Boiler efficiency with multiple heating zones

70sbudgie

I have 3 heating zones plumbed into my gas CH system - two rooms with underfloor heating each with a room thermostat and the rest of the house on radiators with a central room thermostat. All three are programmable. I also have a hot water cylinder that heats for a couple hours each evening.

At the moment the timings for each zone are based on the room occupancy - kitchen first thing in the morning, bedroom early evening. Would the gas consumption be lower if the different zones called for heat at the same or different times? I believe that the WB (Greenstar?) boiler can modulate, but I don't know how I would check that.

peter3hg

Efficiency with a condensing boiler increases as the return flow temperature decreases. In theory then it would be more efficient if all zones called for heat at the same time.

Mstty

Surely it is cheaper to heat each room as you use it over whole house if this works for the OP. Regardless of the return flow 

However it may be worth a test for this household?

70sbudgie

Mstty said:

Surely it is cheaper to heat each room as you use it over whole house if this works for the OP. Regardless of the return flow 

However it may be worth a test for this household?

What sort of test, do you think? It would need to be over a decent stretch of time to remove weather variations.

What about if I can measure the return temp on the boiler? Is it possible to buy a pipe thermometer that stores the readings over perhaps a day?

For the condensing boiler to be most efficient, each of the zone combinations would need to reduce the return temp enough, but I guess not so much that a zone later on the circuit no longer heats sufficiently?

The two UFH zones go through a manifold which I understand reduces the temperature to 40°C. The boiler is set with a relatively low flow temperature - around 55°C, so should be condensing anyway.

I guess with the above logic, the system would be most efficient if all the zones were on at the same time, as this would maximise the reduction in return temp. But not to the extent where a room is being heated unnecessarily.

I guess my boiler has been sized to heat all the zones at the same time, so by not doing so, it is oversized. Hmm, is it better to use more or less of the capacity of the boiler I think I've read that WB Greenstar can modulate down to 5kW? Where could I confirm this? I haven't found anything in my manuals about modulating.

InvertedVee

I have a WB Greenstar 30i and it modulates under the control of my Tado thermostat using the EMS protocol. I can't find what the lowest output is but I think it's about 5 or 6kW.

I've used the installer's instructions https://www.worcester-bosch.co.uk/professional/support/literature/greenstar-i-combi-installation-instructions to range-rate (limit) the maximum CH output to 7.5kW and I run a maximum flow temperature of 55C in winter and 50C in autumn/spring. I have found that my whole home only needed 2kW of heat per hour to maintain 19.5C during the coldest weather in December so I wish I could modulate the boiler down even further like the new Viessmanns.

If your thermostats are wired as simple on/off, rather than EMS, you won't get any modulation from the boiler until the flow reaches its set point. The benefit of using EMS controls is that they call for different flow temperatures according to demand.

You may find that running the house as a single zone is slightly more efficient since each room's emitter will only be heating one space and the boiler flow can therefore be cooler. If you keep adjacent rooms cold, then an emitter will have to be run at a higher temperature as the heat from the heated zone will leak into the unheated zones. But that all depends on the room layouts, insulation and and your usage patterns.

Do remember that maintaining a room's temperature uses much less energy than raising it from cold. Unless you're out the house for a long time each day, it may be better to keep the whole place warmed all the time, especially if it is well-insulated. I nudge the temperature by only a couple of degrees during each 24-hour period.  

70sbudgie

How do I check if my programmable thermostats use EMS (I suspect they don't, as they are marginally older than the boiler)?

If they are not, what should I look for as replacements? The current programmable thermostats are all hard wired and far enough from the boiler that wireless probably wouldn't work well enough (based on the wifi coverage across the house).

I am more comfortable with the house being slightly cooler, so I only vary the temperatures from about 16°C at night to 18.5°C during the day. (No-one else has complained enough yet for me to change it!) The kitchen only heats in the morning as it is a large concrete slab, so retains enough heat during the day to not often need topping up again. I don't try to maintain the temperature in the bedroom because it is mostly only occupied for 8-9 hours of sleeping plus faffing before and after.

InvertedVee

70sbudgie said:

How do I check if my programmable thermostats use EMS (I suspect they don't, as they are marginally older than the boiler)?

You could either:

• do an online search for the models
• turn off the power and look at the wiring at the back of the thermostats

I think you're probably right about them being non-EMS and I don't know if it's even possible to connect three EMS thermostats to the same WB boiler.

70sbudgie said:

If they are not, what should I look for as replacements? The current programmable thermostats are all hard wired and far enough from the boiler that wireless probably wouldn't work well enough (based on the wifi coverage across the house).

It would be best to ask a heating engineer. There a lot to be said for using Worcester Bosch controls, as they are designed to work with your boiler. I used a Tado because I brought it with me from my previous house. (Note also that not all Tados are EMS compatible).

70sbudgie said:

I am more comfortable with the house being slightly cooler, so I only vary the temperatures from about 16°C at night to 18.5°C during the day. (No-one else has complained enough yet for me to change it!) The kitchen only heats in the morning as it is a large concrete slab, so retains enough heat during the day to not often need topping up again. I don't try to maintain the temperature in the bedroom because it is mostly only occupied for 8-9 hours of sleeping plus faffing before and after.

Then probably best to stick with your existing arrangements which seem to be working for you. EMS-compatible thermostats aren't cheap and if it is possible to connect three to your boiler, you'd end up spending much more than £500 if you got someone to install them for you.

BUFF

InvertedVee said:

It would be best to ask a heating engineer. There a lot to be said for using Worcester Bosch controls, as they are designed to work with your boiler. I used a Tado because I brought it with me from my previous house. (Note also that not all Tados are EMS compatible).u.

& afaik even Tado EMS compatibility varies with which version of EMS the boiler is running ...

FreeBear

70sbudgie said: What about if I can measure the return temp on the boiler? Is it possible to buy a pipe thermometer that stores the readings over perhaps a day?

You may find something in the menu on the boiler that displays the return temperature.

It is certainly possible to purchase a thermometer to attach to the pipe(s) and log the temperatures to say an SD card. Or if you have a small computer to hand (a Raspberry Pi), that would do the job of logging the data. Depending on how much time & effort you wish to invest, there is an open source project called EMS ESP that provides an interface between a WB boiler (with a small circuit board) and a computer. Be warned though, that is a slippery slope in to a very deep rabbit hole of home automation and data logging..

I know. Been sliding down that slope for a couple of years now and have the graphs to show for it.

Reed_Richards

70sbudgie said:

How do I check if my programmable thermostats use EMS (I suspect they don't, as they are marginally older than the boiler)?

Isn't this given the name OpenTherm?  OpenTherm certainly has the same capabilities as are being described and it's a standard used by various manufacturers of gas boilers and controllers.

InvertedVee

Reed_Richards said:

70sbudgie said:

How do I check if my programmable thermostats use EMS (I suspect they don't, as they are marginally older than the boiler)?

Isn't this given the name OpenTherm?  OpenTherm certainly has the same capabilities as are being described and it's a standard used by various manufacturers of gas boilers and controllers.

OpenTherm is one of the control protocols but there are others too. Worcester Bosch use their own EMS protocol which is not OpenTherm compatible without an additional interface https://myboiler.com/opentherm/worcester-bosch-opentherm/.