Results 1 to 3 of 3

Thread: evohome and S-Plan

  1. #1
    Automated Home Jr Member
    Join Date
    Jul 2016
    Posts
    24

    Default evohome and S-Plan

    I've been looking at Evohome for sometime and came very near to purchasing earlier this year before I got cold feet around my current setup. The recently announced evohome security system has got my interest going again, but I'm still concerned that it would be right for me.

    I currently have a two-zone heating system with a combi boiler, two wireless heatmiser thermostats, and two two-port valves control the heating in an S-Plan configuration. One zone is a single (large) UFH room the other zone is for the rest of the house fitted with radiators (and the thermostat mounted in the hall). The majority of radiators are fitted with TRV's except two towel rails in the two bathrooms. There is also no ABV currently fitted, the boiler is around 12 years old and does not support opentherm.

    Two things originally put me off evohome, and I'd appreciate your views on these:

    The first ... My current setup has real issues controlling the UFH temperature. It takes at least a couple of hours to warm up the room from cold, and also can overrun excessively i.e the room temperature will keep rising after the thermostat has reached the set temperature and turned the boiler off - the result is that the room can become uncomfortably hot.

    I was originally attracted to evohome due to the optimize start feature, as it would be able to work out to start heating a couple of hours before we are up in the morning, and know to cut the heat before it reaches the set temperature so the room doesn't become too hot. However, I read somewhere (I thought it was on this forum, but can't find the thread now) that the optimize start is not suitable for UFH due to the large offsets required. Is that correct, or am I getting concerned over nothing?

    The second ... I understand that evohome won't be able to drive both the radiator valves (HR92UK) as well as the second two-port valve for the radiator zone. So I'd need to fix the second two-port valve to be always open. Not a problem as the TRV's will stop heated water flowing through the radiators when only the UFH is calling for heat ... except for the two towel rails that I can't fit TRV's to (the current pipework does not allow TRV's to be fitted). Maybe this isn't a concern as it provides a bypass loop (no ABV required?), and means the towel rails will likely always be warm ... however I am concerned about the inefficiency of the system as one towel rail in particular is on a very long pipe run from the boiler. Is this a justified concern, or will the gains with evohome far out way the inefficiency here?

    Don

  2. #2
    Automated Home Legend
    Join Date
    Sep 2014
    Location
    Scotland
    Posts
    2,094

    Default

    Quote Originally Posted by doni View Post
    The first ... My current setup has real issues controlling the UFH temperature. It takes at least a couple of hours to warm up the room from cold, and also can overrun excessively i.e the room temperature will keep rising after the thermostat has reached the set temperature and turned the boiler off - the result is that the room can become uncomfortably hot.

    I was originally attracted to evohome due to the optimize start feature, as it would be able to work out to start heating a couple of hours before we are up in the morning, and know to cut the heat before it reaches the set temperature so the room doesn't become too hot. However, I read somewhere (I thought it was on this forum, but can't find the thread now) that the optimize start is not suitable for UFH due to the large offsets required. Is that correct, or am I getting concerned over nothing?
    I don't have UFH but I know that optimal start will not operate if the current temperature is within about one degree of the set point.

    So for example if your set point changes from an overnight 5 degrees (off, with frost protect effectively) to 20 degrees at 8am, if the measured temperature is already between 19 and 20 degrees it will not come on early as it normally would with optimal start, instead it will come on at exactly 8am as if optimal start was disabled.

    If the temperature was below 19 degrees it would come on early (optimal start) to reach 20 by 8am. It's unclear why Honeywell decided to disable optimal start when the temperature differential was small but its probably on the basis that there's no point coming on early if the temperature is already "close enough". With radiators its fine as they heat the room quickly but for UFH where the temperature rise can be very slow it might be an issue.

    It's a common misconception that optimal start/optimal stop have something to do with turning off early before the set point is reached to prevent overshoot - this is not the case. This "anti-overshoot" behaviour is built in behaviour of the PID (proportional integral differential) feedback control loop that each zone has.

    Whether or not you enable optimal start or optimal stop, if you have a system that has a large thermal mass and tends to overshoot the target easily (which describes both radiator systems and UFH) the Honeywell system will learn over time (only takes a few days usually) that it must start to turn back the heat output (or turn it right off) before the set point is reached and by how much early it must do this to hit the target on the button.

    It looks at not only the current temperature ("switch off if temperature is over 20") but also the rate of temperature rise, and its previously learnt memory of how much thermal overshoot there is in the system. ("If I turn off now, how long will the temperature keep rising at by how much")

    So you always get the benefit of this. How well it works in practice on UFH which has very slow temperature rise times and very large thermal masses I'm not sure, but it certainly works well with radiators, and it keeps adapting day from day as conditions gradually change.

    The second ... I understand that evohome won't be able to drive both the radiator valves (HR92UK) as well as the second two-port valve for the radiator zone. So I'd need to fix the second two-port valve to be always open. Not a problem as the TRV's will stop heated water flowing through the radiators when only the UFH is calling for heat ... except for the two towel rails that I can't fit TRV's to (the current pipework does not allow TRV's to be fitted). Maybe this isn't a concern as it provides a bypass loop (no ABV required?), and means the towel rails will likely always be warm ... however I am concerned about the inefficiency of the system as one towel rail in particular is on a very long pipe run from the boiler. Is this a justified concern, or will the gains with evohome far out way the inefficiency here?
    If it were me I'd rather have all radiators controlled - I like the idea of being able to schedule rooms fully independently and have no heat waste from rooms such as bathrooms heating just because I'm trying to warm the bedroom at night or dry some clothes in a spare room. If it's physically impossible to mount a TRV on the towel rails due to how the piping runs there are some other possibilities:

    You could install a motorised zone valve in the pipe work feeding only the bathroom radiators (under the floor, in the ceiling, wherever it is) assuming that is physically feasible - it might not be depending on your house...

    You then use a BDR91 to control the zone valve and add a "zone valve" zone on the controller for the bathroom. You would also need a wireless thermostat such as a DTS92 mounted in the bathroom so it has some way to measure the temperature in the room. It will look like a normal zone on the touch panel and in scheduling and you'd be able to view and adjust the temperature on the thermostat if it is one with controls such as a DTS92. (You could use one without any controls and still control the temperature from the main touch panel)

    Temperature control won't be as precise as the zone valve will be modulated on and off using TPI when the room is near the correct temperature, it would also cost a lot more than an HR92 since you have zone valve + BDR91 + wall stat.

    Another unconventional approach if the pipe work to the bathroom is relatively accessible somewhere else like a drying cupboard would be to fit an HR92 remotely there in the flow path of the radiators and again use a wall mounted thermostat like a DTS92 in the bathroom to measure the temperature. So even though you can't get at the HR92 except to change batteries, you could view and control the room temperature on the DTS92 or via the main controller.

    This would give the advantage that you would get the much smoother more precise temperature control of an HR92 (with its variable opening valve) and also not require any power to the location as the HR92 is battery powered, whereas the zone valve + BDR91 combination requires mains power. As long as it had a good wireless signal in the location and you could get to it once every year or two to change the batteries it should be quite workable.
    Last edited by DBMandrake; 16th July 2016 at 11:34 AM.

  3. #3
    Automated Home Jr Member
    Join Date
    Jul 2016
    Posts
    24

    Post

    Thanks for your detailed reply, this has helped significant

    Quote Originally Posted by DBMandrake View Post
    I don't have UFH but I know that optimal start will not operate if the current temperature is within about one degree of the set point.
    So it sounds like optimal start won't help with getting my room to temperature by a set time in the morning, but evohome could help with the over-shoot due to it's learning nature.

    Do people use optimal start with radiators? Presumably this one degree of temperature difference could be an issue with radiators as well?

    Quote Originally Posted by DBMandrake View Post
    If it were me I'd rather have all radiators controlled - I like the idea of being able to schedule rooms fully independently and have no heat waste from rooms such as bathrooms heating just because I'm trying to warm the bedroom at night or dry some clothes in a spare room. If it's physically impossible to mount a TRV on the towel rails due to how the piping runs there are some other possibilities:
    At the moment both towel rails are duel fuel. I don't run the central heating during the summer, just have the towel rail electic elements come on for a couple of hours in the morning and evening to keep the towels dry. This is all controlled by lightwaverf relays which actually works surprisingly well. Part of the issue with altering pipework is down to this setup and that I'd end up need to pull tiles off walls to move the position of the pipes to accommodate (or move the radiator up the wall drilling now holes in the tiles).

    Both pipe runs aren't particularly accessible without lifting floor boards (we have engineered wooden floor throughout ), or cutting holes in ceilings. Both of which seem a lot of upheaval and cost for not a lot of gain.

    A couple of thoughts:

    1. Leave the towel rails as is, so that they are always open. There would be some additional cost or wasted heat but I'm guessing the cost would not be significant?

    2. Turn the towel rails off completely and only heat them through the electric side.

    Am I best to install an ABV even if the towel rail circuits are always open?

    Don

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •