Heating and DHW for our cottage

[joe90]

Dear all, now the builders are laying bricks it's time for me to plan details. Generally, like pro Dave, I will be using an ASHP to drive UFH downstairs via a buffer tank and also to heat a DHW tank ( to a temp of 40-45 degrees only to save defrost cycles with the ASHP and excess heat losses from the well insulated DHW tank). As per JSH,s original idea I will install a modulating inline electric water heater to boost DHW should it need it due to hoards of visitors or simply not enough hot water in the tank. I am hoping to run the ASHP on E7 to get the slab and DHW tank up to temp overnight and only switch it on during the rest of the day if required. We are having a woodstove ( small, efficient, room sealed) in the lounge as both err indoors and I love real flames ( this is not negotiable?) and should give heat during cold evenings. 

 

I am also tempted to run a hot water feed from the buffer tank to the loft in case we use a wet duct heater for the MVHR. I will also be running cables there in case we use resistance heating instead.

 

with regard to the hot water cylinder I have read that sealed systems are more efficient than vented systems although they need signing off I have a tame plumber/neighbour who will do this for me.

 

The inline heaters come in 9.6 Kw or 12Kw, any advise on which is more applicable in this situation?

 

In short I would welcome any advise as I am capable of plumbing but not a plumbing designer.

 

joe90

 

[TerryE]

John, if you aren't having a UVC then you'd probably want a TS.  If you are driving your buffer tank to 40°C then as far as I can see there are too possible scenarios for using it:

• Passing the cold feed into the UVC though a heat exchanger driver by the buffer tank.  In this scenario if the buffer tank is charged then the cold fill will be at 35+°C rather than 10°C say, and so if you are running your hot tank normally at 50°C, say, then you will only need to heat the water 15° rather than 40°.
• Using the buffer tank to preheat the UVC with a  bottom coil if fitted, but this will only be useful if the tank around the coil is already below 35°C, say.  But this would heat the entire tank in this scenario.

I am planning to use a SunStack, so only the first option is available for me.  The issue that have with this is that I am only looking at 90 or so heating days a year and the bookend 30 days or so are pretty marginal.  So for ¾ of the year, I won't be using the buffer tank unless I keep it charged just for priming the H/W and if I do then I've got all of the heat waste associated with keeping the buffer tank hot all of the time just in case I want to run some hot water.  When I do the sums I doubt whether I will get the payback to justify the complexity.

 

The problem that I have is that I haven't got the data to optimise our system, and I don't want to over-engineer it unnecessarily.   After having talked the pros and cons through with Jan, what we hare planning to do is to have as simple an electric only system for the first year with a double SunAmp and no buffer tank.  I'll use a simple electric heater for the slab and an inline boost for the SunAmp -- and instrument it properly to work our true usage patterns, demands temperatures, etc.  I estimate something like 90% of my draw will be on the E7 reduced tariff.  Once I have a full years data, I will then be able to design an optimised system and work out the expected payback periods based on hard data.

[Stones]

John, like you I went round in circles with my DHW and heating provision.  In the end I decided to go with a Mitsubishi Ecodan ASHP and pre-plumbed cylinder.  Several reasons for doing so.

 

1. Having done my heating calcs and knowing our DHW useage, I costed out a variety of different solutions.  ASHP had the lowest running costs and over both a 10 and 15 year period had the best return on capital.

2. The ASHP system I have chosen can be programmed (easily) to provide cooling if required

3. A bolt on kit is available for the pre plumbed cylinder which would allow the installation of a secondary heating circuit if we find we need a wet duct heater / cooler for the MVHR ductwork or went for a fan coil heater.

 

I'm planning to heat my DHW to 50C (legionella regime every week).  With a 300 litre cylinder, this should give us more than enough for our requirements relative to our usage pattern, without the need to top up using immersion or an inline heater - I'm happy to accept the reduced COP for DHW.  A 300 litre cylinder also gives me options should I choose to put in some Solar PV and a diversion device at a later date.

 

We had planned a WBS, but I finally managed to persuade my good lady that it wasn't going to be needed, and in any case, there are not that many trees up here to burn!

 

 

[ProDave]

My thoughts on the Thermals store Vs Unvented hot water tank.

 

An unvented tank heated to 45 degrees, will deliver hot water at pretty much 45 degrees until the tank is empty, when it will go cold very quickly.

A thermal store heated to 45 degrees will initially deliver water at 45 degrees but the delivered water temperature will start to fall off unless heat input can keep up with the rate of delivery.

So for practical purposes, to achieve the same useful hot water delivery, a thermal store needs to run at a hotter temperature than an unvented cylinder.

 

I did an experiment recently, I ran a kitchen sink full of water that was as hot as I could tolerate. It was on the verge of burning to keep your hands in it for more than a few seconds. I then measured the water temperature, and it was 45 degrees. So that indicates 45 degrees is plenty hot enough for dishwashing, hand washing etc (mixed with cold water)

 

About the only "need" for water hotter than 45 degrees might be for thermal mixing showers. That is only because to achieve say a 40 degree operating temperature, they might not be abler to blend so far towards hot and maintain stability, so it may be necessary to have hot water for a shower hotter than 45 degrees, just so the mixer can blend it back down properly. That will only be found by experiment and is likely to vary from one shower mixer to another.

 

Therefore my plan is deliver raw 45 degree tank water to all sinks and basins, and just have the in line boost heater for showers, if it proves necessary. Some experimenting will be needed.

 

An obvious disadvantage of storing hot water at a lower temperature is there is less energy stored in the tank. So you will probably want a larger tank than would ordinarily be recommended for the usage.

 

Re unvented tanks and sign off. I installed the last one myself. Building control were happy with that though they did check the discharge arrangements. There is also a guidance note from nhbc that explains how to use a waterless trap to take the discharge into a drain stack to avoid a vent pipe to the outside. that is what I will be doing, and referring building control to that document if the disagree. then arguing it up the chain if they still disagree.

 

I have attached the nhbc waterless trap discharge note.

 

NHBC_discharge_guidance.pdf

[Stones]

1 hour ago, ProDave said:

My thoughts on the Thermals store Vs Unvented hot water tank.

 

An unvented tank heated to 45 degrees, will deliver hot water at pretty much 45 degrees until the tank is empty, when it will go cold very quickly.

A thermal store heated to 45 degrees will initially deliver water at 45 degrees but the delivered water temperature will start to fall off unless heat input can keep up with the rate of delivery.

So for practical purposes, to achieve the same useful hot water delivery, a thermal store needs to run at a hotter temperature than an unvented cylinder.

 

I did an experiment recently, I ran a kitchen sink full of water that was as hot as I could tolerate. It was on the verge of burning to keep your hands in it for more than a few seconds. I then measured the water temperature, and it was 45 degrees. So that indicates 45 degrees is plenty hot enough for dishwashing, hand washing etc (mixed with cold water)

 

About the only "need" for water hotter than 45 degrees might be for thermal mixing showers. That is only because to achieve say a 40 degree operating temperature, they might not be abler to blend so far towards hot and maintain stability, so it may be necessary to have hot water for a shower hotter than 45 degrees, just so the mixer can blend it back down properly. That will only be found by experiment and is likely to vary from one shower mixer to another.

 

Therefore my plan is deliver raw 45 degree tank water to all sinks and basins, and just have the in line boost heater for showers, if it proves necessary. Some experimenting will be needed.

 

An obvious disadvantage of storing hot water at a lower temperature is there is less energy stored in the tank. So you will probably want a larger tank than would ordinarily be recommended for the usage.

 

Re unvented tanks and sign off. I installed the last one myself. Building control were happy with that though they did check the discharge arrangements. There is also a guidance note from nhbc that explains how to use a waterless trap to take the discharge into a drain stack to avoid a vent pipe to the outside. that is what I will be doing, and referring building control to that document if the disagree. then arguing it up the chain if they still disagree.

 

I have attached the nhbc waterless trap discharge note.

 

NHBC_discharge_guidance.pdf

 

It's a very good point that you make about storing DHW at lower temperatures requiring a larger cylinder.

 

 We had 230 litre DHW capacity in our last house, stored at 49C.  Enough for 2 showers, but not much more, hence my decision to increase capacity up to 300 litres.

 

Storage temperature was dictated purely in terms of the minimum temperature required to allow thermostat shower mixers to produce a hot enough shower.

 

 

[joe90]

Thanks guys for all this info, a lot to take in, keep it coming .

[PeterW]

3 hours ago, ProDave said:

I have attached the nhbc waterless trap discharge note.

 

NHBC_discharge_guidance.pdf

 

That is missing a fundamental point ..!

 

HepVo can only be used on a tank where there is a temperature AND pressure relief valve. If it's pressure only you cannot use HepVo as its not certified under the regs for use as it can be in excess of 100c

[ProDave]

9 hours ago, PeterW said:

 

That is missing a fundamental point ..!

 

HepVo can only be used on a tank where there is a temperature AND pressure relief valve. If it's pressure only you cannot use HepVo as its not certified under the regs for use as it can be in excess of 100c

I believe all unvented cylinders have pressure and temperature relief valves so there is no problem.
 

[SteamyTea]

10 hours ago, Stones said:

We had 230 litre DHW capacity in our last house, stored at 49C.  Enough for 2 showers, but not much more, hence my decision to increase capacity up to 300 litres.

Do you have long showers and full baths.

I run my 200 lt DHW at 50°C (ish) and can get 4 showers out of it (or a half filled bath and 2 showers) usually.

A couple of things may account for this.

I have E7 and usually use the water within a couple of hours of the power going off.

I have put a lot more insulation around the cylinder, reducing the losses from about 3 or more kWh/day down to about 1 kWh/day.

 

There has been times in the past when my old lodger would stand under the shower until it goes cold, leaving me with a very luke warm shower at the end of the day, but generally that was the exception, not the norm.

 

13 hours ago, joe90 said:

The inline heaters come in 9.6 Kw or 12Kw, any advise on which is more applicable in this situation?

Are the modulating?  If so, then, assuming the price difference is not ridiculous, go for the larger one.

 

With regards to the efficiency differences of vented or unventilated, I fail to see the difference if everything else is equal i.e. insulation levels, surface areas, pipe lagging and run length.

I may be missing something there (not being a plumber), but a small 'hole' to allow for ventilation is not going to account for much.

 

Knowing what the weather is like down in Cornwall (he says sitting in a friends unheated house in North Shropshire), there are so few days when the temperature goes below 5°C in a year, and hardly any when it is below 0°C in a decade, you have to question the cost benefit of installing 'emergency weather heating'.

Last year I only used a fan heater (was a mild year), this year was a little bit chillier, a little bit earlier, so the fan heater came out a week or two earlier, but it then warmed up again.

I usually 'turn the heating on' when the daily mean temperature falls below 9°C.

[ProDave]

40 minutes ago, SteamyTea said:

 

Knowing what the weather is like down in Cornwall (he says sitting in a friends unheated house in North Shropshire), there are so few days when the temperature goes below 5°C in a year, and hardly any when it is below 0°C in a decade, you have to question the cost benefit of installing 'emergency weather heating'.

Last year I only used a fan heater (was a mild year), this year was a little bit chillier, a little bit earlier, so the fan heater came out a week or two earlier, but it then warmed up again.

I usually 'turn the heating on' when the daily mean temperature falls below 9°C.

What a different climate you live in.

A couple of weeks in winter when the daytime temperature does not get above -10 is not unusual here.
 

 

Re number of showers. Is that "man showers" or "woman showers"  It takes me about 5 minutes to have a shower and wash everywhere and wash my hair (granted I don't have a lot of that left so that bit doesn't take long)  It would be a "short" shower if SWMBO is out in less than 20 minutes.
 

[SteamyTea]

14 minutes ago, ProDave said:

It takes me about 5 minutes to have a shower

What are the extra 2 minutes for? or is it best not to ask

[joe90]

It's just struck me that the COP of the ASHP in colder/damp weather is dependant on if it defrosts, is it not possible to get the ASHP to simply stop producing heat if it is about to defrost?. That way you could get hotter water if the weather is favourable and use the inline heater if it's cold/damp?

[ProDave]

7 minutes ago, joe90 said:

It's just struck me that the COP of the ASHP in colder/damp weather is dependant on if it defrosts, is it not possible to get the ASHP to simply stop producing heat if it is about to defrost?. That way you could get hotter water if the weather is favourable and use the inline heater if it's cold/damp?

That is something I have wondered about and will investigate when I get a system running.

 

My thought if your heating demand is low, and you have all day to heat a tank of water, is in such conditions run the heat pump for say 20 minutes then off for say 40.  That would have to be by an external timer. If doing that, and giving it time to defrost on it's own would avoid a defrost cycle then it would be worth doing.

 

The other efficiency related thing i want to look at is separate the heating of the DHW tank and the heating of the buffer tank, so it only ever heats one at a time. When it is heating the buffer tank for the UFH then I would want to set point temperature of the heat pump set very much lower, than when it is heating the DHW tank. Again that will need some custom controls.
 

[gravelld]

8 hours ago, SteamyTea said:

With regards to the efficiency differences of vented or unventilated, I fail to see the difference if everything else is equal i.e. insulation levels, surface areas, pipe lagging and run length.

I may be missing something there (not being a plumber), but a small 'hole' to allow for ventilation is not going to account for much.

 

I haven't measured it, but I feel the opposite. And surely it depends on the situation. In our house, the cold water tank was almost uninsulated in the loft. There's a clear path for energy to travel from the tank up into that tank. I reckon there'd be loads of heat loss from that. More pipework means more heat loss.