Two central heating zones

Uncle Pierre

A fairly basic question from somebody with little or no knowledge of plumbing & heating systems. I hope any answers don't get too technical!

We have oil fired central heating, and the house is split into two basic zones (upstairs & downstairs). Question is, if both zones are turned on at the same time:

• does it use twice as much oil since it has to run both at the same time, or
• does it use the same amount of oil, but deliver less heat to each room?

TIA.

Lenar3556

Assuming the system is balanced correctly and the boiler is correctly sized, the answer is more like twice as much oil, the boiler will be working harder and running longer to heat both zones simultaneously.

MicktheMan

https://www.boards.ie/discussion/comment/123168874#Comment_123168874

I don't agree. The oil burner is set up to consume a fixed amount of oil per house regardless whether one, two or more zones are calling for heat.

So @Uncle Pierre, all else being equal, your second answer is correct (i.e. for a fixed period of burn time, the same amount of oil is consumed to deliver heat to a larger area, thus the heat up rate is lower)

jimf

do you have any other form of heat controls

like room stats or trv valves on the rads

Uncle Pierre

Conflicting answers above! Wonder if anybody might have a casting vote??? :)

@jimf - there's just one overall stat in both zones ( one upstairs on the landing, one downstairs in the main open plan kitchen/living room area). Downstairs one is generally set to 21 degrees. Upstairs one is set to about 19, as the landing is a wide open space, and my thought process is that if the landing heats up to 19, then all bedrooms will have been heated warmer, especially if doors have been kept closed.

All the radiators have TRV valves, but we don't adjust them. All are turned up to between 4 and 5, with 5 being the highest they'll go.

Don't know if any of that makes a difference to the overall question? Thanks again to all anyway.

declan b

You wont get an exact answer on this one. There are alot of variables i.e , the amount of rads upstairs and downstairs,what the wall stats set to,are the trvs coming into play.

if the boiler is sized correctly and only downstairs is on the boiler could reach stat temperature in 15 mins. if both zones are on the boiler could take nearly 30 mins to reach boiler stat temperature. so the burner is on nearly twice the length of time which is nearly twice as much oil.

jimf

it would def help if you tuned down the trv to a lower setting in rooms that may not need full heat

Lenar3556

https://www.boards.ie/discussion/comment/123169113#Comment_123169113

That’s not the case.

You are correct that the burner doesn’t have a facility to modulate based on load, but it will run longer, and the consumption of oil will be roughly proportional to the number of radiators switched on.

In simple terms, if you switch on 13KW of radiators down stairs and the burner runs to 20 minutes in an hour to keep these hot, switching on another 13KW of radiators (upstairs) will cause the boiler to run for more like 40 minutes.

In the unlikely event that the boiler was undersized and only capable of heating one zone at a time, then your argument would be true. All radiators in the house would run at a lower temp, and the same amount of heat would be emitted, just more slowly and over a wider area.

User567363

Set both zones to on manually and disconnect the power from them

If it's a condensing boiler set it to the lowest temp on the boiler

Dont use a time switch or a manual switch, use a stat at bottom of the stairs set around 17

When you get home close the curtains, tuck curtain behind the rads then crank stat up to 19

Set back to 17 when going to bed

Set trvs on rads to III or III except rad nearest stat, leave that fully on

If it's a non condensing boiler then buy a condensing boiler and go back to the start

I have been doing this for years and it's the cheapest and nicest way to heat the place, and every inch of every room is warm, prevents mould too

Lenar3556

https://www.boards.ie/discussion/comment/123169141#Comment_123169141

As a very rough guide (because there are several variables), an average 1200mm x 600mm double panel radiator will need about 0.25litres of kerosene per hour to keep it hot. If you have 10 of these on for an hour the boiler will consume about 2.5litres.

The setting of the wall mounted stat in each zone, and to a lesser extent the TRV’s will play a very significant role in how many radiators are switched on and for how long, and consequently how much oil is being used.

Your thought process on the stat in the landing and impact on the bedrooms is valid. I would usually put the stat in the master bedroom. Do you also have separate time control on these zones? Having the upstairs zone on for two or three hours a day might be enough? - at other times it would be switched off regardless of what the stat is set for.

Lenar3556

https://www.boards.ie/discussion/comment/123169305#Comment_123169305

I wouldn’t necessarily agree with some of what you have said, but I think the key thing is to trial options and evaluate the results.

With some trial and error, everyone will find a solution that works well for their particular use case.

Uncle Pierre

https://www.boards.ie/discussion/comment/123169261#Comment_123169261

Some of the other answers above - while probably straightforward to the experts here - are already getting technical for a layman like me!

Maybe I should clarify that my question is not about how to get best value or best use from the system. It's simply about if both zone are on at same time (and all else being equal), does it use twice as much oil as if just one zone was on.

The reply here probably comes closest to answering it, even if it tells me there won't be an exact answer. Seems it's not the case that the boiler would use twice as much oil per minute while it's running - but that it could be running for twice as long.

Thanks all.

Wearb

(all else being equal) Seems it's not the case that the boiler would use twice as much oil per minute while it's running - but that it could be running for twice as long.

Yes This ^^^^