How Does a Boiler Cycle?

Joined
20 Mar 2018
Messages
1,730
Reaction score
228
Location
Stoke-on-Trent
Country
United Kingdom
Going back to my query, if the roomstat was set higher than the TRVs cutting demand, then the cycling would still continue as the system (pipe) cools below SP-5(say). The only way to totally stop my (simplistic) boiler cycling would be when the roomstat cuts out, and being a switch in series with the timer, cuts the power.
Usual way is to put the roomstat in an area where is no TRV, typically the hall. And if necessary throttle that rad down so that the TRVs do something before the roomstat is satisfied. Roomstat in the hall is common, but IMO it would be more logical to put it where you spend most time and are most concerned about comfort, usually the living room or lounge (etc, depending where you're from.)

"And if TRVs and cycling could run the heating demand, it doesn't feel like the roomstat makes much of a fuel saving." If there were no roomstat the boiler would cycle on its controlstat the whole time CH was called on the timer. That can't be economical.
 
Last edited:
Sponsored Links
Joined
15 May 2022
Messages
858
Reaction score
215
Location
Cork
Country
Ireland
Smart TRVs or whatever can maintain very tight temperature control AS MEASURED AT THE STAT but I never see any reference to the location of the stat, ie installed on the rad flow or return, I have used the standard liquid filled TRVs for many years but I installed them horizontally on the rads return as I feel it gives far more accurate measurement of the air temperature than when installed vertically on the flow, I'd say around 3C difference, another reason for installing them horizontally on the return (be they uni or bi directional) is that IMO they give a more uniform flow as the flow is in the right direction, ie through the valve from under the valve seat.
The liquid filled ones give (me) very reasonable control, +or- 1Cto1.5C if the TRV is in control (see below), they do get a bit sluggish if they close off with reduced heat demand.
If your rad(s) are sized correctly for say -5C ambient/20C room temperature then a boiler flow temp of 75C will require a return of 65C to give 100% output, a minimum rad return temp of 25C, 5C > the 20C SP temperature is probably required to keep the TRV in (modulating) control mode, this means that at a fixed boiler flow temp of 75 then the TRV is only in control mode if the rad output is not < 51.5% of its rated output, it will be a "30deg" rad with flow/return temps of 75/25C, If only 25% rad(s) output is required then the boiler flow temperature must be reduced to 49.4C, to give a "17.2 deg" rad, flow/return temps of 49.4/25C. I often reduce my (oil) boiler flow temperature to 55C which gives excellent TRV control and anyone with a gas fired boiler with outside temperature compensation and TRVs should IMO get excellent control even with the standard TRVs. I presume the smart "TRVs" should control even better but without having to fire the boiler up every 1 minute in 10?.
 
Last edited:
Joined
9 Nov 2009
Messages
347
Reaction score
17
Location
Edinburgh
Country
United Kingdom
Usual way is to put the roomstat in an area where is no TRV, typically the hall. And if necessary throttle that rad down so that the TRVs do something before the roomstat is satisfied. Roomstat in the hall is common, but IMO it would be more logical to put it where you spend most time and are most concerned about comfort, usually the living room or lounge (etc, depending where you're from.)

"And if TRVs and cycling could run the heating demand, it doesn't feel like the roomstat makes much of a fuel saving." If there were no roomstat the boiler would cycle on its controlstat the whole time CH was called on the timer. That can't be economical.
So going with your description, the rad in the hall is strangled so the TRVs in all the other rooms manage the room temperatures. At some point, all of those rads are closed, or as near as damn it, and the majority of the boiler output is going to the hall rad.

Now there are two cases:
1) Hall warms up, room stat disconnects power to the boiler. Now wait until the hall cools enough for the room stat to kick in, and heat goes where it's needed. This feels like a race between the hall cooling down vs other rooms not cooling as fast else the other room temps will swing excessively.
2) Hall warms up, but not enough for the room stat to kick in. Now we're back to TRVs maintaining temp, perhaps with less swing, and boiler cycling.

All of this just feels like room stats are a hangover from the pre-TRV days when they were hardwired and in the hall. Nowadays room stats have been replaced with overly complex wireless controllers.

I'm guessing there's a controller system out there that has done away with the room stat concept and has TRV heads smart enough to be set remotely, but also signal back to a virtual room stat that turns the boiler off when ass smart TRVs are closed, and back on when just one demands heat.
 
Sponsored Links
Joined
20 Mar 2018
Messages
1,730
Reaction score
228
Location
Stoke-on-Trent
Country
United Kingdom
So going with your description, the rad in the hall is strangled so the TRVs in all the other rooms manage the room temperatures. At some point, all of those rads are closed, or as near as damn it, and the majority of the boiler output is going to the hall rad.
That's right, and the boiler will modulate down if it's that type, or cycle on/off on its control-stat if not
Now there are two cases:
1) Hall warms up, room stat disconnects power to the boiler. Now wait until the hall cools enough for the room stat to kick in, and heat goes where it's needed. This feels like a race between the hall cooling down vs other rooms not cooling as fast else the other room temps will swing excessively.
Yes, I suppose if the hall cools slowly other rooms could get too cold before CH is called again. But I have roomstat in the hall and in practice mine works OK.
2) Hall warms up, but not enough for the room stat to kick in. Now we're back to TRVs maintaining temp, perhaps with less swing, and boiler cycling.
The hall rad would have to be designed big enough, or the system wouldn't work as intended. You'd end up with boiler and pump running the whole time CH on on the timer. I doubt if that would give good economy, and be a waste of a good roomstat!
Nowadays room stats have been replaced with overly complex wireless controllers.
I don't know much about them, but I agree they do seem very complex.
I'm guessing there's a controller system out there that has done away with the room stat concept and has TRV heads smart enough to be set remotely, but also signal back to a virtual room stat that turns the boiler off when ass smart TRVs are closed, and back on when just one demands heat.
I believe there is (was) a system with all TRVs and a HW cylinder valve, with a bypass from pump back to the boiler return. A flow switch downstream of the bypass take-off. Every 10 minutes or so the pump starts, if no flow detected after a few seconds it stops. If flow is detected the boiler fires, and stays on till flow goes below the flow switch setting (because all TRVs have closed). Perhaps the heating engineers on here can say whether they've seen or heard of this set-up.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Top