Why loft vents for boiler and immersion cylinders?

[Andy Hall]

Yes.

I have the MAN (now MHG) Micromat EC and it will do precisely that.

the UK, this unit is sold by Eco Hometec and targetted (among other places) for the homebuild market. MHS boilers sell it as the Strata

1 more for commercial applications where several can be clustered for more output.

mention the MHS site because it has much better brochures and technical manuals and the product is the same.

This boiler has a wide modulating range - down to 3kW on some models - and a very good build quality. Servicability is also good in that all major components push fit and clip onto the back plate and can be swapped very quickly.

The internal controller has

- Weather compensation. A sensor located outside provides the temperature and the boiler will adjust output as that changes. For UFH this can be helpful because heat output from the floor can't be quickly adjusted. Because of the insulating effect of the house an inside sensor knows about outside temperature change much later than if it is measured directly

- Analogue sensing of HW cylinder/store temperature. There is a sensor which fits into a pocket in the cylinder which places it in the middle rather than the surface. The boiler knows the store temperature and a reheat cycle can be triggered at a programmable temperature.

- Optionally, analogue sensing of room temperature. There is a programmer made by Siemens for this boiler which gives it the actual room temperature to use rather than just a controller telling the boiler to switch on and off. This provides better results when combined with the weather compensator.

- Analogue control of the pump. Water flow is optimised to match the boiler output and thus the temperature difference across the heat exchanger is optimised.

- Control of motorised valves. There is an option on the boiler to have an internal diverter valve, but a better solution is to use either an external diverter valve or zone valves (S plan). There are installer settings on the controller which allow the boiler to control either scenario directly - i.e. you don't need an external controller.

The controller has about 40 different combinations of operational settings for different max flow temperatures, valve operations, external controls. You can set the weather compensation curve, which basically means the weather compensated flow temperatures at 20 and -1 degrees outside.

Max. flow temperatures in CH mode can be set for systems with radiator outputs designed for conventional boilers (85), condensing (70) and UFH (55). My system had a conventional boiler originally and I changed radiators where needed to give enough output at 70.

If you want to run radiators and UFH, then the normal way is to run either a separate zone or one derived from the main CH with a separate pump and mixing valve which will blend some of the UFH return water with flow at 70 degrees to provide a low flow temperature for the UFH.

In CH operation, I find that the boiler will modulate down to a flow temperature of about 40 degrees or so when the weather is relatively warm and will sit all day doing that. In colder weather, it's rare that it goes over 60 degrees. There can be as much as a 25 degree difference across the heat exchanger. The pump speed drops to as low as 35% of max output.

HW reheat operation depends on one of two things. The cylinder temperature has dropped below the low set point (in my case 55 degrees and a storage temperature of 60 degrees) - it would probably be 85 and

80 or possibly 85 and 75 for a store depending on size. If the boiler detects the cylinder temperature dropping rapidly - i.e you just started running a bath and a shower, then it will start much earlier.

This prevents regular reheats when small amounts of water are being used, but begins reheat earlier when there are large amounts.

The boiler controller switches over the zone valves and winds the boiler and pump up to full power. The return temperature is low until the last couple of minutes of the cycle, whereupon the power level and pump speeds are gradually reduced. This prevents the cylinder temperature from overshooting and maintains the boiler return temperature as low as possible.

In practice all of this works very well. However the boiler does cost around 60-100% more than other good quality boilers.

[Doctor Drivel]

Please use paragraphs :( The great thing about a heat bank is that a simpler, cheaper, less complicated boiler can be fitted. The way the cylinder is arranged ensures stratification has low temperatures at the bottom of the cylinder. The 75C is only at the top section of the cylinder, for DHW purposes. The lower section will always be much lower temperature than this. There are baffles in the cylinder to prevent water mixing. The plate heat exchanger can ensure that water fed back into the bottom of the heat bank is around 25C. You could insist on a double pass plate heat exchanger be fitted which would ensure this sort of return temperature and high flowrates in DHW on full load.

Another point about a heat banks is that a smaller boiler can be used. Boilers operating directly on heating systems are sized for "peak" use. On heat banks/thermal stores only "average" use is required. The thermal storage fills the peaks. But also when fitting a boiler "directly" to a heat bank, not via a coil, a "very large" boiler can be fitted too. This will give pronto DHW recovery. So you have the best of both to choose from. Most boilers modulate their output these days, and the most cost effective boilers are in the 25 - 30kW range. So getting the largest you can at a good price is worth it as the heat bank will not complain and still no cycling.

I personally would consider a Glow HXi30, 30kW, boiler mated to a heat bank. It is a Vaillant underneath, stainless heat exchanger, cheap enough, quiet and very, very good. They do have different sizes and the boiler keeps a constant output modulating along the way. Going over in kW size is not a problem

not use S plans and the likes. Take everything off the heat bank using pumps which don't restrict flow. It is a wonderful central neutral spot.

Insist on "two" cylinder anti-cycle stats on the heat bank. This prevents anti-cycling. When heating, the store is heated to 75C, then it cools to

60C before the boiler cuts in. So no on-off, on-off of the boiler on the one cylinder stat. The stats are placed so that the bottom of the store still remains very, very cool. Look at the DPS diagrams professional people who say a condensing boiler will not condense when mated to a thermal store/heat bank are just plainly thick.

[Christian McArdle]

Nope. If you take water out at 25C and return it at 45C, then it will heat the heatbank, even if most of it was previously at 75C and the hot water is deposited at the top.

Worcester Bosch Greenstar System with the optional diverter valve allows separate setting of water and CH temperatures. You can further subzone the heating side with S-Plan if you have multiple zones. I have the boiler without the diverter and was only aware of it from reading the instructions. I seriously wish I had bought the option, as it would have enabled me to run the heating at a much lower temp.

Christian.

[Doctor Drivel]

The W-Bosch setup. Do they extra controls for DHW and CH and it diverts to suit? I can't remember now.

IIRC, the W-B maintains a constant flow temp and modulates to maintain that. You can install your own diverter valve and fit a weather compensator. It will switch in and out the boiler to maintain the compensator setpoint, which will be low most of the time. Compensators have anti-cycle control on them.

[Christian McArdle]

It's a diverter, rather than a 3 way. DHW priority and separate flow temp controls. The DHW side is limited to 75C. The CH side to 85C. I would have preferred it the other way round as I have indirect heat bank heating and would have liked 82-85C for the hot water and 70C for the heating, but it should be possible to swap the sides (possibly with a relay to invert the priority).

That is my understanding and why I think it would be possible to run the hot water from the CH side.

How would you get it to go hotter for the water with an external diverter?

Christian.

[Doctor Drivel]

Is this two separate knobs on teh panel: DHW and CH temps?

Sounds Greek to me. Oh being PR here is that racist?

See the tech dept, they will proabbly tell you how to reverse

Set the boiler to maximum temp. When DHW calls, the compensator is switched out via a relay. When DHW is satisfied back onto compensator control. The compensator switches the burner (could be in the room stat circuit), the relay just takes this in and out to what flow temp it wants to maintain. The boiler should have anti-cycle control, so no constant click-click, of the burner, and the compensator should have anti-cycle control too. But it may not modulate the burner as the setpoint is maximum on the boiler, and may only modulate to what that is set to.

Some boilers have all this built-in. The MAN/Eco-Hometec, Viessmann ranges do. But these are serious money, but with quality to suit. It would be cheaper, and simpler, to have two cheaper quality boilers, such as the Glow Worms (Vaillants), which are small in physical size too. One for DHW only set to max temperature, and one for CH only on a weather compensator. No

3-way valves. Having a few isolation valves could bring one in to do both if one is down, so backup if needed.

[Christian McArdle]

Yes. The combi and system both have the same panel with DHW and CH temp controls. In normal use, the system boiler DHW temp knob has no function unless the diverter is specified. The new range may be different, I have no idea. The system boiler still needs the DHW temp control even without the diverter, as it is used by the software to control the installer menus.

In actual fact, both DHW and CH are normally limited to 75C. To get more on the CH side, you have to remove a stop on the knob that physically prevents you turning it up further. Unfortunately, no such stop exists to remove on the DHW side. Presumably they didn't conceive of heat banks and TMVs in the design.

Christian.

[Doctor Drivel]

On the heating boiler there is no DHW know at all, so I assume this cannot have this three way valve.

Does the DHW mode modulate? The boiler doesn't really care if water is sent to CH or DHW. Changing over the CH and DHW pots so that the DHW markings turn the CH and reversing the ports on the 3-way valve should get around the low temp settings on the DHW.

Is this 3-ways valve a kit to add on? Or factory fitted?

[legin]

Now I am totally confused! Assuming that we are starting from cold, UFH is on and as you have stated the return temp is at 25C. Boiler lifts the temp by 20C so we now have a flow at 45C. How can that possibly heat the bank to any higher than 45C. Lets assume no UFH and we are calling for hot water. The heat bank is heated to say 75C. UFH now calling; If water is subsequently passed through this at 45C how can it still heat the bank. My initial assumption is that it would draw heat from the bank. If not then how does a thermal store actually heat the hot water as it passes through its coil? Thanks Legin

[Doctor Drivel]

The water requires another pass through the boiler to gain take it to 65C You could put two boilers in series so the first raise to 45C and then input to the second boiler which then raise it again. Realistically, the boiler will raise about 35C, so nearly two passes of heat bank water through the boiler.

So 75C water passing thought the plate heat exchanger and cooling the bottom of the cylinder.

So water out the top at 75C for DHW and water out the bottom UFH section at about 50C - 60C.

I don't know what you mean. I think you mean that the return water (water in the bottom of the heat bank) is about 25C. The boiler only comes in when the store of water cools significantly to reheat in one long efficient burn. The store temp would be around 60C at the top before the boiler comes in. So, water enters the boiler at say 25C the boiler raises it 35C, that is 60C entering the top of the store, then as the store water is passed through the boiler it re-heats. A tall thin store may mean the top temp never gets below 65C, and most of the store water is much less than this. But if you want 75C entering the top of the store you fit a blending valve on the boilers flow and return pipe. This ensures the store water passes only once through the boiler, and only water of 75C enters the top of the store.

See:

On the boiler return there is a blending valve.

See above.

[Doctor Drivel]

I checked this out. It cannot operate at different temperatures on CH and DHW. The tech man on the phone was a bit slow with poor product knowledge, but he said the boiler can only give out one temperature on CH and DHW with the diverting valves fitted, which can only be fitted on system boilers.

It can be done externally, but the modulation would not work that well, as you are just switching in and out the boiler to weather compensator or pipe stat. You can brake into the control circuitry, if you know what you are doing, and ad another temp pot, but that is very much another thing and the guarantee would be void.