely ubiquitous. Virtually all single family houses will have the the same gas heater for tap water and central heating. They're separated using a water/water heat exchanger, all the answers here that claim this is difficult are just plain wrong. Furthermore, gas in the Netherlands is much cheaper than electricity, and gas heaters are extremely efficient, so at first glance it seems silly to use a separate boiler for your faucet.

There are a few reasons why your apartment building may be different. They are:

  • distance between boiler and faucet. The entire pipe of cold water needs to be flushed before you can get to hot water. This is already a problem in larger single family houses, which would use close in electric boilers to bridge the gap. This is the most likely reason
  • retrofit hot water. Many cheaper housing complexes, especially those aimed at students, will be nowhere near their original state. Especially with the recent shift towards studio apartments instead of shared facilities, a lot of small kitchens and bathrooms are being installed. It's often much cheaper to buy a heater than to retrofit long lengths of piping in (concrete!) walls. Especially since typically the building owner has to pay for the piping but the tenants pay for their own electricity.
  • legacy systems. Maybe the building was built before combined gas heaters were so ubiquitous, and your cheap landlord will only replace it once it finally breaks down, even if that means your pay double the price for heating your apartment with an old non-condensing boiler - in which case your electric heater might even be cheaper to run. I'm not saying all landlords are selfish money grabbing cheapskates, just that some are.

You are talking about hydronic systems.

First, separating the hot potable water from furnace/ boiler heat isn't hard. That can be done with a water-water heat exchanger. This can even be near the faucet for quick hot water.

The big problem is, tanked dedicated water heaters are very efficient, because they are well-insulated and have no external piping to lose heat through. The insulation is quite good and they can be shut off for 12 hours and still have plenty of usable hot water. In fact it's probably the #1 appliance they tamper with when doing the primitive form of demand-side management known as "shutting off consumer loads". Some people choose time-of-day rates and intentionally run their water heater at night.

Now if you do a combined system, you must keep the whole-house furnace/boiler spun up and heated at all times on the off-chance that someone opens a hot spigot in the next minute. This costs fuel/energy and someone must pay not only for that but also maintenance of that system under 24x7 stress.

Whereas with a separate furnace/boiler, that can sit there all summer pulling literally 1 watt - 1/2 watt of losses in the 24V transformer, and 1/2 watt to power your smart thermostat if you even have one. It does not use any energy at all until the thermostat makes a call for heat. And when the cycle is complete it is back to 1 watt.

There's another issue still, and we run into this one a lot. Someone wants to switch to heat pumps, either to get more efficient heat, stop using costly Putin Juice, or simply to get air conditioning. Often this uses "mini-split" systems that require only line-sets through the wall rather than monstrous ducting. But this creates a lose/lose dilemma. If they tear out their hydronic system, they lose their water heating. But if they keep it for water heat, they're running the world's most expensive water heater. So then water heat has to come into scope.

Also, a hydronic combined system is incapable of air conditioning. It also doesn't have ducts, so you can't just bolt on a cheap American style barrel A/C that relies on the forced-air furnace for air handling. Combined hydronic systems made sense when A/C was realistically out of reach for most home buyers... but now that the average blown up Ukrainian tower block apartment has heat pumps on half the apartments in a country that doesn't make heat pumps domestically and has per capita international PPP of about $4000/year, the "A/C is not affordable" issue is clearly not a problem.

  • I suspect it is possible to do air conditioning on a combination hydronic system, but it'd take some very tricky footwork with a reverse cycle chiller Commented Jun 16, 2023 at 3:04

Sweden: Many (I haven't checked all) heat pump based central heating systems supply the water from the same source and one big hot water tank, sometimes they have a different heat exchanger per use, sometimes there is a smaller tank connected in series using the same heat source, and sometimes one of the uses can be disconnected or limited independently, but the energy comes from the same source.

A reasoning I saw for the separation or limitation is for cases where there is not enough energy for all uses- suppose the whole family wants to take a shower while the house is cold and needs heating, the separation allows prioritization of resources.

enter image description here


Just guessing here:

Could it be that they don't have enough space in the heating room for an proper dimensioned buffer?

A gas burner won't necessarily need an buffer for heating. But for warm faucet water (what's the proper term for that?), efficiency requires an buffer (wide spans of flows, intermittent use).

Should be buffer be missing, it would be likely that they decided to "outsource" that to the individual appartments.

  • It is normal to have a separate boiler or whatever per household. Commented Jun 14, 2023 at 16:48
  • @JeremyBoden in case of OP apparently not
    – Martin
    Commented Jun 14, 2023 at 21:46
  • 2
    RE: "Warm faucet water," in the USA we call it "domestic hot water" (DHW). Commented Jun 16, 2023 at 5:52

I believe at-least part of the answer is that the heating system needs to be pressurized to account for varying temperature causing expansion etc..., where as when a boiler supplies a faucet, a system can just run without those concerns.

In the UK we have combi-boilers which can take into account both situations, but I imagine this could also be done with separate systems, which may explain the system you have.

  • 1
    FYI: Domestic water supply is at a much higher pressure (50+ PSI) than closed-loop hydronic (15 PSI). Commented Jun 16, 2023 at 5:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.