downrate oil boiler?

thrope

I have a 1 year old grant vortex boiler which can be set up with 15, 21 or 26kw nozzles. It currently has the 21, 3 bedroom 1.5 storey house with 11 rads, 200L invented cylinder.

Oil consumption over winter was around 11L/day which I'm a bit disappointed with as it's the same as the old non condensing combi it replaced used the winter before,.with a sticky valve for a month that meant the radiators heat up every time hot water was used.

I monitor the flow and return temps and I think that most of the time the boiler is short cycling on the boiler thermostat and I am wondering if this could impact efficiency. I have a tpi thermostat on 10 minute cycles but still get boler cycling in the on periods on a cold day.

So at the upcoming service I am thinking to switch to a 15kw nozzle with the hope that it will be enough, will short cycle less even if it takes a bit longer to get up to temp.

Does this make sense? Upstairs rads usually cut off with trvs pretty quick as heat goes up so thinking 15 might be ok even though 21 usually recommended for 11+ rads.

Thanks for any thoughts

FreeBear

Measure the size of the radiators and make a note of the type (i.e. single panel or double, single row of fins or two). Go to a web site such as Screwfix, look up the radiator sizes, and make a note of the heat output (use the KW number rather than BTU). Add up the numbers for all 11 radiators, and if the total is less than 15KW, then swapping the jet may well be worthwhile.

Note - Ideally, you also need to know if the radiators are appropriately sized for the rooms. Quite often, they are oversized for various reasons. Either laziness on the part of who ever installed the system, or there was an intention to run at a lower flow temperature.

Reed_Richards

PhilHornby said:

I swapped the original upstairs/downstairs wall-stats for lots and lots of Wiser iTRVs and Roomstats - so the boiler is invariably only heating one room at a time. It short-cycles every time it is called upon to heat a room .

Off topic but as you mentioned it:  the problem with the Drayton Wiser system is that it gives priority to the TRVs over the room thermostats.  So it will heat the room with the roomstat until such time as none of the TRVs on the same zone are calling for heat.  If you have a room with an undersized radiator that is slow to heat up then the room with the roomstat can end up way too hot (hotter than the set temperature).  This could be a very simple option in software, you pick a zone and specify whether that zone should have TRV priority or roomstat priority.  I've asked Wiser if they can do this but they haven't.   

Olly_J

Reed_Richards said:

Off topic but as you mentioned it:  the problem with the Drayton Wiser system is that it gives priority to the TRVs over the room thermostats.  So it will heat the room with the roomstat until such time as none of the TRVs on the same zone are calling for heat.  If you have a room with an undersized radiator that is slow to heat up then the room with the roomstat can end up way too hot (hotter than the set temperature).  This could be a very simple option in software, you pick a zone and specify whether that zone should have TRV priority or roomstat priority.  I've asked Wiser if they can do this but they haven't.   

I've got a wiser system, some rooms have smart trv's + a room stat and some just have trvs at the moment. The rooms with the stat only rely on the thermostat calling for heat, even though there are multiple trv's within that room, they do not call for heat individually...I have all my rooms as seperate zones in the app

You can see the answer here too

When placed in the same room, does the Wiser Radiator Thermostat or the Wiser Room Thermostat take precedence? – Drayton By Schneider Electric (zendesk.com)

The only way i can see your issue would occur is if you grouped multiple physical rooms into one logical room in the wiser app perhaps

Reed_Richards

Thank you for your ideas, @Olly_J.  I read your link and I agree with the responses made by Ian Friend.  The system can work for you because you have rooms with both smart TRVs and a room stat.  In those cases the smart TRVs can adjust themselves to achieve the temperature called for by the room stat.  But, as Ian Friend observes, if you have just a room stat then TRVs in other rooms (on the same zone) will take precedence. 

And I don't see defining Zones in the app that are not real zones controlled by real valves can do anything but hinder correct operation so I presume you really do have a large number of real zones..    

Olly_J

Ok I see what you mean I think; where a wiser room has been defined, which combines multiple physical rooms, but the stat is separated from the sTRV's by walls etc, then I could imagine you might get an overheating issue, where the room with the stat is not up to temp, but the room with the sTRV's is too hot, but the stat still calls for heat.

I suppose my question is why not define your room with the sTRV's as a room in wiser, and your room with the stat as another room in wiser, that way they will call for heat independently, as and when they require it.

This is how I have mine defined room wise, each is a 1:1 with a real room..some have sTRV's and stats, some just have sTRV's, I don't have any defined that spans multiple physical rooms.

 

The stand-alone sTRV's control their heat fairly well, just not as accurately as a room stat + sTRV does, but extra room stats are expensive obviously..so I have only brought them for problem areas like bay window radiators where the sTRV is very close to the outer wall leading to it thinking the room is cooling down very fast (which it isn't) so room stats fix that

Reed_Richards

Olly_J said:

I suppose my question is why not define your room with the sTRV's as a room in wiser, and your room with the stat as another room in wiser, that way they will call for heat independently, as and when they require it.

Yes, they will call for heat independently but as they are on the same physical zone then all the rooms on that same physical zone will get hot water sent to their radiators.  Those rooms with TRVs, smart or otherwise, will reach the desired temperature.  But a room with just a room thermostat (and dumb TRVs, if present, set to max. so they don't interfere) will overheat. 

That's what happened.  My kitchen (small radiator with smart TRV) could not keep up with my living room (room thermostat, no smart TRVs) so my living room overheated because the TRV in my kitchen kept calling for heat.  

thrope

The solution to this is to get smart TRVs in the room with the thermostat. If they are registered as the same room the room thermostat will take priority for establishing the temp and the trvs will just work as zone valves.

Olly_J

Reed_Richards said:

Yes, they will call for heat independently but as they are on the same physical zone then all the rooms on that same physical zone will get hot water sent to their radiators.   

A room with just a room thermostat (and dumb TRVs, if present, set to max. so they don't interfere) will overheat. 

That's what happened.  My kitchen (small radiator with smart TRV) could not keep up with my living room (room thermostat, no smart TRVs) so my living room overheated because the TRV in my kitchen kept calling for heat. 

Why not set your dumb TRV to 3 or 4 to prevent the room overheating and set the Wiser thermostat as its own room? Really its no different from a traditional setup, in that you want the thermostat set slightly below the C value if the TRV, so it will be a bit of trail and error, but max will never close the valve.

The kitchen wiser TRV will continue to call for heat when it needs it, and fire the boiler up, but as long as your dumb trv in the living room is set properly, then it should not matter if water is being sent, as it will have closed off the rad valve, the same as a smart TRV would do.

Not to labour the issue, but it seems the main issue here is that you have the dumb TRV set to max.Were you to buy another smart valve, it would close the valve when your thermostats set temp was reached i.e the same as being set to 3 or 4..

Reed_Richards

The situation I want to avoid at all cost is to have the thermostat calling for heat, the pump trying to pump water round the heating but the TRVs all being off because all the radiators are hot enough.

Olly_J

Understand that, but that is no different a situation from a standard wall stat and dumb TRV's, so its not a Wiser issue as such causing it.

As thrope said, the full answer is to buy another smart trv head and put it in the room with your stat, but I ran my system with a stat and Dumb trv's for a while at the start and it was just a case of trial and error find a happy medium.

The only thing that will happen if the stat calls but the valve is closed, is your boiler will cycle as the return temp will be too high, so then you just have to tweak the TRV up a bit or the stat down a bit