Heating system for PH

BigGeorge

Am looking for feedback /opinions on the following suggested heat systems for a passive house. In particular your thoughts on the complexity, effectiveness or cost would be appreciated

8-10sqm, solar thermal array on south facing roof
Large capacity HW tank in the plant room, 750-1000lt
High efficiency condensing gas boiler in plant room
MHRV with water/air post heater unit run off HW tank, approx 2kw ( raises air inlet temp to about 20C when cold)
Zoned low temp underfloor down stairs
No heating upstairs
Hi temperature heat dump on tank to outside for summer overheating

CJhaughey

BigGeorge wrote: »

Am looking for feedback /opinions on the following suggested heat systems for a passive house. In particular your thoughts on the complexity, effectiveness or cost would be appreciated

8-10sqm, solar thermal array on south facing roof
Large capacity HW tank in the plant room, 750-1000lt
High efficiency condensing gas boiler in plant room
MHRV with water/air post heater unit run off HW tank, approx 2kw ( raises air inlet temp to about 20C when cold)
Zoned low temp underfloor down stairs
No heating upstairs
Hi temperature heat dump on tank to outside for summer overheating

Its not a passive house if it has a gas boiler and water powered UFH.
A PH by defination need no external heating.

BigGeorge

Good comment & fair enough point but everyone i'v spoken to who lives in one states that background heat is necessary in the houses, especially in the MHRV supply. Have had the PHPP run with a gas boiler instead of a wood pellet boiler or even a heat pump , and it still passes.

So working on the basis that some background heat is required, I'm looking to maximse the solar and come up with the most economical form of suplemental heat - which is unfortunately coming out as the gas unit at the moment given how little gas will actually be used, in the region of €350 per annum.

morning delight

BigGeorge wrote: »

Am looking for feedback /opinions on the following suggested heat systems for a passive house. In particular your thoughts on the complexity, effectiveness or cost would be appreciated

8-10sqm, solar thermal array on south facing roof
Large capacity HW tank in the plant room, 750-1000lt
High efficiency condensing gas boiler in plant room
MHRV with water/air post heater unit run off HW tank, approx 2kw ( raises air inlet temp to about 20C when cold)
Zoned low temp underfloor down stairs
No heating upstairs
Hi temperature heat dump on tank to outside for summer overheating

Sounds similar to what they've put into the Denby Dale PH in England. They've gas outside the front door. Can you connect to the gas network or will you have to put a tank in (if that is how it's done!)?

BigGeorge

Yep, have mains gas, have space though for a pellet silo so am open to options

sinnerboy

CJhaughey wrote: »

Its not a passive house if it has a gas boiler and water powered UFH.
A PH by defination need no external heating.

That is not correct . With respect to heating certification criteria is
15kwh/m2/year max space heat demand or max heat load of 10w/m2 .
Only if you meet max heat load of 10w/m2 is it possible to provide heat by air alone . Often it is not - if you over heat air it becomes uncomfortably dry .

Certified Passive houses often feature boilers ( all types of fuel ) or heat pumps ( ground source or air ) , electric mat ( watch BER / B Regs compliance with THAT option ) .

Heat distribution may be by rads or ufh . Better to use rads with low/medium thermal mass fabric . Better to use UFH with high thermal mass . (There is a terrific article in the latest Construct Ireland on that very matter )

The typical "traditional" energy domestic requirement has been approx
65-80kW m2/year space heating + 20-35 kW m2/year domestic hot water .
The space heating was by far the bigger part . This flips over with a Passive house - max 15kW m2/year space heating + 20-35 kW m2/year domestic hot water .

You need to generate Domestic Hot Water in the Heating Season with something - even 10m2 of Solar Thermal will not do this in Nov-Dec-Jan .
Thus - boiler ! or UFH . Or immersion ( even )

Gig George it is hard to be definitive with out more information in the form of the PHPP calcs together with quotations from mech installers together with a canny energy advisor . My "nose" tells me that whist the big cylinder will take all the ST's will generate during summer ( you may not need to "dump" hot water ) ..... the tank will demand a lot from your bolier during the depths of Winter . Just a hunch .

.

homewardbound11

I am stuck with this at the moment and have posted on this site. I have tried mears and went back to both my plumber and BEr assessor. This issue is not isolated to passive house but to any low energy house. My plumber has automatically applied a factor of 4 to the cubic foot calcs but these are at least 25 percent over online calculations.
I Approached the SEI on this as my preliminary ber has an A2 rating and my ber calculations had a boiler requirement of 17kw .
This is what the SEI have replied with and i am open to opinions. I am awaiting my BER assessor's reply for a fortnight now.

I dont want to over spec the build or have excess consumption due to oversized radiators. Neither do I want to under specify the build.

Maximum recommended dwelling heating load: Expressed in kW.
For dwelling heat load, the maximum space heat demand is calculated by multiplying the heat loss coefficient by the desired maximum temperature differential ΔT (usually -1 to 21oC). The heat loss coefficient is the heat loss in W/K of the dwelling when accounting for fabric and ventilation losses. The heat loss coefficient is called the "Total Heat Loss" in the DEAP. 2 to 4 kW should be added to the space heating demand for water heating depending on occupancy or floor area.
The total of space and water heating requirement should then be multiplied by 1.1 to account for distribution losses giving the following maximum recommended dwelling heating load:
Dwelling space and water heat requirement (W)
= dwelling space heat loss + dwelling DHW requirement
= Total Heat Loss*22 *1.1 + 4000*1.1

therightangle

sinnerboy wrote: »

....the tank will demand a lot from your bolier during the depths of Winter . Just a hunch .

Im curious as to what you mean here sinnerboy....Is it because of the "need" to bring the entire large tank up to 60 weekly to combat legionella?
Would this be a reason to introduce a tank in tank, or secondary smaller tank for DHW?

Or is it the extra volume of the big tank that the boiler has to heat when trying to satisfy a cylinder thermostat?

Or just that the solar wont be contributing much in winter?

.

sinnerboy

I'm not a heating engineer but ...

PHI advice is that to avoid legionella that the hot water cyilinder must be raised to 60 degrees at least 3 times each day .

The UK Heath service requires that water - in distribution pipes must not drop below 55 degrees - meaning in practice it has to be stored above 60 degrees .

The "need" is real .

In Ireland - my simplistic understanding - open to correction - is that

1. if we size our hot water storage cylinder to take all that our solar panels can offer during summer without dumping any hot water produced --- that
2. we then have a very large cylinder to heat by boiler ( or heat pump ) in Winter .

So we have to balance the size of our cylinder taking into account our low solar radiation winters - and accept an amount of "dumping" of hot water in summer .

Pity .

If I am wrong - I will be happily corrected .

.

RVR

If this is a combination tank with DHW production coil or tank in tank then the boiler won't have to heat the entire tank in wintertime?

1. The boiler only has to heat the top of the tank to 60/65. Domestic hot water is taken care of as it will be indirectly heated to a similar temperature before it leaves the tank.

How hot the DHW will be heated to obviously depends on flowrate. E.g. at 14l/min it could be a delta T of about -3, e.g. 3 degrees less than the surrounding water (this depends on the tank obviously - check with your manufacturer) If this is a major concern then keep the top of the tank at 65/70!!!

If you use a tank with a DHW coil (instead of one with a tank in tank) there will only be about 40 litres stored at any given time so the chance of leigonella is greatly reduced straight away.

2. If the underfloor or rad tappings are also positioned correctly (e.g. towards the top of the tank) then the entire tank doesn't have to be heated. Stratification will cause the hottest water to remain at the top, while where the cooler water just beneath is (35/40) is where you want to tap off your underfloor. The solar coil (unless you're using a stratifier) will be located at the bottom in the coldest (10/15) water so will have a good efficiency.

Cheers

Des

RVR

Ps - If you are using a hot water "battery" heater in your MVHR for upstairs - you would tap off from the hottest part - top of the tank!!!

therightangle

Interesting and worthy references there sinnerboy - the last plumber I spoke to claimed legionella scare was only for hospitals and large cylinders, etc.

An interesting take is SEI/Greener Homes Installer Guidelines for solar thermal:

The controller settings should be set to ensure that
the homeowner gets the optimum performance from
the system; for example the maximum cylinder
temperature may be set to 700C when measured at
the bottom of the cylinder or 800C when measure at
the top of the cylinder. This will ensure operation on

sunny days and ensure energy storage for dull days.

More if you google:
Good_Practices_for_Equipment_GHS_Vol_3.pdf

RVR, do you have more info on tank in tank schematics/design for large(ish) domestic solar installations, so as to reduce winter boiler use, etc.

Or more info on the DHW direct coil option? BTW, why would the 40litre coil be a lesser legionella risk? Is it because the volume is small?

Tanks

RVR

Is it because the volume is small?

Yes - water will spend less time sitting inside the coil than inside a larger tank so less time for the bacteria to grow.

do you have more info on tank in tank schematics/design

I've attached a jpg of a drawing we did for a customer. It has gas boiler, heat pump and solar - however you could remove the heat pump and nothing else would change.

In this case the solar uses a stratification coil which loads the solar in as high up the tank as possible. But using a coil for solar located at the bottom of the tank would also give good results.

How much of the tank the boiler heats is determined by the positioning of the stat on the combination tank. By positioning the stat about 1/4 of the way down from the top the boiler would keep the top of the tank hot for DHW production, rad tappings etc etc.

BigGeorge

Thanks guys - great information.

Has anyone expererience of using a battery / water to air heater on the MHRV ? Suggestions on suppliers or whether to get one of the MHRV units with the rad doing the post heat built in would be great

therightangle

Thanks for the schematic RVR,

Can you explain the relationship between the 2(or 3?) tanks you have shown? The solar looks like it is squashed into a 3rd cylinder - Im not sure why that is...

RVR

Hi therightangle,

There is only one tank in the schematic (at the centre) but it is a solar stratifier tank. In this particular tank the solar is connected through a special external coil which is physically bolted to flanges on the side of the tank. It is designed to allow the solar enter the main body of the tank as high up as possible, depending on the temperature of the tank and the temperature supplied by the solar etc.

I've attached an updated schematic which shows the pipes from the external coil attached to the main body more clearly. The main body then contains the inner DHW production coil.

I hope this helps to explain better. The schematic would be almost exactly the same if the solar was going through an internal coil in the main body of the tank opposed to the external stratifier on this particular tank.

Cheers

Des

sinnerboy

RVR wrote: »

How much of the tank the boiler heats is determined by the positioning of the stat on the combination tank. By positioning the stat about 1/4 of the way down from the top the boiler would keep the top of the tank hot for DHW production, rad tappings etc etc.

Aha - is this the (simple) solution to not overburdening the boiler during Dec/Jan when the solar thermal panels or tubes offer so little RVR ?

What capacity and approx physical size is the cylinder in your schematic

RVR

sinnerboy wrote: »

Aha - is this the (simple) solution to not overburdening the boiler during Dec/Jan when the solar thermal panels or tubes offer so little RVR ?

Yes - and this approach leaves the remaining 3/4 of the tank unheated so that the lower temperature solar / heat pump can contribute lower down the tank (however little solar might contribute in the depths of winter...)

What capacity and approx physical size is the cylinder in your schematic

1000L, approx 2050mm h x 1050 mm w excluding the external stratifier on the side.

therightangle

Thanks for that RVR.


To get this straight, and you seem like the right person to ask, I have a few more q's ....

RVR wrote: »

If you use a tank with a DHW coil (instead of one with a tank in tank) there will only be about 40 litres stored at any given time so the chance of leigonella is greatly reduced straight away.

Are your schematics showing HP, boiler ufloor connected to the cylinder water, and DHW (output) taken from the coil, the opposite of a traditional cylinder?
Is this all one coil, the length of the tank? Sorry, its unclear from the schematic.
Is it accepted then that DHW only needs to reach the 60+ deg for an instant, ie as it is passing out of the cylinder, to kill legionella?
Do you have any references for this RVR?

RVR wrote:

however you could remove the heat pump and nothing else would change.
In this case the solar uses a stratification coil which loads the solar in as high up the tank as possible. But using a coil for solar located at the bottom of the tank would also give good results.

If the Heat Pump, or wood pellet that the OP has mentioned, were excluded and it was just the gas boiler with the solar, would you then just put the solar into the bottom of the main cylinder, with no difference in solar performance? So adding the stratifier in this case does not justify the extra cost and space?

I wonder why add the HP or pellet inputs then, as that amount of solar will surely satisfy Part L for most houses, and the boiler is always there?

Apologies for all the questions but your proposal in general seems to make a lot of sense and Im just wondering are there downsides.

RVR

therightangle wrote: »

Are your schematics showing HP, boiler ufloor connected to the cylinder water, and DHW (output) taken from the coil, the opposite of a traditional cylinder?

Exactly.

Is this all one coil, the length of the tank? Sorry, its unclear from the schematic.

Yes, but most of the surface area of the coil is within the top 1/4 of the tank where the coil is tightly packed.

Is it accepted then that DHW only needs to reach the 60+ deg for an instant, ie as it is passing out of the cylinder, to kill legionella? Do you have any references for this RVR?

Have a look at this link for some information of how soon legionella dies: http://www.docstoc.com/docs/29662993/Safe-Hot-Water-Temperature

At 60 degrees C, legionella dies within 32 minutes. At 66 it is down to 2 minutes. At 70 and above it is much shorter again and they die very rapidly e.g. this is the "disinfection temperature".

If you could bring the domestic water to 70 or above before it exits the tank (e.g. in the top 1/4 of the tank), and use a blending valve to mix down to a safe temperature, this would give excellent protection. The fact that water will spend very little time in the part of the coil which is not in the top 1/4 of the tank means less time for bacteria to grow, which further reduces the risk.

The idea of coil in tank has grown on the continent as opposed to tank in tank due to the relatively low volume of DHW stored and tighter local regulations - there are plenty of these coil in tank products on the market here and abroad for this very reason.

If the Heat Pump, or wood pellet that the OP has mentioned, were excluded and it was just the gas boiler with the solar, would you then just put the solar into the bottom of the main cylinder, with no difference in solar performance?
So adding the stratifier in this case does not justify the extra cost and space?

Yes, this would give good performance also. As you say the cost and space savings mean you would probably just use a coil instead and get similar performance. The stratified tank is just what was used in this particular drawing!

I wonder why add the HP or pellet inputs then, as that amount of solar will surely satisfy Part L for most houses, and the boiler is always there?

Cost of fuel, CO2 emission considerations, budgets, there are plenty of reasons someone might decide to do it this way. Another option might be solid fuel or wood gasification boiler which could be carbon neutral. There are plenty of technologies out there, it's up to the person themselves just how much they want to do and for what reason (for example solar will not be enough to meet part l in very large houses)!

Apologies for all the questions but your proposal in general seems to make a lot of sense and Im just wondering are there downsides.

No problem, glad to help. Only downside is that it is a more costly system than just having a regular boiler connected to rads and cylinder, but otherwise is a good option.

therightangle

Thats excellent RVR, much appreciated!

I suppose the cost downside you mentioned is probably justified for someone who wants to make more efficient use of solar, when it is their sole provision for Part L.