PWM Control of Oil Fired CH?

I am looking at putting a thermostat at the core of the house - which has extremely thick stone solid walls and a huge time constant. Running the wire back to the loft. Yes the boiler and main controls are all in the loft - don't blame me. It was like that when I moved in! (It works well enough that this isn't a problem most of the time)
It also does the hot water so the boiler must run an hour or so morning and evening even if the CH is completely off during the summer.
I was wondering about getting a PWM capable wireless unit instead and using the thermostatic control to modulate the separate CH pump when the main oil fired boiler is active for CH. I think this will result in tighter overall temperature control and less overshoot.
But it will also cause the boiler to burn for shorter times and more often. My question is has anybody tried this and what were the results. Is the effect of more frequent burns less efficiency from the burner starting up from cold more frequently or a net fuel saving? It is fairly obvious for a gas boiler that it should be a win, but for oil it is a lot less clear. Anyone done this and have practical experience?
The wireless controller I was considering was this one - Salus ST320RF or a couple of other more expensive ones which do more complete PID implementations. This one has the nicest styling (bad reason to choose I know but most of the others look rather industrial).
http://www.uk-automation.co.uk/products/Salus-Wireless-Room-Thermostat-with-Receiver-ST320RF.html
What are the pros and cons ? Any better thermostats to consider?
--
Regards,
Martin Brown
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On 23/03/2012 09:39, Martin Brown wrote:

http://www.uk-automation.co.uk/products/Salus-Wireless-Room-Thermostat-with-Receiver-ST320RF.html
I have just installed two of the non-RF versions (the salus 320) (two zone heating, upstairs and downstairs).
These units are simple on-off controllers, they will not do PWM motor control.
On some CH pumps you will see a knob on it for pump speed. If might be possible to fashion up your own pump speed controller by using relays onto the wiring for this speed knob. You would need some kind of modulation control to take an input to then control the required amount of pump speed. That might be a better method than PWM.
You do need to change your mindset regarding using these SAlus St320RF's compared to a conventional timeswitch and wall thermostat.
the smallest time period you can have is 10 minutes and the smallest temperature change you can have is 0.5°C.
For example, you think I want 20°C between 7.00am and 9.00am, (the time between getting up, showering and breakfast and going out) then 5°C from 9.00am till 4.30pm (assuming everyone in house is out at work or school so you only need frost protection) then 20°C from 4.30pm till 10.00pm (everyone comes home has evening meal, watch TV) then 14°C from 10.00pm till 7.00am (to stop you getting hypothermia in bed but dont need full on heating due to use of duvets)
In a modern well insulated house, it does not take long for the house to reach the desired temperature.
In your case with a stone house with long time constant, you would need to establish how long your house takes to heat up and cool down and factor these in to your ST320RF settings by making the temperature transition times earlier by the time constant you've established for your house in order to achieve the desired temperature set profile.
You might want to consider getting a weather compensated model so that the boiler starts earlier in winter and later in the summer so that the weather has little effect on achieving the desired thermal profile inside the house.
Stephen.
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On Fri, 23 Mar 2012 09:56:46 +0000, Stephen H wrote:

morning
That might be wasteful. When the time switch on one system I had broke I started to only heat the DHW when I needed it, reheat time from cold was only about 20mins. The tank would stay hot enough for washing up/hand washing for the best part of 48hrs. There was a very noticeable drop in the gas bill... I was on my own and there was no shower.

Or look for an "Optimum Start Control" feature(*1), that calculates the difference between actual room temp and target temp and in conjuction with a rate of rise fiddle factor automatically turns the heating on early enough to reach the target temp by the target time. This feature is normally only on the first target temperature/time of a day.

With a house with a high termal mass I doubt weather compensation is worth it, the house does that by the sheer amount of heat stored (or not(*2)) in itself. With a high thermal mass property it's best to just keep the heat input ticking along, you really do not want it to cool down significantly as it'll take days to warm back up. BTDTGTTS...
I'm not sure what the OP hopes to gain with the PWM. I get the impression that there is no room stat currently, the addition of one will improve things, particulary if the new stat is a programmable one with OSC.
We really need more information about the boiler, non-condensing/condensing/combi. What the DHW arrangments are. What existing controls there are etc.
(*1) Different makers use a different term "Optimum Start Control" is the Danfoss term and has manual setting for the fiddle factor. Honeywell uses a different term and "learns" how fast the building warms.
(*2) In winter the building has lots of heat relative to the weather, it tends to stay warm. In summer the building is relatively cold so it tends to stay cool.
--
Cheers
Dave.




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On 23/03/2012 10:26, Dave Liquorice wrote:

Hot water tank isn't quite that well insulated but will stay plenty warm for 24+ hours. I should more accurately say the boiler has the option of coming on and heating up the water for about an hour morning and evening. Hot water is on a gravity feed circuit to both the oil boiler and the wood burning stove. There is a manual override switch that can force the main CH pump to be on from downstairs. And also a protective thermostat on the exit pipe from the stove to force CH circulation if the temperature starts to get too high.
There is also a frost stat that defends the loft space pipe runs.

I am a bit surprised at that since I do monitor oil usage fairly carefully and have dropped the amount used by about 30% over the past couple of years by burning more wood and very deep loft insulation.
Can't do anything about cavity wall insulation - no cavities. The guy who drilled the obligatory vent for the modern solid fuel regulations cursed the walls for their thickness and resistance to his core drill.
Winter oil usage rate for space heating is well more than an order of magnitude higher than summertime water heating.

That sounds interesting. The system at present has only the boiler thermostat and a broken sensor in the main room and gives fairly crude levels of control which can be tweaked by manually enabling the pump from downstairs and/or lighting the fire. The main room is no longer a good place for the master sensor as the new stove is way more efficient than an open fire.

I was originally toying with the idea of gating the system with an exterior temperature monitor so that the pump was disabled by it.

Yes. Pretty much exactly describes it. The time constant for the interior structural walls as opposed to the air inside is a few days.

Boiler is an old Worcester Danesmoor 15/19 oil fired boiler dating from I would guess the mid 70's. Touch wood it is well behaved as given the odd location in the loft major rework would be involved to change it. It has to be non-condensing given the age! There is a separate pump.
Pump and boiler have separate independent control and overrides. Timer is a bit simple minded but works OK: On/Off, Once daily, twice daily.

That sounds interesting. The two main aims would be to have the system better at warming up the house after an unusually cold night in winter and to back off heating quicker when in spring the sunshine brings the core up to an acceptable temperature (eg late evening sun into rooms).

Indeed. That describes the situation very well.
I tend to end up thinking about wasted oil more at this time of year since during spring with the odd sharp frost at night it shows up all the weaknesses of the existing mostly timed heating system.
--
Regards,
Martin Brown
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On Fri, 23 Mar 2012 11:17:51 +0000, Martin Brown wrote:

Doubling or tripling the jackets or even fitting a jacket to a preinsulated tank might be worth it. Go to far though and SWMBO'd will complain about a "cold" airing cupboard. B-)

What happens if the power fails? Guess you have to put the fire out just when you could really use it... There ought to be a heat dump radiator or connection into the heating system that will gravity feed controlled by thermostatic valve with its sensor at or near the top of the HW cylinder.

no
Yep, made a noiceable reduction in our oil consumption this year with the installation of insulated dry lining and wood burner. Not sure we have actually saved any money though when you factor in the cost of logs...
It really depends on your demand for hotwater, one person on their own and out at work all day doesn't need a tank of hotwater sitting losing heat. A family of teenagers all showering or bathing every day the heat lost by even a basically insulated tank probably pales into insignificance compared to the use.

Ours was about 6:1, ie winter about 6cm/week in the sight tube. Summer 1cm/week. This winter it's was around 4.5cm/week. Hopefully this summer might drop to 0cm/week as we have also fitted a solar thermal system.

You don't want any thermostat in the same room as a heat source it's not controlling. Like wise you don't want a room stat in a room with a radiator fitted with thermostatic valve unless you disable the thermostatic valve.

So might be a gert lump fo cast iron with high water capacity, long burns are required for best effciency. Our oil boiler is a floorstanding model holding 12 gallons of water and is massive lump of cast iron. From cold it takes quite a while to start churning out hot water even with >35kW of input.

A programmable stat out of the room with the wood burner and in a space with a radiator (any thermostatic valve wound flat out or control head removed) should do the job. Choose one withn OSC or similar. I like the Danfoss TP7000 others swear by the very similar Honeywell versions. Both are available in RF rather than wired versions. RF might be useful if the room with the woodburner is the main room used. You can move the stat into that rooom to give you best comfort when the stove isn't lit and out when it is. Perhaps not every time you light the stove just in general, so out during teh winter when te stove is lit every day, back in during the warmer parts of the year when it isn't. Remember any thermostatic valve on the radiator though...
The gravity DHW is a bit of PITA as you have to heat the water when there is a demand for CH. There are ways around that, not sure if the d-i-y FAQ on heating covers those but it is probably worth a look at those pages.
--
Cheers
Dave.




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On 23/03/2012 12:33, Dave Liquorice wrote:

There is a heat dump radiator with gravity feed on the back side of the fireplace wall in the master bedroom. Part of the reason to look at resiting the thermostat is because with the fire on the house can lock in a bistable mode with the CH pump off and only the main living room and master bedroom being heated - tough for anyone in the rest of the house.
The gravity feed and dump radiator works OK in a power cut although it is wise to back the fire off a bit from maximum output or it and the domestic hot water tank ends up fiercely hot. It turned out that the previous back boiler on the open fire had been plumbed incorrectly so that the dump radiator did not work at all if the CH pump had failed. We never managed to make it kettle though even in long power cuts.

I am seeing £200 for the logs and about £400 for the oil saved. Main inconvenience is having to lug the logs in and stack them in bulk.

It is just how the thermostat was previously installed. The old open fire didn't seem to upset it. Changing it brings various other ideas into play - like the best place to use as a total house control point.

Similarly large chunk of cast iron and built like a battleship. The fire at full bore can output enough heat to completely shut down the main oil boiler so in very cold weather that is what I do.
A few weeks and it will be back to hot water only with just the odd flick of the manual pump switch to stop the thing from seizing up in the summer season when it doesn't get used.

Being able to relocate the stat until it does what I want is one reason why I am tempted to use an RF Wireless based unit. Thanks for the tip of Honeywell TP7000 with OSC that looks like it does what I want.

The hotwater is well enough insulated and consumed in sufficient quantity that I really don't think it is worth me worrying about.
I don't fancy motorised valves. My parents have a complex system like that and the things are forever playing up, sticking open etc. etc.
--
Regards,
Martin Brown
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On Sat, 24 Mar 2012 09:57:28 +0000, Martin Brown wrote:

We've probably burnt about £350 quids worth of logs (5 dumpy bags at £70/bag). Oil saved is a bit harder to work out but if one assumes normal winter useage of 6cm/week since mid November (when the stove was installed) against what we have actually used that's about 700l or £420 @ 60p/l.
Trouble is we are also heating far more of the building than previous winters and the insulation levels in half of it have gone up so there is a lot of mud in the water.

SWMBO'd does that, at least when it's cold and she's lighting the stove every evening. B-) Now it's got a bit warmer she seems to have gone off the idea.

The heating never gets switched off here, the programable stat takes care of it. There might be a period of a week or two in June when the heating never comes on but thats about it. No six month heating lay off at 1400' on the North Pennines. B-) Might be different this summer with 50mm of foam insulation board on all external walls.

*Danfoss* TP7000 ... B-)

when
Trouble is the CH primary is normally rather too hot for DHW and with an open gravity loop the DHW will rise up to that temperature. Having said that one gets used to the possibilty of scalding hot water coming out of the taps PDQ!

Can't say I've ever had any serious problems with ours. One did get a bit sticky in the motor head but a quick strip, clean and lube and it's been fine since.
--
Cheers
Dave.




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On Fri, 30 Mar 2012 08:42:00 +0100, Fred wrote:

on

Internal not external. So 50mm foam board on metal firings with 12.5mm plasterboard bonded to it and plaster skim.
Window reveals where tricky they ended up with 25mm foam plus PB and skim. There isn't a great deal of the window frame showing...
--
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Dave.




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On Wed, 28 Mar 2012 09:13:09 +0100, Martin Brown wrote:

Not tried keeping ours in overnight.

Meh, I buy on function not form... The TP7000 looks nicer than it's predecessor the TP75:
http://heating.danfoss.com/product/19382.jpg
--
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Dave.




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On Mar 23, 10:26 am, "Dave Liquorice"

Honeywell's 'Optimisation', at least on the CM9xx series, is active on any program step where an increase in temperature is set.
Mathew
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For wireless, I always recommend Honeywell. I have had problems with the Salus wireless controllers a number of times because of interference - they worked fine when next to the base station, but in the next room would not work. The Honeywells use a different frequency (833 vs 433Mhz? mHz?) that is less prone to interference, and they are 'paired' att he fatcory, so should work out of the box. The downside is the Honeywells are at least 30% more expensive.
Alan
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To reply by e-mail, change the ' + ' to 'plus'.

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On Fri, 23 Mar 2012 10:12:10 +0000, A.Lee wrote:

Not had any problems with the two wireless Danfoss programmable stats we have here. They are on 433MHz.

I'd be wary of anything using 800 to 863MHz that is about to be sold off to the mobile phone operators for 4G... 863 to 865 is used for all manner of kit some use a constant carrier (wirelss headphones) which will muck up anything that expects a clear band.

Nope, M is right but hz isn't.

Nope, m is wrong (milli, 1/1000th) Hz is right.
You are looking for MHz. Unit symbols based on peoples names are, generally, capitalised:
K for Kelvin, William Thomson, 1st Baron Kelvin Hz for Hertz, Heinrich Hertz H for Henry, Joseph Henry T for Tesla, Nikola Tesla W for Watt, James Watt A for Ampere, Andre-Marie Ampere Pa for Pascal, Blaise Pascal F for Farad, Michael Faraday
--
Cheers
Dave.




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On Fri, 23 Mar 2012 10:49:27 +0000 (GMT), "Dave Liquorice"

Only the more modern ones.
The CM6x series uses 433 ish

It'll be 868MHz or thereabouts, an ISM band with the same 'open' status for low power devices as 433 ish. The only difference being that 433MHz devices can transmit continuously, the 868MHz ones have a lower (10%?) duty cycle.
Neither band is immune from interference, the only thing that might ensure that is a more suitable modulation scheme using multiple carriers, but both bands in built up areas will be full of so much crap its a wonder they work at all.
There is more detail in this OFCOM document
http://stakeholders.ofcom.org.uk/binaries/spectrum/spectrum-policy-area/spectrum-management/research-guidelines-tech-info/interface-requirements/IR_2030.pdf
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On 23/03/2012 10:12, A.Lee wrote:

I always try and avoid RF full stop for the aforementioned interference reasons. So I went for wired salus controls.
All my landline phones are corded, I don't do wireless networking or bluetooth.
Regards,
Stephen
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On 23/03/2012 09:39, Martin Brown wrote:

I use a Danfoss TP9 programmable thermostat to control my oil fired CH. It is a two channel programmer and uses a wired remote sensor located in my sitting room. If provides much better temperature control than the previous mechanical thermostat and programmer. Ideally I would like to reduce the number of heating cycles per hour, but it is not a configurable item on this programmer.
--
Michael Chare

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I have serious reservations about the benefit of proportional (PWM) control on some heating systems.
I have a Honeywell CM907 wired stat controlling my main living area. It has fixed proportional control and it does maintain the temperature with high accuracy but I'm not happy that it forces a minimum of 3 switching periods per hour, as this results in the boiler thrashing at full power to achieve circulation temperature at switch on and more often than not is shut down just a few minutes after the rads get warm.
This is particularly noticeable in those borderline seasons when you want the heating on but you really don't need that much heat, you seem to be cycling the boiler (and controls) unnecessarily.
Also, one of the benefits of programmable thermostats is that you don't really need to turn them off completely, you just set them to a lower, setback temperature where they will come on in a cold snap or save the temperature of your high thermal mass space dropping too low. Often this is a sort of don't care temperature, say you set it to 16degC but you don't really care if it drops to 15 or rises to 17, but you can't tell the propo stat not to care about the accuracy of the setback temperature, it will religiously try to control it to within half a degree, cycling your boiler 3 times an hour, 24/7 whether it needs to or not.
I think an alternative to this is to use a stat with simple on-off control which has high accuracy and hysteresis that can be varied to suit system needs.
My Honeywell stat is on the way out and I am looking at alternatives that can be configured either as proportional or on-off controllers. The Siemens REV24 has caught my eye, it has high accuracy and can have hysteresis set as low as half a degree if that suits the system. In a high thermal mass building (as mine is), I would be happy to have the stat switching one an hour and give the boiler a run at a long slow burn when it will be at it's most efficient.
The benefit of having the control method configurable is that you can try it one way and swap to the other if it doesn't work for you.
FWIW, my system ran the smoothest before I installed the thermostat, the boiler has a wide range of modulation and I set it low to get a very long and slow burn. Throughout the first winter the boiler was rarely cycled when I had it on, it just tickled along, giving out about 6kW or so on low, a bit like a smooth country drive vs boy racing between traffic lights. I only turned up the modulation when it got really cold.
My boiler is gas btw so it may have a wider range of modulation than yours but I'd guess that oil burners like cycling even less that gas so the argument above probably still applies.
Siemens stats are a bit pricey (shop around) and unfortunately have a certain look of German utilitarianism about them so may not be liked by everyone.
Actually, prices not as bad as I thought, REV24-RF, £110 inc vat which is about the same as the wireless honeywell (CM927), £80 for the wired vs £62 for the honeywell.
Watch out for RF drop off through thick walls, I have put in 2 wireless systems for friends but have wired at home ;-).
HTH
--
fred
it's a ba-na-na . . . .
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On Friday, March 23, 2012 9:39:33 AM UTC, Martin Brown wrote:

You're doing it wrong, IMHO. Time proportional (electronic) control is (was) used for room temperature control, but with on-off durations of minutes; with PWM I believe it's milliseconds (no experience). Rapid on-off usually results in premature death of the motor. It's a cheap & cheerful way of regulating the heat flow with just a pump. A thermostat does much the same.
Better to modulate the CH flow temperature, i.e., weather compensation. If not a modulating burner, then a mixing control from a heat store. It needs some thermal mass to give the burner a decent run time during periods of low-load, or else you get rapid on-off cycles and early burner burn-out, as above.
A new cylinder would probably pay for itself by reducing the oil boiler run times.
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On Sat, 24 Mar 2012 07:45:59 -0700 (PDT), Onetap wrote:

and

...
Oh I think we need to sort out what we are talking about, there are three different things here:
PWM of the power to the CH pump to vary it's speed in relation to heat demand.
Proportional time control of the boiler & pump. The lot comes on a fixed number of times/hour with varying duration dependant on heat demand.
Start optimisation. The heating is switched on by a variable amout of time before the target time such that the target temperature is reached by that time.
I'm not aware of any domestic stats that offer PWM power. Some pumps do "modulate" but AFAIK that is all builtin. Fixed cycles/hour proportional control is not recomended for systems with motorised valves. So that leaves the start optimistaion and a thermostat as being most useful in a domestic situation. Using a programable thermostatic brings in the fine temperature control (against mechanical stat) of an electronic sensor along with the abilty to have the temperature appropiate for the time of day.
--
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Dave.




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