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Thread: HR92 - open/close 100% or modulating?

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    Default HR92 - open/close 100% or modulating?

    So, im now about a week in with my new evohome system and things have definitely changed / been learnt with by the system about the rooms etc.

    One thing i have noticed is that when a valve calls for heat it doenst necessarily open the valve from 0% to 100%, it sounds like its controlling the opening position, perhaps based on current room temp versus required room temp. i.e. it can go from 0% to 40% etc. The reason i ask is that i noticed my farthest rad on the system (living room) had a small whooshing noise last night when operating even though the lockshield is 100% open, this wasnt happening previously either using the HR92 or with the standard TRV head.

    Its not a problem, just wanted to know more about the control philosophy.

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    Automated Home Legend top brake's Avatar
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    Quote Originally Posted by Leigh View Post
    So, im now about a week in with my new evohome system and things have definitely changed / been learnt with by the system about the rooms etc.

    One thing i have noticed is that when a valve calls for heat it doenst necessarily open the valve from 0% to 100%, it sounds like its controlling the opening position, perhaps based on current room temp versus required room temp. i.e. it can go from 0% to 40% etc. The reason i ask is that i noticed my farthest rad on the system (living room) had a small whooshing noise last night when operating even though the lockshield is 100% open, this wasnt happening previously either using the HR92 or with the standard TRV head.

    Its not a problem, just wanted to know more about the control philosophy.
    Hi

    The HR92 are modulating and can be fully open, fully closed or at a % open position between

    To prevent system noise it is recommended that you have a ABV Automatic Bypass Valve
    I work for Resideo, posts are personal and my own views.

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    Thanks, there is an ABV installed as noted in my other thread but i will review the setpoint again in light of this. The manual says set at 0.2 if there are TRVs on the system and its about 0.22-0.25 at the moment.

    Thanks.

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    Automated Home Legend top brake's Avatar
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    sounds like the issue is with velocity rather than pressure. what system and pump do you have, and what setting is the pump on?
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    Quote Originally Posted by Leigh View Post
    So, im now about a week in with my new evohome system and things have definitely changed / been learnt with by the system about the rooms etc.

    One thing i have noticed is that when a valve calls for heat it doenst necessarily open the valve from 0% to 100%, it sounds like its controlling the opening position, perhaps based on current room temp versus required room temp. i.e. it can go from 0% to 40% etc. The reason i ask is that i noticed my farthest rad on the system (living room) had a small whooshing noise last night when operating even though the lockshield is 100% open, this wasnt happening previously either using the HR92 or with the standard TRV head.

    Its not a problem, just wanted to know more about the control philosophy.
    The HR92 is a PI or "Proportional Integral" controller.

    This means that the valve opening is proportional to both the difference between measured and set point temperatures and the length of time at which it has been away from that temperature. The proportional band isn't documented but seems to be about +/- 1.5 degrees.

    This means if the set point is more than 1.5 degrees above the current temperature the valve will open fully, and if the set point is more than 1.5 degrees below the current temperature the valve will fully close. However within that +/- 1.5 degree "proportional band" the valve will adjust to a partial opening somewhere in between to reduce water flow (and thus radiator temperature) but not completely shut it off.

    If you're observant and sitting nearby you will hear it make very small, very brief, (half a second) adjustments to the valve position about every 4 minutes until it homes in on an equilibrium that maintains the room temperature steady and at the target. The ideal situation is that it finds a valve opening that lets just enough water through to make the radiator just hot enough to balance the heat loss of the room - thus the room remains at a constant temperature without oscillating up and down. It's a delicate balance but one that it is often able to manage.

    If there are things changing in the environment such as people entering and leaving, doors being opened and closed, appliances being turned on and off etc, then obviously it would need to make adjustments to compensate for those, so it would make small adjustments every now and then to do so.

    If the valve opening required to maintain this balance is quite small you will hear a small hiss through the valve where it is restricting the water flow. This is regardless of where your lock shield valve is set. If the hiss is unusually loud then it could be too high a flow velocity as top brake suggests, although with your ABV set to 0.25 that seems unlikely. (0.25 actually seems quite low to me)

    The reason you wouldn't often hear a noise like this with a conventional TRV is they are just not very accurate and are unable to maintain such a delicate balance for any length of time like the HR92 can - a conventional TRV will tend to overshoot the target temperature which causes it to fully close, then it undershoots the target in the other direction and opens wide - wide enough that you don't hear a hiss. It spends little if any time at the exact partial opening where you would notice a hiss, but your room temperature is not staying steady either.

    With the HR92 if you hear a gentle but steady hiss from the valve after the room is up to temperature that lasts a long time that is actually a pretty good sign that it has found the correct equilibrium point on the valve to maintain the room at the desired temperature!
    Last edited by DBMandrake; 28th January 2016 at 10:45 AM.

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    I think one of the important factors that people seem to (initially) misunderstand is that once a room is up to temperature, with an Evohome system the valve won't them completely close, and the boiler shut down. It keeps a small amount of heat trickling into the radiator to maintain the room heat, compensating for any heat losses. This avoids the see-sawing of temperatures that you get with conventional controls.

    Many of the posts on this forum seem to be from people querying why their heating is still running once their rooms are warm.

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    Quote Originally Posted by paulockenden View Post
    I think one of the important factors that people seem to (initially) misunderstand is that once a room is up to temperature, with an Evohome system the valve won't them completely close, and the boiler shut down. It keeps a small amount of heat trickling into the radiator to maintain the room heat, compensating for any heat losses. This avoids the see-sawing of temperatures that you get with conventional controls.

    Many of the posts on this forum seem to be from people querying why their heating is still running once their rooms are warm.
    That's a very good point.

    As well as controlling the valve openings it does modulate the boiler as well of course, either by directly reducing the flow temperature via OpenTherm, or using duty cycle modulation via a BDR91 as a crude method of flow temperature modulation.

    I've found that if any zone is out of the proportional band in the "needs more heat" direction, the boiler relay will just go to 100% duty cycle. However if all active zones are in the proportional band near their set points, the boiler duty cycle will drop way down to only that which is necessary to replace the lost heat in the rooms.

    For example in recent weather conditions our boiler has been running at roughly 30% duty cycle on the BDR91 in the evenings once the zones are up to temperature. So during this maintenance period where it is just trying to maintain the room temperatures not only are the HR92 valves only partially open, the boiler is only running part time too, so the average flow temperature is also reduced.
    Last edited by DBMandrake; 28th January 2016 at 11:37 AM.

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    Thank you all, thats is very helpful. I can indeed hear the valve for around half a second so it is doing exactly as you say DBMandrake and Paul. It doesnt bother me at all i just wanted to understand how the system operates better.

    As for my pump its a Grundfos Alpha 2 which i changed last week to contant pressure mode as per the suggestion in one of my other threads as i have one rad with the lockshield open only a fraction as it got all the flow before the system was balanced and it was noticeably audible. Changing to CP mode has made a lot of difference.

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    Best description of how the HR92s work that I've read.

    My system (now about 3 months) has settled down and it operates as stated above. We had a completely new heating/hot water system installed - new boiler / rads / pipes / pump /valves / ABV / filter [think that's all ] and of course, Evohome. All Rads were correctly sized and also the Boiler.

    The Alpha 2 Constant Pressure setting is an absolute *MUST* - just won't work properly on any other setting (and the CP2 setting for our 12 Rads and Hot Tank). The Proportional Pressure is an absolute disaster - will even negate the correct working of the ABV.

    The way the temperature is now constant, room by room, is amazing. It simply manages itself. We have none of the problems that others appear to have, but I have to say it was a relatively steep learning curve - nothing onerous, but just that you have forget everything you know about "old style" heating and boilers.
    Last edited by MrB; 28th January 2016 at 03:39 PM.

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    Quote Originally Posted by MrB View Post
    My system (now about 3 months) has settled down and it operates as stated above. We had a completely new heating/hot water system installed - new boiler / rads / pipes / pump /valves / ABV / filter [think that's all ] and of course, Evohome. All Rads were correctly sized and also the Boiler.
    Would you mind telling us what boiler and hot water tank you bought, and why? I'm thinking of replacing my one, and would appreciate someone else's (also an Evohome owner) perspective. Thanks!

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