My understanding is that most domestic unvented heating systems use the boiler's pump to circulate water and on/off valves to divert water to the hot water cylinder or heating zones.
Under what circumstances should the zone valves be replaced by zone pumps with a mixing header between the pumps and the boiler? What's the function of the mixing header, and do you always need it if you have any heating circuit pumps or DHW pumps external to the boiler?
Does a zone pump arrangement confer any control advantages over using a boiler pump + zone valves?
(I'm currently considering buying a 38kW boiler which offers a vertical low velocity header as an accessory:
A heat bank/thermal store. Off this you can have the UFH and rad circuits, each one taken directly off the herat bank, so influence from one to the other. The UFH is taken off the cooler bottom, the rads off the centre and the DHW off the hotter top It also provides instant mains pressure DHW. It provides a neutral point that solves many problems with mixed circuits too. See
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for an explanation. Other companies make heat banks too. The boiler only heats the heat bank and is best with two cylinder stats to eliminate inefficient boiler cycling. The mains pressure DHW means that power shower pumps are not required. Check out your mains pressure. It may be worth your while replacing the existing mains pipe with a new plastic larger bore pipe.
A simpler and much cheaper condensing boiler: Worcester Bosch Greenstar, Ideal Icos, Glow Worm, are three good candidates and can be had from £500-£750. The advantage of an heat bank is that cheaper and much simpler boiler can be used as they only heat the water in the heat bank as fast as possible.
A control system of a Landis & Staefa 2 stage controller and modulating valves. (see my other post on this)
This would be a pretty cost effective solution and provide exactly what you need giving excellent efficiency and economy.
The Landis & Staefa controller does most of the control, seamlessly controlling the two differing heating systems of UFH and rads brilliantly, rather than have a control system in an expensive boiler that only half does the job.
According to the boiler installation instructions on mhs' web site you use the mixing header if the resistance of the heating system to be supplied is more than the boiler's built-in pump can handle.
don't need to ;-). Your house must be (a) ginormous (b) draughty and/or (c) badly insulated to warrant a 38kW non-combi boiler, but if you need the output for either of (b) or (c) then spending £x on dealing with these losses is likely to give a better return on investment than spending the same amount on a more sophisticated boiler. For smaller and/or more energy-efficient houses the energy savings from installing even a cheap condensing boiler probably won't actually repay the extra purchase cost of the beast (though hopefully the prices of condensing boilers will come down next year when the big builders start buying
The main advantage of this is cheapness. One pump costs less than several pumps and the associated controls. One pump is generally adequate for most UK domestic installations. Many UK 'heating engineers' cannot comprehend anything more complex than one pump and a
3-port mid-position valve.
Shutting a zone-valve will cause an increase in the flow rates to all the other open zones. This may cause problems with intermittent noise, especially from TRVs. It will mess up any intelligent control systems, anything with sensors and modulating valves.
Yes. You can get a more consistent flow rate to each zone. Shutting off any one zone won't affect the others. It costs more. I'm not sure why they call it a mixing header, I don't have time to look at the link. It's more usually a low-loss header, with no mixing of the water taking place before the returns. A thermal store/buffer vessel would also perform this function, as mentioned in the post about the heating controls.
Try also a Google search for primary/secondary pumping and "closely spaced tees".
If you're using a mixing device, for UFH for example, and are not using a boiler-reset type system, then you have to have a second circulator. The mixing valve might, for example, require 90% return water mixed with 10% hotter flow water. There will be 10% of the return from the mixed circuit returning to the boiler and this will probably be inadequate for proper boiler operation. I've seen mixing valves piped like this, though. They worked ok, unless they started mixing.
In the above case you might have a primary pump circulating water from the boiler around a low-loss header and to the DHWS. You'd then have one or more secondary circuits, with their own pumps drawing water as required from the primary circuit. The secondary circuits would all return the same volume of return water to the primary as they draw off, so they won't affect the primary flow rate.
OK, but why not simply put another primary pump in series with the boiler?
(a) 890 cu.m. and 'yes' to (b) and (c). The property is an old listed building and I can't change 20 large single-glazed sash windows for double-glazed ones...but that's a topic for a whole new thread. I'm doing what the Conservation Officer will allow me in terms of insulation.
I did the Google search, and in doing so realised that the "low velocity mixing header" actually forms a primary loop, with the secondary pumped heating loops teed off primary loop. Contrast this with the normal domestic single pump and multiple zone valve arrangement, which is more like a radial circuit (sort of).
It seems that primary/secondary pumping is used predominantly in larger (commercial) heating installations whereas domestic heating systems just have a primary pump and zone valves. I don't know at what point you should change from zone valves to p/s pumping. Perhaps, when you want more control than the 'crude' on/off systems provided by Danfoss Randall.
Doesn't an automatic bypass valve stop this problem?
With the DHWS controlled by a zone valve, I guess.
That sounds so reasonable. Unfortunately, none of the domestic heating design books I've read refer to anything other than one primary pump plus zone valves. Being a beginner, I don't know if I'd have the confidence to go for pumps instead of valves.
Use a Landis & Steafa RWC82 controller. Two stage with "switching in each stage, no modulation.
use a load compensating control combi for UFH only.
Use a cheapish combi, not even a condensing model for the rads, as it will not be used as often as the UFH.
Have the condensing combi (UFH) do the DHW kitchen tap and other most used taps in the house. Have the other combi do the other taps. Combine the two hot outlets using non-return valves and a small shock arrestor, just before the high flow tap, which is the bath. The two combined should give over 20 litres per minute. Have one combi do one shower, one combi do the other, so no interference from one to the other.
The RWC82 controller will switch in the UFH combi which is set to no more than 55C and it will modulate accordingly, so lowering the UFH temp to suit. The UFH is now sorted. When the UFH can't cope the rads combi will be switched in on the 2nd stage. This is boost so no real need for sophisticated modulation.
This is a very cheap and effective solution to your needs. Seemless control of the UFH and rads to maintain the room temperatures. No expensive boilers, No expensive modulating valves. DHW instantly and to high flowrates by combining the two combi outlets.
The set up is simple: one two stage controller and two combi's. The two combi's may set you back no more than £1400-1500. The 2nd stage rads combi can be had for under £400, The controller is £250. You may want to have a high limit pipe stat on the UFH flow set to 56C. When over 56C the UFH combi cuts out to protect the UFH circuits. You also have eredundancy. If one boiler is down you have have another.
So, one controller, controlling two boilers to maintain a room temperature setpoint. The controllers are highly accurate and will maintain the room temp to what you set it to.
NB: 4 room temperature sensors located around the house can be wired up to "average" between them. This gives a more realistic house temperature. I would not have them on rads in rooms where the sensors are.
If the DHW flowrate is fine, 20 l/min will do even two bathrooms at a push, then I would go this route for cost effectiveness.
Looks like an ultra-efficient condensing boiler _is_ what you need then! Other makes/models you might look at (if you haven't already done so) are Keston and MAN or whatever they're called now. I daresay Andy H will be along in a few milliseconds to tell you all about his, though you could always ask that nice Mr Google :-)
The first paragraph referred to a heating system with a mixing valve, but only one pump, i.e., the way NOT to do it. The mixing valve was between the pump and the boiler. At low loads, most of the flow produced by the pump was re-circulated around the heating circuit leaving only a small amount to return to the boiler. The boiler did not like this.
The second paragraph ( "In the above case you might have a primary pump???etc..") described how the system should have been set up, with primary boiler pump and a secondary (variable temperature) pump downstream of the mixing valve.
On a conventional 1-pump system, an automatic by-pass valve might be connected between the pump's flow and return pipes, to maintain a minimum flow through the boiler if TRVs are mostly shut down. It's operated by the increase in the pump delivery pressure as the flow rate reduces.
Yes. You'd probably need a regulating valve in the heating return to the boiler, which would be a straight pipe with little hydraulic resistance. The water would otherwise by-pass the DHWS branch, which would have a higher resistance. It could also be controlled by another secondary pump but generally the domestic DHWS flow rate is too small. My one's got a 3-port mid-position valve.
heating design books I've read refer to anything other than one primary pump plus zone valves. Being a beginner, I don't know if I'd have the confidence to go for pumps instead of valves.
It (primary/secondary pumping) is very rare on UK domestic installations, mainly because of the costs. IMM has mentioned a £250 controller; most domestic customers cringe at £50 for a replacement heating timeswitch/on-off controller. Whether IMM's recommended controller is applicable to this is another convoluted question, but £250 is probably on the low side. With commercial stuff it's tax-deductible, so they're less price-sensitive and are prepared to pay for effective control.
You would be very fortunate to find a UK ?heating engineer' who understood such a system.
Also, the Americans generally have bigger houses, so it's more common in the US. See ? Primary-Secondary Pumping made Easy' by Dan Holohan, available from HeatingHelp.com. It's written for home owners, so it's fairly free from techno-jargon. It's in US/Imperial units (gpm, BTU/hr, degF) but it's easy to grasp.
Not really, I did also mention a £160 controller with limited weather compensation.
The £250 "two stage switched" controller plus the room sensor, or 4 of them if you want to average temps, would be the only extra cost when using two combi's. NO valves of any description to buy. NO tanks or cylinders. NO pumps to buy as the boilers come with them. Just a Honeywell CM67, or better still a Landis & Staefa equiv.
Each stage of the controller switches a combi. The 1st UFH stage a condensing combi set to 55C and it will also modulate. The seconds stage will also modulate, but supplies the rads. Seamless control..
Cost:
Two Biasi Riva condensing combi's can be had for just over £500 each. OK these are at the lower end. Or more upmarket. An Ideal Icos condensing combi for £750 and a regular non-condensing Worcester Bosch Junior for around £560. So, for £1300 for two combi's.
The controls, let's say £320 in all.
A total of just over £1600 and then the rads and pipes on top. That is very, very good and a top rated commercial controller thrown in.. If you want to save on the boilers then A condensing Biasi at £500 plus a £381 regular Biasi. that works out at: £1200 all in plus pipes. NB: I would not go for the condensing Biasi. The regular is fine, just making a point.
Once understood it is quite easy. The £160 controller I mentioned is quite easy to understand.
Interestingly, having spoken to their technical departments, both Viessman and MHS *require* the installation of low loss headers if any pumps are installed in the heating system (other than the boiler's pump, of course) running off their modulated boilers.
MHS supply a Strata 1 boiler. It is a re-badged version of the Eco Hometec Micromat EC. The Eco Hometec literature shows a Y-plan zone valve installation with a supplementary pump in the radiator circuit. The MHS literature shows almost the same arrangement as an example of an incorrect installation!
(For flow control, Viessman Vitodens can be connected to a sensor that monitors the header temperature. The Strata 1 just monitors the temperature within the boiler.)
I think that I've got enough information to come up with a draft heating system design. I'll probably post it on the web and invite comments. This newsgroup has been an excellent source of help in that respect.
Actually, cost isn't a major factor. Not because I'm loaded but because my renovated house will be a family home for a considerable time so I've got time to "enjoy" the capital outlay. Something that is clear for me to understand is one of the key requirements for the new heating system.
Unfortunately, I'm a plumbing novice with that very dangerous thing: a little bit of knowledge.
With condensing boilers headers reduce efficiency. With a condenser you don't want very hot water feeding right back into the return. This raises the return temp which reduces efficiency. You want the return temp as low as possible. Best use a heat bank or thermal store, which is a great neutral point, and have the boiler heat that directly. For most of the re-heat the return temp will be low.
All headers are neutral points for circuiots to tee into. A heat bank gives you that and a DHW store with instant high pressure DHW. You only need cheap simple boilers with a heat bank, one of its selling points.
I'll try and come up with some alternative systems.
Not if you are using a condensing boiler with wide modulating range such as the MAN Micromat or Viessmann.
Putting a heat bank in the middle will do one of two things.
a) If the heatbank is used for HW as well as CH, and there is a thermostat arrangement on the heatbank, the effect will be to prevent the boiler from operating at the lower end of its temperature range and will cause it to cycle on and off.
The whole point of this type of boiler is to have high output for a short period to heat the heatbank/cylinder quickly for DHW use, and to drive the heating system directly to allow modulation down to low output levels.
or
b) If the heatbank is used solely for CH and no thermostat, it will effectively create an equilibrium situation with heat in and out and no real advantage over a much smaller header.
It is far more efficient to run this type of boiler continuously at lower output than in bursts of high output.
Won't a modulating boiler adjust its flow temperature and velocity to reduce the return temperature though? I thought that that was the purpose of the modulating control.
Here is Andy still attempting to justify an expensive purchase. He goes on....
All that does is lower the flow temp in the header. If a secondary circuit kicks in it may require hotter water than what is in the header, then the boiler has to react, which can take far too much time. Fan coil units require high temps of around 80C pretty instantly, so having a header stting there at 40C is no use. Also the primary and secondary pumping has to be setup correctly. If not you may have problems.
Not if anti-cycle stats are fitted. Simple thing to do. Putting a MAN boiler on a heat bank will do the same thing as if it is on a header, except the return temp will be cooler most of run time as speaders are inside the heat bank to ensure hot water goes to the top and stays there, preventing mixing of the store, and the heat bank heats up top down. It is possible for the water at the top of the heat bank to be 80C and 20C at the bottom. Which is not the case with a header which sends heated water from the boiler directly back to the return.
In commercial systems cylinders are used as neutral points to replace headers, similar to Dunsley neutralisers and heat banks. The Germans tend to use these. Cylinder neutral points, similar to a heat bank, are better when using condensing boilers. The cylinder water is stratified with say hot water for fan coil units and DHW at the top and cooler water at the bottom for background rads circuits controlled weather compensators. When the boiler reheats the neutral point cylinder, the return temp is invariable lower than using a header promoting efficiency. Also not having to set the primary and secondary pumps up is great bonus.
In is usually better to divide and rule and have dedicated boilers heating to the temperatures of the various circuits. One boiler heating only high temp DHW, one high efficiency condensing boiler only heating a low temp background rad circuit etc. Boilers can then be matched for maximum efficiency. I have seen one demostic boioer heat only the fan coilo battery on an air handfling unit. There misy have been 20 or 30 of these in the place. They had a cuborad of spare for the bopioer type and they were never down for long if part were replaced. No expensive mixing valves were used and large harders and long large expensive pipes. Quite cheap to do they told me. The problem is redundancy in commercial systems, that is why boilers are sequenced together. If open drops out the other(s) will cope. Then headers, neutral points, mixing circuits etc, are incorporated to make it all work.
Many commercial boilers are frames with smaller boilers on the frame, called modules, each with its own burner. The size of the whole "boiler" can be specified by adding modules. Having many modules reduces large boiler cycling as smaller burners (boilers in effect) are brought in to match the heat demand. If one module drops out the others are still operative. The "modules" are switched in to demand by a controller similar in operation to the two stage Landis & Staefa one I mentioned, only it will have stages to suit the number of burners.
So, dividing and ruling when boilers are cheap enough makes a lot of sense in a small comercial large domestic setup.
Is it? Look at what I write and take note. Nah, don't bother.
No DHW?
There is. The return is cooler most of the run time compared to a straight through header.
So, you use a cheaper and simpler boiler. The MAN type of boilers will be more efficient on a heat bank rather than on a straight through header, because of the top down heating of the heat bank.
Most commercial system are still non-condensing and use headers. Where commerical condesners are used they still use the same temps as in non condensers, and gain about 5-7% because of the larger heat exchangers. Headers are commercial and are intended as:
A neutral point where different circuits may take off hot water to suit. The different circuits then mix to lower temps to suit.
Headers also are used to keep the return temp up on the boiler to prevent back end condensation.
A central point for all circuits.
It will be and you will be educated somewhat. Imagine allowing people like you to design a system? The thought of it.....
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